FEARING, J.

¶ 1 This appeal primarily asks us to address conditions precedent to a medical expert rendering opinions during a medical malpractice trial. The trial court excluded opinions of plaintiff Joshua Driggs' foremost medical expert because the physician did not commit, when asked, to base opinions on reasonable medical probability, because he testified to a national standard of care, because he conceded in cross-examination that his opinions were personal, and because he did not provide a percentage for the increased risk of a fracture resulting from the lack of fixation for an allograft. We agree with Driggs that the trial court committed harmful error, and we remand for a new trial.

FACTS

¶ 2 Joshua Driggs sues Providence Physician Services and its employee, Dr. Andrew Howlett. We refer to the respondents collectively as Providence Physician Services or Providence.

¶ 3 Appellant Joshua Driggs asserts errors during the course of trial. Therefore, we briefly outline the facts in this section of the opinion and later provide extended details of the facts when reviewing trial rulings. Joshua Driggs limits his suit for medical malpractice to claims of negligence by two employees of Providence Physician Services, Orthopedist Andrew Howlett and Physician's Assistant Brandi DeSaveur, during 2009. The story of Driggs' medical care begins earlier.

¶ 4 In 2004, health care professionals diagnosed fifteen-year-old Joshua Driggs with osteosarcoma above the ankle in his right distal tibia. Osteosarcoma is a common form of bone cancer in children. Instead of amputating the leg, Dr. Ernest Conrad removed the cancerous fragment of the bone and inserted an allograft, or bone segment from a cadaver. Dr. Conrad attached the allograft to the remaining tibia by screwing a metal plate to the tibia and allograft. The plate supports the allograft because the cadaver bone lacks the strength of a living bone. The metal plate is called fixation or hardware and serves as a support for the allograft. In this suit, Joshua Driggs and his experts promote the need of fixation to an allograft.

¶ 5 The cadaver bone inserted into Joshua Driggs' tibia extended six and a one-half centimeters, which equates to three and one-half to four inches. Driggs' surgeon, Dr. Ernest Conrad, would not remove the metal plate from a patient's allograft without substituting another plate or other form of fixation unless the graft is "very small" and has vigorous growth around it. Conrad defines "very small" in this context as "four or five centimeters or smaller." Clerk's Papers (CP) at 1552.

¶ 6 Although properly aligned initially, Driggs' allograft later twisted and required additional surgery. In January 2006, Dr. Andrew Howlett, of Providence Physician Services, assumed care of Joshua Driggs' right leg. In January 2006, Dr. Howlett performed an ankle fusion and osteotomy on Driggs to correct malalignment in the leg, improve mechanics in the foot, and decrease arthritic pain. In November 2006, Dr. Howlett performed another surgery to alleviate pain in the ankle caused by two screws.

¶ 7 In January 2008, Joshua Driggs' right ankle pain returned. Dr. Andrew Howlett discussed with Driggs another surgery to remove the plate inserted by Ernest Conrad and replace it with an intramedullary rod. A rod may substitute for a plate in supporting the allograft. According to Howlett, he discussed with Driggs, before the surgery, the possibility of not replacing the plate with a rod, because of deleterious effects of a rod. An intramedullary rod runs through the inside of the bone, and the rod's installation requires destruction of existing allograft and live bone.

¶ 8 During a March 6, 2009 surgery, Dr. Andrew Howlett removed the plate and screws from Joshua Driggs' cadaver bone. Dr. Howlett did not replace the plate with an intramedullary rod. Driggs claims Dr. Howlett violated the standard of care by failing to install fixation and breached his right to informed consent by failing to explain the risk to him of the omission of fixation.

¶ 9 After the March 2009 surgery, Joshua Driggs underwent physical therapy, but continued to suffer pain and swelling. In May, while crossing his yard, Driggs experienced a shooting pain in his right leg.

¶ 10 On May 27, 2009, Joshua Driggs visited Dr. Howlett's office at Providence Physician Services and met with Physician's Assistant (PA) Brandi DeSaveur. Driggs reported the increased pain and swelling to DeSaveur. PA DeSauver X rayed the tibia and diagnosed a possible sprain. DeSaveur failed to note a subtle fracture. Driggs contends that DeSaveur violated the standard of care by failing to identify the fracture and Andrew Howlett violated the standard of care by failing to properly supervise Brandi DeSaveur. Driggs posits that his condition worsened as the result of the failure to promptly diagnose the fracture. On May 27, DeSaveur instructed Driggs to discontinue physical therapy for one week.

