BERRY, J.

This case involves a series of four controlled cocaine "buys" by a State trooper, acting undercover, from a man known as "Carlos" — but later identified as the defendant, Adolfo Marte. Based on these controlled buys, a jury convicted the defendant of four counts of trafficking in cocaine in excess of twenty-eight grams. The defendant was also convicted of a single count of trafficking in cocaine in excess of 200 grams based on seizures pursuant to a search warrant executed at the defendant's apartment. See G. L. c. 94C, § 32E(b)(4). The search warrant was obtained on the same day as the defendant's arrest, following the fourth controlled buy of cocaine.

At trial, over the defendant's objection, the Commonwealth introduced certificates of drug analysis, without trial testimony from the chemical analyst, to support each of the five trafficking charges. This was constitutional error violating confrontation rights. See Melendez-Diaz v. Massachusetts, 557 U.S. 305, 310-311 (2009). The principal issue in this appeal1 is whether the Commonwealth's evidence — including field testing of the cocaine bought in the controlled buys and lack of field testing of the cocaine seized in the apartment — rendered the error in the admission of the drug certificates harmless beyond a reasonable doubt. See generally Commonwealth v. Vasquez, 456 Mass. 350, 352, 360-362 (2010).2

For the reasons that follow, we affirm the four convictions based on the controlled buys. However, the conviction based on the seizure from the defendant's apartment cannot stand.

1. Background. The following is a brief summary of the trial evidence. The circumstances underlying each of the controlled buys were, reduced to essential acts, nearly identical. Prior to each controlled buy, Trooper Daniel Tucker would contact the defendant (who used the name "Carlos") requesting to purchase two ounces of cocaine. The defendant set a price of $2,200 for each two-ounce sale. The defendant would instruct Trooper Tucker to proceed to the parking lot of a nearby Friendly's restaurant. During the first and second controlled buys, an associate of the defendant known as "Nieves" arrived by car at the Friendly's parking lot and delivered to Trooper Tucker a plastic bag containing approximately two ounces of a white powdery substance. After the second buy, the defendant, in a telephone conversation, inquired whether the trooper was satisfied with the quality of the substance delivered and boasted, "You never have to worry about my packages, I've been around a long time." The defendant himself made the third delivery. After the trooper's call to the defendant, another man known as "Andino" made the fourth delivery at the Friendly's, but shortly thereafter was joined by the defendant.

After each controlled buy, Trooper Tucker immediately performed a field test on the purchased substance. Each of the four field tests indicated the presumptive presence of cocaine. As previously noted, the defendant was arrested after Trooper Tucker made the fourth and final controlled buy. That same day, police executed a search warrant at the defendant's apartment and seized a large quantity of a white powdery substance from a secret compartment, what the police expert characterized as a drug "hide," under the kitchen sink.3 Also seized were two scales and two boxes of plastic sandwich bags from the kitchen area of the apartment. Field tests were not performed on the substances found at the defendant's apartment.

2. Standard of review. "The `essential question' in analyzing harmlessness beyond a reasonable doubt is `whether the error had, or might have had, an effect on the [fact finder] and whether the error contributed to or might have contributed to the [findings of guilty].'" Vasquez, 456 Mass. at 360, quoting from Commonwealth v. Perrot, 407 Mass. 539, 549 (1990). "[I]t is not enough for the Commonwealth to demonstrate that its other, properly admitted evidence was `sufficient' to convict the defendant or that the inadmissible evidence was `consistent' with the admissible evidence." Commonwealth v. Tyree, 455 Mass. 676, 701 (2010), quoting from Commonwealth v. Dagraca, 447 Mass. 546, 554-555 (2006). "Rather, to establish harmlessness beyond a reasonable doubt, the Commonwealth must show that other properly admitted evidence of guilt is `overwhelming,' in the sense that it is `so powerful as to nullify any effect' that the improperly admitted evidence `might have had' on the fact finder or the findings." Vasquez, supra at 362, quoting from Tyree, supra at 704 n.44.

Against this backdrop, we conclude that the totality of the Commonwealth's evidence, including but not limited to the positive field test for cocaine, was sufficient to render the admission of the drug certificates harmless error with respect to the four controlled buys of cocaine delivered to Trooper Tucker which were the predicates for the four convictions for trafficking in cocaine in excess of twenty-eight grams. However, our review of the trial record, as described in part 4, infra, reflects little evidence which would have established that the composition of the substance seized from the apartment was cocaine, except for the erroneously admitted certificate of analysis. Hence, we conclude the Commonwealth's evidence did not overcome the Melendez-Diaz error with respect to the cocaine seized in the apartment — which was the predicate for the conviction for trafficking in excess of 200 grams.

