Appellant Rickie Albert Scalia entered a conditional guilty plea, see Fed.R.Crim.P. 11(a)(2), following the district court's denial of his motion to suppress evidence seized from his residence pursuant to a search warrant. He now appeals, see id., his conviction for unlawful "manufacture" of marijuana, 21 U.S.C. § 841(a)(1); 18 U.S.C. § 2, and a criminal forfeiture conviction under 21 U.S.C. § 853. Scalia also challenges the mandatory minimum five-year sentence imposed pursuant to 21 U.S.C. § 841(b)(1)(B)(vii) (minimum sentence of five years for "manufacture" of one hundred or more marijuana plants). Scalia contends that the affidavit supporting the search warrant application was insufficient to establish probable cause, and that the district court lacked sufficient reliable evidence on which to find that more than one hundred marijuana plants were seized from his residence. Finding no error, we affirm.

On February 14, 1992, Agent Kenneth MacMaster of the Maine Bureau of Intergovernmental Drug Enforcement (BIDE) applied for a state court warrant to search appellant's residence for marijuana and related paraphernalia. MacMaster's supporting affidavit relied upon, inter alia, information provided by a confidential informant described as a "young concerned citizen." The informant told MacMaster that he had visited the Scalia residence on numerous occasions and as recently as ten days before coming to MacMaster. The informant said that he had observed two marijuana plants a foot tall in appellant's living room, five eighteen-inch plants in the bedroom, and from forty-five to fifty plants of various sizes in a basement walk-in cooler. The informant told MacMaster that s/he was able to recognize the plants because s/he had "received instruction from his/her school concerning various drugs," and that some of the informant's family and friends were casual marijuana users. The marijuana plants in the walk-in cooler were being grown under artificial lights operated by switches installed outside the padlocked cooler door. The informant observed that other rooms in appellant's residence and rooms in an adjacent horse barn were padlocked as well, and that Scalia kept several shotguns and a handgun on the premises. On at least four occasions, the informant observed Scalia selling marijuana at either his residence or his business premises.

The primary contention Scalia makes on appeal is that the MacMaster affidavit did not establish the reliability and veracity of the informant because (1) MacMaster did not explicitly attest that the informant had no prior criminal record; (2) the informant apparently had not provided information to law enforcement officials previously; and (3) MacMaster did not attempt to corroborate the informant's tip through follow-up surveillance efforts at appellant's residence.1

We review the issuance of a search warrant with "great deference," United States v. Ciampa, 793 F.2d 19, 22 (1st Cir.1986), to verify that there existed a "substantial basis" for the judicial officer's common-sense determination that, "given all the circumstances set forth in the affidavit ..., including the 'veracity' and 'basis of knowledge' of persons supplying hearsay information, there [was] a fair probability that contraband or evidence of a crime [would] be found in a particular place." United States v. Caggiano, 899 F.2d 99, 102 (1st Cir.1990) (quoting Illinois v. Gates, 462 U.S. 213, 238-39, 103 S. Ct. 2317, 2332, 76 L. Ed. 2d 527 (1983)).

The MacMaster affidavit stated that the informant was "not currently facing any criminal or juvenile charges nor is he/she under suspicion for any wrongdoing." (Emphasis added.) Appellant first suggests that MacMaster's use of the word "currently" was deliberate wordplay--a statement which was technically true but designed to camouflage the fact that the informant had confronted criminal or juvenile charges in the past. Generally speaking, the representations contained in a search warrant affidavit are presumed valid and truthful. United States v. Spinosa, 982 F.2d 620, 626 (1st Cir.1992). To mount an effective challenge based on an alleged use of deliberate or reckless falsehoods by an affiant, a defendant must request an evidentiary hearing pursuant to Franks v. Delaware, 438 U.S. 154, 98 S. Ct. 2674, 57 L. Ed. 2d 667 (1978). A Franks hearing is required only if the defendant makes a "substantial preliminary showing (1) that a false statement in the affidavit has been made knowingly and intentionally, and (2) that the false statement is necessary for the finding of probable cause." United States v. Paradis, 802 F.2d 553, 558 (1st Cir.1986). The defendant's offer of proof must be "more than conclusory" and should be supported by "[a]ffidavits or sworn or otherwise reliable statements of witnesses." Franks, 438 U.S. at 171, 98 S.Ct. at 2684. A comparable showing is required if the defendant would establish that technically accurate statements by an affiant have been rendered misleading by material omissions. See United States v. Rumney, 867 F.2d 714, 720 (1st Cir.), cert. denied, 491 U.S. 908, 109 S. Ct. 3194, 105 L. Ed. 2d 702 (1989).