¶ 11 On June 7, 2009, Joshua Driggs visited a hospital emergency room due to pain in his right leg. He received a shot and a prescription for pain medication. On June 8, Driggs returned to Dr. Andrew Howlett's office. During the appointment, Howlett X rayed Driggs' tibia and noticed the bone fracture.

¶ 12 On June 11, 2009, Dr. Andrew Howlett performed another surgery and inserted a tibial intramedullary rod into Joshua Driggs' right lower leg. Despite the rod, the cadaver bone failed to fuse with Driggs' live bone.

¶ 13 On December 11, 2009, Dr. Howlett, during another surgery, placed a rod with intermittent screws through the entire tibia. The December 2009 surgery necessarily destroyed Driggs' subtalar joint in his ankle. The surgery also caused equinas, a condition by which Driggs' toes touch the floor but his heel rests two and one-half inches above the floor. Driggs thereafter walked on his right toes.

¶ 14 In July 2010, Dr. Brian Padrta performed an operation to remove two remaining screws and correct the equinas. Nevertheless, as of March 25, 2013, Driggs continued to suffer from severe equinas, numbness in his right foot, and a limp.

PROCEDURE

¶ 15 On January 17, 2012, Joshua Driggs sued Dr. Andrew Howlett and his employer, Providence Physician Services. Driggs asserted professional negligence and lack of informed consent. The complaint alleged:

CP at 10-11, 13-14. The complaint did not specifically identify Physician's Assistant Brandi DeSaveur as a negligent actor. In their answer to the complaint, Dr. Andrew Howlett and Providence Physician Services admitted that Providence employed Dr. Howlett, Howlett acted within the scope of his employment when treating Driggs, and Providence was vicariously liable for any negligence committed by Howlett.

¶ 16 Both parties engaged expert medical witnesses. Joshua Driggs hired two witnesses, Drs. Steven Graboff and Lawrence Menendez. Providence engaged five experts, but only Drs. James Bruckner and Brian Padrta testified at trial.

¶ 17 On May 3 and September 20, 2013, Joshua Driggs deposed Dr. Andrew Howlett. Driggs' counsel addressed, with Howlett, Brandi DeSaveur's failure to diagnose the tibia fracture on May 27, 2009. A portion of the September deposition of Dr. Howlett follows:

CP at 1270-71.

¶ 18 The parties filed proposed jury instructions and a trial management report weeks before a January 6, 2014 trial. On December 9, 2013, Joshua Driggs proposed a jury instruction that declared Brandi DeSaveur to be an agent of Providence and any act or omission of DeSaveur was an act or omission of Providence. On December 12, 2013, Driggs filed a joint trial management report, which read in part: "[The Plaintiff also contents [sic] that Dr. Howlett and coemployees failed to follow the standard of care in their follow up treatment under the circumstances. (The Defendant objects to this statement.)]." CP at 112 (alterations in original). The trial management report did not name Brandi DeSaveur as a coemployee who breached the standard of care.

¶ 19 Joshua Driggs' feature witness, Dr. Lawrence Menendez, has served on the University of Southern California Keck School of Medicine staff since 1985. He specializes in the care of bone tumors. He teaches orthopedic oncology to medical students. He has been board certified since 1987. Menendez is a member of the American Academy of Orthopedic Surgeons, Musculoskeletal Tumor Society, and International Society on Limb Salvage and sometimes lectures at the respective organizations' meetings.

¶ 20 Dr. Lawrence Menendez could not attend trial to testify. On December 12, 2013, Joshua Driggs conducted a video recorded perpetuation deposition of Menendez to play to the jury. Near the beginning of Dr. Lawrence Menendez's deposition, Joshua Driggs' counsel remarked and asked:

CP at 1347. Note that Dr. Menendez did not respond to counsel's direction to base his opinions on reasonable medical certainty.