3. The controlled buys and the four trafficking convictions therefor. With respect to the controlled buys, our analysis begins with whether the error arising out of the admission of the drug certificates was harmless by considering the significance of the fact that immediately following each buy, Trooper Tucker conducted an on-site field test. The trooper's testimony was clear and direct that he personally conducted each field test and that each of the white substances purchased in the controlled buys tested positive for the presumptive presence of cocaine. At trial, the trooper described in simple terms how he performed the field test. He also described what the result looked like after a series of vials were broken open and applied to the seized substance, yielding colors which changed to reflect the presence of cocaine — a testing procedure, as described in the direct examination, which could have been, of course, further probed in cross-examination. Trooper Tucker had past experience in conducting such field tests and had participated in approximately one hundred cocaine seizures during his eight years on the State police force, including the Cape Cod Drug Task Force, where he was working at the time of the events in this case and conducted the field tests at issue. As noted, his testimony concerning the field tests was open to cross-examination confrontation by the defendant. Accord Commonwealth v. Connolly, 454 Mass. 808, 831-832 (2009) (field-testing officers had many years of experience in narcotics investigation and were available for cross-examination at trial; error harmless). Cf. Commonwealth v. Rodriguez, 75 Mass.App.Ct. 235, 243 (2009) ("[S]ome of the substances were field tested and found to be cocaine. [The detective] so testified on cross-examination without objection").

Since 2009, when the Connolly case was decided, there have been significant case developments involving field testing that further calibrate and refine the weight to be given field testing in reviewing whether the constitutionally erroneous admission of drug certificates of analysis is harmless beyond a reasonable doubt. Among the leading post-Connolly cases are Commonwealth v. Fernandez, 458 Mass. 137, 151-153 (2010); Commonwealth v. King, 461 Mass. 354, 357-361 (2012); and Commonwealth v. Billings, 461 Mass. 362, 364-365 (2012). The framework of these cases, decided in the wake of Connolly, is double edged, imposing more exacting scrutiny of the foundational evidence required concerning field testing, but endorsing, as did the Connolly case, the inclusion of a positive field test in the harmless error equation — so long as such field tests are properly conducted, and so long as the testing is supported by background foundational evidence.

The post-Connolly field test framework may be further detailed as follows. On one side, the evidentiary standards for properly admitted field tests are heightened. In this respect, the post-Connolly cases make clear that, where the trial evidence concerning a field test is not demonstrated to have the requisite foundation — which includes the experience of the officer(s) conducting the test, the methodology of the testing, and definitive identification of the substance, i.e., as cocaine, heroin, or another controlled substance — the field test yields no real offset to the constitutional error in the admission of the drug certificate of analysis. See, e.g., Commonwealth v. Nelson, 460 Mass. 564, 577 (2011) (admission of certificate of drug analysis identifying marijuana not harmless beyond reasonable doubt where neither investigating officer nor drug distribution expert identified substance based on training and experience); Billings, supra (error in drug certificate admission not harmless where officer conducting field test did not have requisite experience, and where result of field test regarding composition of substance was left unidentified in officer's trial testimony). See generally Commonwealth v. Fernandez, 458 Mass. 137 (2010) (admission of certificates of analysis not harmless beyond reasonable doubt).

However, on the other side, these field testing cases reaffirm the general proposition (as recognized in Connolly),4 that a presumptively positive field test, with proper foundation, may be of persuasive weight. Aggregated with other corroborative and circumstantial evidence that identifies substance composition (apart from the drug certificate admitted), the Melendez-Diaz error may be overridden and render that error harmless. See Fernandez, supra at 150 n.4. Restated another way, a positive field test that affirms the presumptive presence of a controlled substance, which does not stand alone,5 but rather which is buttressed by other incriminatory evidence and expert drug testimony, which also tends to prove the substance composition, is to be tallied up as a whole and may render the Commonwealth's evidence "so powerful as to `nullify any effect' that the improperly admitted evidence `might have had' on the fact finder or the findings." Vasquez, 456 Mass. at 362, quoting from Tyree, 455 Mass. at 704 n.44.