Appellant neither requested a Franks hearing nor attempted an offer of proof relating to any material omission from the MacMaster affidavit. Moreover, on appeal there has been no showing that the informant ever had a criminal or juvenile record, or any other involvement with the law, which might undermine the reliability of the affidavit. We therefore find no basis for concluding that the informant had a prior record.

Next, appellant suggests that the reliability of first-time information provided by a "concerned citizen" should be considered inherently suspect, since law enforcement officials can have had no "track record" against which to assess the informant's competence to convey accurate intelligence relating to criminal activities, or the trustworthiness of the informant's motives in volunteering information. We disagree. "[A] warrant affidavit [need not contain] an averment of previous reliability, the appropriate inquiry always being whether the informant's present information is truthful or reliable." United States v. Cochrane, 896 F.2d 635, 641 (1st Cir.) (citing United States v. Harris, 403 U.S. 573, 581-82, 91 S. Ct. 2075, 2080-81, 29 L. Ed. 2d 723 (1971)) (emphasis added), cert. denied, 496 U.S. 929, 110 S. Ct. 2627, 110 L. Ed. 2d 647 (1990). In the absence of a prior record of reliability, we have recognized that, where the informant was "not a professional ... but a private citizen with no known criminal record or other criminal contacts, who came forward on his own ... [,] the informant's story may be more easily accepted...." United States v. Campbell, 732 F.2d 1017, 1019 (1st Cir.1984) (citing United States v. Burke, 517 F.2d 377, 379-81 (2nd Cir.1975)) (emphasis added). Since there is no evidence that the informant who came to MacMaster either had a criminal record, or was suspected of current criminal activity, a neutral judicial officer fairly could find that the informant was a "private citizen" who volunteered the information to MacMaster.

Furthermore, "an informant's reliability need not invariably be demonstrated through the detailed narration of the information previously furnished to law enforcement.... Rather, the affidavit may disclose an adequate basis for evaluating the informant's veracity through the very specificity and detail with which it relates the informant's first-hand description of the place to be searched or the items to be seized." United States v. Taylor, 985 F.2d 3, 6 (1st Cir.1993) (citing Caggiano, 899 F.2d at 102-03) (emphasis added); see also Cochrane, 896 F.2d at 641 ("[A]n important indicia (sic) of reliability is the fact that the informant's knowledge was based upon personal observation rather than hearsay."). As is apparent from our recitation of the confidential information provided to MacMaster, the particularity of the informant's description of the interior and contents of the residence (e.g., padlocked doors, multiple firearms), the location, size, and number of marijuana plants, the marijuana growing apparatus (e.g., basement cooler, artificial lights), and the illegal marijuana sales conducted by appellant inside the search premises, provided considerable intrinsic support for the informant's capacity to convey reliable intelligence relating to the criminal activity attested to in the affidavit.2

Finally, appellant argues that MacMaster should have corroborated the informant's tip before applying for a search warrant, preferably through surveillance of Scalia's residence. Corroboration may take various forms, however, and we have never intimated that surveillance is mandatory. See, e.g., Taylor, 985 F.2d at 6 (finding probable cause where corroboration of informant's tip did not go beyond check of target's criminal record). The confidential informant's tip to MacMaster comported with information received from other sources, including previous BIDE debriefing interviews with several of Scalia's former associates, see supra note 1, as well as informal complaints lodged with the Maine State Police by eight unidentified citizens expressing concerns about Scalia's drug trafficking activity.

The combined force of the informant's detailed tip, MacMaster's expert assessment, and the corroborative police reports provided substantial support for a common-sense determination by the issuing judge that there existed a fair probability that marijuana and related paraphernalia would be found in appellant's residence.

A defendant convicted under 21 U.S.C. § 841(a)(1) is subject to a mandatory five-year minimum sentence if the court finds by a preponderance of the evidence that the defendant manufactured or possessed "100 or more marijuana plants regardless of weight." 21 U.S.C. § 841(b)(1)(B)(vii). See United States v. McMahon, 935 F.2d 397, 400 (1st Cir.), cert. denied, --- U.S. ----, 112 S. Ct. 272, 116 L. Ed. 2d 224 (1991). Were it not for the mandatory statutory minimum, appellant would have been exposed to a 46-to-57 month guideline sentencing range (Level 23, Criminal History Category I), based on the total weight of the marijuana plants (111.8 kilograms) seized at his residence. See U.S.S.G. § 2D1.1(c)(9) (100 KG-400 KG marijuana; level 26); id. § 3E1.1(a), (b) (3-level reduction for acceptance of responsibility); id. § 5G1.1(b) ("Where a statutorily required minimum sentence is greater than the maximum of the applicable guideline range, the statutorily required minimum sentence shall be the guideline sentence.").