¶ 21 During the deposition, Dr. Lawrence Menendez spoke about failings of allografts from weakness and incapacity to incorporate into the host bone. He promoted the need to affix an allograft with plates and screws to promote strength in the allograft. He commented on holes or weak areas in the allograft called stress risers, which fracture without protection from hardware. Dr. Menendez testified that

CP at 1343.

¶ 22 During his deposition, Dr. Lawrence Menendez testified that in 2009 the national standard of care for removing fixation from an allograft required replacement of the fixation. Joshua Driggs then questioned Dr. Menendez about whether a fracture will result from the lack of fixation:

CP at 1343-47, 1350-51.

¶ 23 Providence Physician Services also questioned Dr. Menendez during the perpetuation deposition:

CP at 1411.

¶ 24 On December 20, 2013, Providence Physician Services moved to exclude evidence of the circumstances leading to Providence's termination of Brandi DeSaveur. In response, Joshua Driggs commented that evidence established that DeSaveur should have, but failed to, discern fractures present on the May 27, 2009 X ray, and, as a result, Driggs' fractures worsened and complicated his treatment.

¶ 25 On December 31, 2013, Providence Physician Services filed a supplemental motion in limine to preclude testimony by Dr. Lawrence Menendez regarding the standard of care, medical causation, and medical risk for informed consent. Providence underscored that Dr. Menendez is from California and he testified, during his deposition, to a "national standard of care" rather than a Washington standard of care. Joshua Driggs responded that an out-of-state expert may testify to the national standard of care as long as other evidence shows the standard of care in Washington to be a national standard. Driggs submitted a declaration by Dr. Menendez stating that he conferred with experts within Washington and determined that the Washington standard of care was equivalent to the national standard. In response, Providence argued that submission of a supplemental declaration amounted to an ambush, did not allow for cross-examination, was inadmissible, and should not be considered by the trial court when ruling on its motion. The trial court agreed with Providence's characterization of the declaration as an "ambush" and refused to consider it. After hearing the parties' arguments on January 2, 2014, the court reserved ruling on the motion.

¶ 26 On January 7, 2014, Joshua Driggs began presenting his case to the jury. That day, Driggs submitted to the court an affidavit from Dr. Lawrence Menendez averring that he contacted medical colleagues in the state of Washington to confirm that the practices in Washington echoed the national standards of the American Orthopedic Association, that the standard of care applicable in this case is a national standard, and that he is aware of the standard of care in Washington.

¶ 27 On January 9, 2014, orthopedic surgeon Steven Graboff testified at trial as an expert for Joshua Driggs. When Dr. Graboff first sought entry into medical school, no United States school admitted him. He, therefore, began medical school in Guadalajara, Mexico. After four years and three months of schooling in Mexico, Graboff transferred to the University of California at Irvine School of Medicine, where he received a medical degree in 1980. Graboff is also board certified, although, according to Graboff, he has encountered difficulty on occasion in retaining certification. Dr. Graboff is not a specialist in tumors, but has experience with limb salvage through allografts. The American Academy of Orthopedic Surgeons suspended Graboff for two years for testimony he gave in a medical malpractice suit.

¶ 28 Dr. Steven Graboff's trial testimony included:

3 Verbatim Report of Proceedings (VRP) (Jan. 9, 2014) at 376-77. Dr. Graboff later testified that a physician violates the standard of care if he does not replace fixation for an allograft with other fixation.

¶ 29 Concerning the care provided by Physician Assistant Brandi DeSaveur, Dr. Steven Graboff testified:

3 VRP (Jan. 9, 2014) at 399-402, 405.

¶ 30 On January 13, 2014, the trial court entertained additional argument on Providence Physician Services' motion to exclude portions of Dr. Lawrence Menendez's testimony. Providence contended that Dr. Menendez must know the Washington standard of care and may not rely on other experts to establish the foundation for his testimony. Also, Providence argued that all of Dr. Menendez's opinions were personal opinions and did not meet the testimonial requirement that a medical expert's testimony be based on a degree of reasonable medical probability. Providence also asked to exclude Dr. Menendez's testimony on whether removing the plate without installing a rod was a material risk and required Joshua Driggs' informed consent. Providence argued that Dr. Menendez never provided any testimony as to the scientific nature of the risk and the likelihood of its occurrence.