Buttressing evidence to support the composition identification in a presumptively positive field test case may include, but is not limited to, testimony and other evidence regarding: the type of field test used; the method of testing; whether the testing officer had received training in drug investigations, particularly in field testing drugs; any incriminatory admission by a defendant as to what the substance is (cocaine, heroin, etc.); and any other direct or circumstantial evidence relevant and tending to demonstrate the chemical composition of the substances at issue. See note 4, supra. This evidentiary construct in which a positive field test is buttressed by other corroborative, circumstantial evidence is in accord with the law that "[p]roof that a substance is a particular drug need not be made by chemical analysis and may be made by circumstantial evidence." Commonwealth v. Dawson, 399 Mass. 465, 467 (1987). See Commonwealth v. Charles, 456 Mass. 378, 381-382 (2010).

We turn to the circumstantial evidence in this case, which, in our view, was strongly corroborative and fully buttressed Trooper Tucker's clear and direct testimony,6 reporting positive cocaine results in the field tests performed on the substance purchased in the controlled buys. First, the entire pattern of, and repetitive manner of, the drug sales by the defendant — that is, telephone call by the undercover trooper to the defendant asking to buy two ounces of cocaine, price of $2,200 set by the defendant, and telephone calls immediately followed by delivery by car of the negotiated-for cocaine at the off-site restaurant designated by the defendant — were probative that the substance the defendant was selling to the undercover trooper was cocaine. Second, corroborative that the substance delivered was cocaine were the defendant's incriminatory admissions following one transaction, when the defendant boasted to the trooper, "[Y]ou never have to worry about my package, I've been around a long time." Third, the drug evidence expert, Sergeant Brooks, testified that the sale price of $2,200 for each of the two-ounce purchases corresponded precisely with the market price of cocaine at the time. Fourth, Brooks also testified that the manner of packaging and sale of the white substance collected in the small, clear plastic bags — packaging which was similar in each of the four controlled buys — is typical of street-level cocaine distribution. All of this evidence had independent bases and corroborates the field test evidence that the substance was cocaine, apart from the admitted drug certificates of analysis admitted in conflict with Melandez-Diaz.

Related to proving the substance purchased as cocaine, it was also the Commonwealth's burden to establish that the seized substance amounted to more than twenty-eight grams for each of the controlled buys. Here, the substances themselves were admitted in evidence for jury review of volume and heft. Furthermore, the quantity of cocaine in each bag amounted to approximately two ounces — i.e., approximately twice the weight required by the relevant twenty-eight gram subsection of the trafficking statute, so there was a particularly large amount for the jury to review.

4. Substance seized from the apartment hide. As noted, the police seized two plastic bags containing white rock powder from the hide beneath the kitchen sink in the defendant's apartment. The drug certificate indicated that the white rock was cocaine, weighed 465.18 grams, and was seventy-two percent pure cocaine. This was the basis for the defendant's conviction for trafficking in excess of 200 grams of cocaine. However, aside from this erroneously admitted drug certificate, there was not very much evidence bearing on the composition of the substance.

As previously noted, no field tests were performed on the substances seized from the apartment, including this large white block. See Vasquez, 456 Mass. at 364-366 (noting lack of field testing; admission of drug certificates not harmless beyond reasonable doubt); Commonwealth v. Fluellen, 456 Mass. 517, 527 (2010) (same). In addition, unlike the four controlled buys, there was no valuation evidence concerning potential value or sale price on the market price of cocaine. Nor was there anything like the defendant's incriminatory admission about the "package" and its quality.

Indeed, apart from the erroneously admitted drug certificate, the Commonwealth's trial evidence devolved to Sergeant Brook's expert opinions about the manner and means of drug dealing generally. Reduced to essentials, this drug expert opinion evidence was to the effect that the defendant's sparsely furnished apartment appeared to be a "stash pad" (as, to which the sergeant opined, is commonly used by drug dealers), that the sink compartment appeared to be a "hide" (again, in the sergeant's opinion, such as might be of the type commonly used by drug dealers to conceal drugs or proceeds), and that there were two scales and plastic bags (which, again, would be such as those used by drug dealers). These pieces of expert opinion, while clearly relevant to provide information on how the drug trade works, did not serve to identity the seized substance itself as cocaine. Cf. Vasquez, supra at 366-367 (rubber bands, cash, sandwich bags, and walkie-talkie all relevant on the issue of distribution, but not on chemical composition).

5. Conclusion. Based on the foregoing, the judgments on the four charges of trafficking in excess of twenty-eight grams of cocaine are affirmed. The judgment on the charge of trafficking in excess of 200 grams of cocaine is reversed, and the verdict is set aside.7 The case is remanded to the Superior court for further proceedings as may be appropriate and consistent with this opinion.8

So ordered.