At the time the search warrant was executed, an agent for the Maine Drug Enforcement Agency, Bruce Bristow, visually examined the marijuana plants found on the premises. Based on his education and experience, Bristow concluded that all 112 plants were marijuana. He randomly selected fifteen plants, and submitted these representative plants (ranging from one and one-half inches to two feet tall) for chemical analysis.3 The chemical analysis confirmed that all fifteen samples were marijuana plants. Following an evidentiary hearing, the district court found that all 112 plants were marijuana. Scalia was sentenced to the mandatory minimum five-year term.

The district court's drug-quantity finding was based on four factors: (1) Bristow's trained visual identification of all plants seized at the search scene as marijuana; (2) the fact that all the plants were seized contemporaneously from the same location; (3) the positive chemical analysis on all fifteen sample plants tested, and (4) the absence of evidence that appellant was growing anything other than marijuana. Appellant challenges only the first factor relied on by the district court, by attempting to undercut the reliability of Bristow's visual identification. Appellant argues that Bristow conceded that at least two other plant species which grow in Maine look like marijuana when they are only one and one-half inches tall. Appellant's claim fails for two reasons.

First, while the record reflects that Bristow testified that two marijuana look-alikes do grow in Maine, he never stated that his training and experience left him incapable of distinguishing these look-alikes from marijuana.4 4] Rather, Bristow stood by his visual identification. Cf. United States v. Maceo, 873 F.2d 1, 6 (1st Cir.) (extrapolations of drug quantity involve issues of credibility and weight of evidence reserved for trier of fact), cert. denied, 493 U.S. 840, 110 S. Ct. 125, 107 L. Ed. 2d 86 (1989); United States v. Echeverri, 982 F.2d 675, 677 (1st Cir.1993) (qualification of "expert" witness reviewed only for manifest abuse of discretion).

Second, the other factors relied on by the district court strongly support its drug-quantity determination as well. See United States v. Akitoye, 923 F.2d 221, 227 (1st Cir.1991) (factual findings on drug quantity reviewed for "clear error"). "Although the sentencing court must 'err on the side of caution[,]' " United States v. Barnett, 989 F.2d 546, 552 (1st Cir.1993) (approximating producible quantity of controlled substance based in part on amount of precursor chemicals seized) (quoting United States v. Sklar, 920 F.2d 107, 113 (1st Cir.1990)), drug-quantity estimations need not be statistically or scientifically precise. See U.S.S.G. § 6A1.3 ("In resolving any reasonable dispute concerning a factor important to the sentencing determination, the court may consider relevant information without regard to its admissibility under the rules of evidence applicable at trial, provided that the information has sufficient indicia of reliability to support its probable accuracy.") (emphasis added); United States v. Uwaeme, 975 F.2d 1016, 1018 (4th Cir.1992) (noting that drug-quantity estimations may be based on hearsay, nonscientific testimony, defendant's notebook entries, street values of drugs equivalent to money seized from defendant, or on extrapolations of potential drug output from seized components).

More to the present point, courts have endorsed statistically based drug-quantity extrapolations predicated on random test samples in circumstances where the government was able to demonstrate an "adequate basis in fact for the extrapolation and that the quantity was determined in a manner consistent with the accepted standards of [reasonable] reliability." United States v. McCutchen, 992 F.2d 22, 23 (3d Cir.1993); United States v. Pirre, 927 F.2d 694, 697 (2d Cir.1991) ("It is sufficient for the government to show that its method of estimating the total [amount of drugs] is grounded in fact and is carried out in a manner consistent with accepted standards of reliability."); see also Barnett, 989 F.2d at 553 ("We must determine whether the government presented sufficient reliable evidence to permit the court reasonably to conclude that appellants were responsible for a quantity of drugs at least equal to the quantity threshold for the assigned base offense level."). For example, sufficient indicia of reliability may be found where a preponderance of the evidence demonstrates that (1) a proper "random" selection procedure was employed; (2) the chemical testing method conformed with an accepted methodology; (3) the tested and untested samples were sufficiently similar in physical appearance; and (4) the tested and untested samples were contemporaneously seized at the search scene. See McCutchen, 992 F.2d at 23-24 (3d Cir.1993). In sum, the overall margin of reliability in a drug-quantity approximation must be adequate to afford reasonable assurance that the defendant is not subjected to a guideline sentence or a mandated minimum sentence greater than warranted by the reliable evidence.5 Given the relatively light burden of proof, the reasonable reliability of the drug-quantity sampling and extrapolation procedures employed by the government, and the utter absence of countervailing drug-quantity evidence, we affirm the district court's findings.

Affirmed.