¶ 31 The trial court granted Providence's motion to exclude Dr. Menendez's testimony about the standard of care, medical causation, and risk for informed consent. The court observed that no law supported Joshua Driggs' reliance on another physician's testimony to lay the foundation for Dr. Lawrence Menendez's opinion that the Washington and national standards of care correspond. In its ruling, the trial court noted that Lawrence Menendez never agreed, in response to counsel's direction, to base his opinion on reasonable medical probability. The trial court also noted that Dr. Menendez, in response to questioning by defense counsel, commented that his opinions are personal opinions.

¶ 32 Providence Physician Services called to testify Orthopedic Surgeon James Bruckner, of Bellevue, Washington. Despite Dr. Steven Graboff earlier claiming to the contrary, Dr. Bruckner, at trial, denied testifying in his deposition that the national and Washington standards of care corresponded. Bruckner testified at trial that he had no knowledge of whether the state of Washington standard equated with the national standard. Dr. Bruckner testified that, under the standard applied in Washington, if not nationally, an orthopedist exercises discretion as to whether or not fixation is needed for the allograft. In other words, fixation is not always demanded. Dr. Bruckner conceded, nonetheless, that he has never removed fixation for an allograft without substituting other fixation.

¶ 33 On January 15, 2014, Joshua Driggs submitted additional and alternative jury instructions and a proposed jury verdict form that allowed the jury to find Providence Physician Services liable if the jury found negligence by Dr. Andrew Howlett or Physician Assistant Brandi DeSaveur. On January 17, 2014, Driggs requested the trial court to reverse its decision to exclude Dr. Menendez's testimony. Driggs contemporaneously submitted supplemental declarations of Dr. Menendez stating the standard of care in Washington is equivalent to the national standard of care and that he based his testimony on a more probable than not basis to a reasonable degree of medical certainty. The trial court refused to reverse its ruling.

¶ 34 On January 23, 2014, the trial court hosted exceptions and objections to the proposed jury instructions. Providence Physician Services objected to Joshua Driggs' proposed jury instruction thirteen, which allowed the jury to find Providence liable if it found Brandi DeSaveur, as an agent of Providence, negligent. In turn, Driggs argued:

10 VRP (Jan. 23, 2014) at 1597.

¶ 35 The trial court rejected Joshua Driggs' instruction thirteen and approved a jury verdict form that identified only Dr. Andrew Howlett as an agent of Providence. During argument on the instruction, the trial court stated:

10 VRP (Jan. 23, 2014) at 1598-99, 1604.

¶ 36 During closing arguments, Providence Physician Services focused on the dearth of reliable expert testimony supporting Joshua Driggs' case. Providence attacked the credibility of Dr. Steven Graboff and emphasized the lack of opinions from Dr. Lawrence Menendez on the standard of care, causation, and risks:

10 VRP (Jan. 23, 2014) at 1687-91.

¶ 37 The jury entered a verdict in favor of Providence Physician Services.

LAW AND ANALYSIS

¶ 38 On appeal, Joshua Driggs contends that the trial court abused its discretion by excluding expert testimony of Dr. Lawrence Menendez. He also maintains that the trial court erred in refusing its proposed jury instruction naming Brandi DeSaveur as an employee accused of negligence. Driggs claims either error by itself constituted harm that requires remand for a new trial. We focus on the exclusion of Lawrence Menendez's testimony and opinions.

Lawrence Menendez's Testimony

¶ 39 The trial court excluded Dr. Lawrence Menendez's opinions regarding the standard of care for fixation of allografts, the absence of fixation causing a fracture and other complications in Joshua Driggs's right leg, and the risks attended to the absence of fixation. The trial court excluded testimony on the standard of care because Dr. Menendez did not testify to a Washington standard of care. The court barred testimony on causation because Menendez never stated that he based his opinion on reasonable medical probability. The trial court rejected testimony on the risk for purposes of informed consent because Menendez did not reference percentages of the risk. In its ruling, the trial court also mentioned that Menendez agreed with defense counsel that his opinions were personal opinions. An expert's testimony as to his personal opinions could be a basis to reject all opinions stated, although the record does not show which of the opinions the trial court excluded on this basis.

¶ 40 In the published portion of this opinion, we review whether Dr. Lawrence Menendez could testify to a national standard of care if another physician testifies that the Washington standard equates to the national standard. We also review whether the exclusion of Dr. Menendez's many opinions was harmless to Joshua Driggs. In the unpublished portion of the opinion, we examine whether Dr. Menendez testified on the basis of reasonable medical probability, whether Menendez's opinions are inadmissible as personal opinions, and whether Lawrence Menendez could testify to Driggs' informed consent cause of action.

¶ 41 We review the decision to exclude an expert witness's testimony for abuse of discretion. Winkler v. Giddings, 146 Wn.App. 387, 392, 190 P.3d 117 (2008). Discretion is abused if it is exercised on untenable grounds or for untenable reasons. Morin v. Burris, 160 Wn.2d 745, 753, 161 P.3d 956 (2007). Important for this appeal is the rule that a decision is based on untenable grounds or made for untenable reasons if it was reached by applying the wrong legal standard. Mitchell v. Wash. State Inst. of Pub. Policy, 153 Wn.App. 803, 821-22, 225 P.3d 280 (2009). A trial court that misunderstands or misapplies the law bases its decision on untenable grounds. Little v. King, 160 Wn.2d 696, 703, 161 P.3d 345 (2007). In reviewing a ruling for abuse of discretion, this court will often separate questions of fact from the conclusions of law that they support and refuse to defer to the trial court on conclusions of law. Bartlett v. Betlach, 136 Wn.App. 8, 18, 146 P.3d 1235 (2006).

State Standard of Care

¶ 42 As a preliminary matter, the parties dispute whether the trial court correctly disallowed a supplemental declaration from Dr. Lawrence Menendez. Joshua Driggs filed the declaration after the perpetuation deposition of Dr. Menendez and in response to Providence Physician Services' motion to strike testimony of Menendez. In the declaration, Dr. Menendez disclosed that he conferred with experts within Washington State and determined that the Washington standard of care was equivalent to the national standard concerning the need for fixation of an allograft. We decline to resolve whether the trial court abused its discretion when refusing to consider this additional testimony of Menendez. Even without the declaration testimony, we rule that Dr. Menendez's opinion on the standard of care should have been heard by the jury. Principles of judicial restraint dictate that if resolution of another issue effectively disposes of a case, we should resolve the case on that basis without reaching the first issue presented. Wash. State Farm Bureau Fed'n v. Gregoire, 162 Wn.2d 284, 307, 174 P.3d 1142 (2007); Hayden v. Mut. of Enumclaw Ins. Co., 141 Wn.2d 55, 68, 1 P.3d 1167 (2000).

¶ 43 In a medical malpractice claim, a plaintiff must show that the health care provider violated the relevant standard of care. A plaintiff must prove the relevant standard of care through the presentation of expert testimony, unless a limited exception applies. Volk v. Demeerleer, 184 Wn.App. 389, 430-31, 337 P.3d 372 (2014), review granted, 183 Wn.2d 1007, 352 P.3d 188 (2015). In turn, the trial judge must make a preliminary finding of fact under ER 104(a) as to whether an expert is qualified to express an opinion on the standard of care in Washington. Winkler v. Giddings, 146 Wash.App. at 392, 190 P.3d 117 (2008).

¶ 44 By Washington statute, the standard of care is the degree of "care, skill, and learning expected of a reasonably prudent health care provider at that time in the profession or class to which he belongs, in the state of Washington, acting in the same or similar circumstances." RCW 7.70.040 (emphasis added). One might question if the standard of care in Washington ever differs from the standard of care throughout the nation. Law changes from state to state, but medical care holds constant throughout America, at least outside rural areas. Increasingly, medical experts testify that Washington follows a national standard of care. We remain bound, however, by our legislature's declaration that the trier of fact must find and apply a state standard of care.

¶ 45 Joshua Driggs suggests that a trier of fact may assume that the national standard of care and the state standard of care coalesce unless one witness testifies to a differing standard. A lax reading of Winkler v. Giddings, 146 Wn.App. 387, 190 P.3d 117 (2008) and Pon Kwock Eng v. Klein, 127 Wn.App. 171, 110 P.3d 844 (2005) could support such a rule. In each decision, this court underlined the absence of testimony from the defending physician that Washington retained a variant standard of care. Nevertheless, neither case expressly adopted such a rule. We need not decide whether to adopt such a rule because we may rest our decision on other grounds.

¶ 46 A physician licensed in another state may provide admissible testimony that a national standard of care exists in this state and that the defendant physician violated that standard. Elber v. Larson, 142 Wn.App. 243, 248, 173 P.3d 990 (2007); Pon Kwock Eng v. Klein, 127 Wash.App. at 179, 110 P.3d 844. In his deposition, Dr. Lawrence Menendez testified that Dr. Andrew Howlett violated the national standard of care of an orthopedist oncologist by failing to affix the allograft during the March 6, 2009 surgery. Menendez did not affirm that the Washington standard of care followed the national standard of care. Nevertheless, Dr. Steven Graboff at trial averred that the national and Washington standards of care are equivalent. We rule that Dr. Graboff's testimony lays a sufficient predicate for Dr. Menendez's opinion.

¶ 47 Providence Physician Services contends that the only type of expert competent to testify as to the standard of care required of a practitioner in the state of Washington is an expert who knows the practice and standard of care in Washington. Providence cites McKee v. American Home Products Corporation, 113 Wn.2d 701, 706-07, 782 P.2d 1045 (1989) for this proposition. McKee included a claim of pharmacist malpractice. The only evidence provided by the plaintiff concerning the standard of care of a pharmacist practicing in Washington was an affidavit of an Arizona physician. The Supreme Court disregarded the opinion of the physician because he was not a pharmacist. The Supreme Court also rejected the opinion because the physician did not reference the standard of care of a pharmacist in this state. McKee v. American Home Products Corporation does not address our issue: whether one physician may testify solely to a national standard of care when another physician testifies that the Washington standard echoes the national standard.

¶ 48 Providence Physician Services argues that allowing a witness's familiarity with the applicable standard of care to be established through other witnesses would subvert the process of expert witness qualification. In other words, Providence advocates a rule that would require that an expert in a medical malpractice case know the standard of care in Washington State before the trial court accepts the witness as an expert. In so arguing, Providence may confusingly conflate the qualifications of an expert witness to testify with the opinions to which the witness may testify. No rule requires that an expert possess, within his personal knowledge, all information necessary to qualify him as an expert witness. No rule precludes a party from relying on one expert witness for a portion of needed evidence and another expert witness for another segment of required testimony. RCW 7.70.040 does not preclude a party from relying on more than one medical expert to establish that the defendant health care provider violated the standard of care in Washington. Thus, based on Washington case law discussed below, we hold that a qualified medical expert may testify to a national standard of care alone if another qualified medical expert at the same trial testifies that the Washington standard parallels the national standard.

¶ 49 One expert may rely on the opinions of another expert when formulating opinions. State v. Russell, 125 Wn.2d 24, 69, 882 P.2d 747 (1994); Volk v. Demeerleer, 184 Wash.App. at 430-31, 337 P.3d 372 (2014); Deep Water Brewing, LLC v. Fairway Res. Ltd., 152 Wn.App. 229, 271, 215 P.3d 990 (2009). This rule may assume that the testifying expert has contacted another expert and gained information from the second expert before testifying. Dr. Menendez did not learn, before his perpetuation deposition, from another physician that the national standard of care and state standard coincided. We see no difference however, for practical purposes, if the litigant, rather than the expert witness, presents such information at trial through the second expert. In other words, we conclude the plaintiff may call to the stand the second expert to notify the jury that the state standard echoes the national standard particularly when the first expert could have called the second expert on the phone to learn of the state standard and repeat the second expert's comments to the jury. If anything, the evidence for the plaintiff strengthens if the second witness provides the foundation during trial testimony, since the jury hears the additional information directly from the second expert rather than through the first witness's hearsay statement of what another expert told him. The second expert's opinion that the Washington standard equates to the United States standard also then become subject to cross-examination by the defense. Joshua Driggs' jury heard first hand from Dr. Steven Graboff that the national standard of care and state standard of care conflate. Driggs accomplished directly what he could have achieved indirectly through an earlier phone conversation between Dr. Lawrence Menendez and Dr. Steven Graboff. Thus, Lawrence Menendez's testimony as to the national standard was admissible.

¶ 50 Three decisions, Hill v. Sacred Heart Medical Center, 143 Wn.App. 438, 177 P.3d 1152 (2008), Winkler v. Giddings, 146 Wn.App. 387, 190 P.3d 117 (2008), and Elber v. Larson, 142 Wn.App. 243, 173 P.3d 990 (2007) discuss to varying extents the issue presented in this appeal. Providence Physician Services relies on Winkler v. Giddings. In Winkler, plaintiff's expert testified to an "educated assumption that the standard of care was the same across the country." 146 Wash.App. at 392, 190 P.3d 117. Plaintiff presented no other evidence that the Washington standard of care followed the national standard. Dr. Neil Giddings presented testimony that the relevant standard of care differed depending on the area of the country.

¶ 51 Winkler is easily distinguishable because Joshua Driggs presented the additional testimony from Steven Graboff. Dr. Andrew Howlett presented no testimony of a varying standard of care from one region to another.

¶ 52 In Elber v. Larson, the physician in a medical malpractice suit moved for summary judgment. The physician contended that plaintiff's witness, Dr. Daniel Meub, was not qualified as an expert witness because Meub lacked background, training or experience in Washington. The trial court granted summary judgment. This court reversed and held that a medical expert is qualified to testify to the Washington standard of care if he offers uncontradicted testimony that he is familiar with the standard of care and that the standard is a national standard.

¶ 53 A compelling decision is Hill v. Sacred Heart Medical Center. Plaintiff John Hill presented testimony from two physicians. One physician testified that the national standard of care controlled the conduct of the defendants, but did not expressly state that he knew the Washington standard of care to coincide with the national standard. A second physician testified that she knew the Washington standard to parallel the national standard. This court relied on both physicians' testimony when reversing a summary judgment dismissal of the medical malpractice suit.

¶ 54 We recognize that Joshua Driggs presented at trial the deposition of Dr. Lawrence Menendez before presenting Dr. Steven Graboff to testify. Nevertheless, an evidence rule allows testimony to be presented at trial when that testimony is admissible only on the assumption that later testimony is presented. ER 104(b).

¶ 55 We recognize the need to defer to the trial court in evidentiary rulings. Nevertheless, the trial court's exclusion of Dr. Lawrence Menendez's testimony resulted from a misapplication of the law. Thus, we rule the trial court abused its discretion.

¶ 56 Providence Physician Services distinguishes Elber v. Larson and Hill v. Sacred Heart Medical Center on the ground that the trial court disregarded expert opinions when addressing a summary judgment motion and this court reviews evidentiary rulings de novo when examining a summary judgment ruling. We recognize this distinction, but we may still reverse a trial court evidentiary ruling at trial based on a misperception of the law.

Harmless Error

¶ 57 Providence Physician Services contends that, assuming the trial court's exclusion of Dr. Lawrence Menendez's opinions is error, the error was harmless. Providence underscores the fact that Dr. Steven Graboff testified to each opinion of Lawrence Menendez that the trial court excluded. Therefore, Providence argues the excluded testimony of Menendez would have been cumulative.

¶ 58 When a trial court makes an erroneous evidentiary ruling, the question on appeal becomes whether the error was prejudicial, for error without prejudice is not grounds for reversal. Brown v. Spokane County Fire Prot. Dist. No. 1, 100 Wn.2d 188, 196, 668 P.2d 571 (1983); Mut. of Enumclaw Ins. Co. v. Gregg Roofing, Inc., 178 Wn.App. 702, 728-29, 315 P.3d 1143 (2013). An error will be considered harmless unless it affects the outcome of the case. State v. Jackson, 102 Wn.2d 689, 695, 689 P.2d 76 (1984); Brown, 100 Wash.2d at 196, 668 P.2d 571. A harmless error is an error which is trivial, or formal, or merely academic, and was not prejudicial to the substantial rights of the party assigning it, and in no way affected the final outcome of the case. Anfinson v. FedEx Ground Package Sys., Inc., 159 Wn.App. 35, 44, 244 P.3d 32 (2010), aff'd, 174 Wn.2d 851, 281 P.3d 289 (2012).

¶ 59 Error will be considered prejudicial if it presumptively affects the outcome of the trial. James S. Black & Co. v. P & R Co., 12 Wn.App. 533, 537, 530 P.2d 722 (1975). When the reviewing court is unable to know what value the jury placed on the improperly admitted evidence, a new trial is necessary. Thomas v. French, 99 Wn.2d 95, 105, 659 P.2d 1097 (1983); Smith v. Ernst Hardware Co., 61 Wn.2d 75, 80, 377 P.2d 258 (1962); State v. Murphy, 7 Wn.App. 505, 508-10, 500 P.2d 1276 (1972). The rule should be the same when the appeals court may not judge what value a jury may place on improperly excluded evidence.

¶ 60 We agree with Providence Physician Services that normally exclusion of cumulative evidence is harmless. Improper admission of evidence constitutes harmless error if the evidence is cumulative. Hoskins v. Reich, 142 Wn.App. 557, 570, 174 P.3d 1250 (2008). A factor to consider when determining harmless error is whether excluded evidence involved cumulative evidence. State v. Johnson, 124 Wn.2d 57, 76, 873 P.2d 514 (1994); Kimball v. Otis Elevator Co., 89 Wn.App. 169, 178, 947 P.2d 1275 (1997). Probably all American jurisdictions follow this rule. People v. Fletcher, 328 Ill.App.3d 1062, 1071-72, 768 N.E.2d 72, 263 Ill.Dec. 312 (2002); Gonzalez v. Stevenson, 791 S.W.2d 250, 253 (Tex.App.1990).

¶ 61 We decline to follow the cumulative evidence rule in this appeal for three reasons. First, the excluded opinions of Dr. Lawrence Menendez probed the central issues in dispute in this case. Menendez's opinions included whether Dr. Andrew Howlett's treatment fell below the standard of care, whether that negligent treatment proximately caused Driggs' harm, and whether the risk of harm from the surgery was so material that a reasonable patient would have wanted disclosure and would have chosen different treatment.

¶ 62 Second, the jury garnered the misimpression that Dr. Lawrence Menendez lacked opinions on these key issues. During closing argument, Providence Physician Services highlighted that Lawrence Menendez provided no opinions on these key issues. Of course, Providence did not disclose to the jury that Menendez held opinions but the opinions were excluded from their hearing.

¶ 63 Third, in addition to noting the absence of opinions from Dr. Menendez, Providence excoriated the credentials and credibility of Dr. Steven Graboff, the other physician who provided the same favorable testimony as Lawrence Menendez. Providence's own words during closing argument concerning Graboff's character as an unqualified, disgraced, hired gun, whose opinions cannot be believed, illustrate the harm of the exclusion of Lawrence Menendez's opinions. None of the reported decisions, wherein the court holds harmless exclusionary error with regard to cumulative evidence, concern these three factors.

¶ 64 One foreign decision of limited relevance is Harper v. Roberts, 173 Ohio App.3d 560, 2007-Ohio-5726, 879 N.E.2d 264. Home owners sued roof contractors for breach of contract in connection with the construction of a new roof that leaked during rainfall. The jury ruled in favor of the contractors and the Court of Appeals reversed and remanded for a new trial. The reviewing court held that the trial court committed error during a "biased" questioning of the owners and their expert witness. More importantly, the error was prejudicial because the case depended on the credibility of the witnesses.

¶ 65 In this appeal, with the credibility of Steven Graboff impugned, Dr. Lawrence Menendez's testimony grew critical. The error in excluding Dr. Menendez's testimony was not harmless. On this basis, we reverse the jury's verdict and remand for a new trial consistent with this opinion.

¶ 66 A majority of the panel having determined that only the forgoing portion of this opinion will be printed in the Washington Appellate Reports and that the remainder having no precedential value shall be filed for public record pursuant to RCW 2.06.040, it is so ordered.

WE CONCUR: SIDDOWAY, C.J., and KORSMO, J.