Kenneth Barker was president and general manager of Lionsgate Corporation, a family construction business. The Army Corps of Engineers contracted with Lions-gate to construct a quarter mile concrete channel for flood control purposes. Lions-gate performed work beyond the scope of the contract because of changed site conditions, design errors, and changes in government plans. Barker submitted 74 claims to the government for claimed" extra costs.
The government, in turn, charged Barker with sixty-four counts of presenting false, fictitious, or fraudulent claims to the United States, in violation of 18 U.S.C. § 287.1 A jury found Barker guilty on three counts2, was unable to reach a verdict on twenty-two counts, and acquitted him on the remaining counts. The district court granted a judgment of acquittal on two of the counts on which the jury reached no verdict. On the three counts of conviction, *1410the district court denied Barker’s post-verdict motion for a judgment of acquittal and his alternative motion for a new trial.
Barker contends that the evidence is insufficient to support the conviction, and that the district court therefore improperly denied his motion for judgment of acquittal and his motion for a new trial. Barker further contends that the jury should have been instructed that section 287 is not violated when a false charge is offset by an undercharge on the same claim. In addition, Barker argues that the district court erred by allowing an unqualified witness to testify as an expert. And finally, Barker contends that section 287 violates his First Amendment right to petition the government for redress of grievances.
We affirm the judgment of the district court.
ANALYSIS
I. Sufficiency of the Evidence
Barker contends that there was insufficient evidence to support a conviction on counts 33(b), 48(b), and 57(b). In addressing this contention, we must determine whether the evidence, viewed in the light most favorable to the Government, would permit any rational trier of fact to conclude that the defendant was guilty beyond a reasonable doubt. Jackson v. Virginia, 443 U.S. 307, 318-19, 99 S.Ct. 2781, 2788-89, 61 L.Ed.2d 560 (1979); United States v. Nelson, 419 F.2d 1237, 1241 (9th Cir.1969).
Count 33(b) involves a claim made by Barker that he and his sons, Wayne and Paul, worked on Sunday, May 25, 1990. The Barkers testified that all three of them had worked all that weekend, but the government introduced documentary evidence to the contrary. Barker responds that the daily job report, which indicates that only one person had worked that day, also shows three pickup trucks at the site. Barker argues that the entry showing one worker must be in error because one worker could not use three trucks. The jury was entitled, however, to conclude that if there was any error it was in the entry regarding the pickup trucks. Its conclusion was supported as well by a quality control report indicating that only one supervisor worked all three days of the Memorial Day weekend in issue.
Counts 48(b) and 57(b) required the jury to determine whether Barker was double billing the government by calculating his compensation as both a direct cost and as part of overhead. The jury reviewed Lionsgate’s accounting documents prepared in the regular course of business as well as one created for an appearance before the government Contract Board of Appeals, and these documents were capable of being construed to support the government’s double-billing contention. In addition, the jury heard testimony by a government’s witness and by Lionsgate’s accountant regarding the computation of overhead and direct charges. The government’s witness testified that Lionsgate’s accountant had in an earlier proceeding admitted that Barker’s salary was included in overhead.3 Lionsgate’s accountant testified that, when a person who is normally charged in overhead is being charged as a direct cost, the percent of overhead for the project should show a decrease, yet exhibits here showed no decrease in the overhead percentage when Barker’s services were directly billed.
Although the evidence presented at trial was open to alternative interpretations, it was not so unreliable as to cause us to depart from the rule that determining the credibility of witnesses and assessing conflicting evidence is a matter for the jury. See United States v. Taylor, 716 F.2d 701, 711 (9th Cir.1983). The evidence was sufficient to support the verdicts against Barker. Likewise, the weight of the evidence, though not overwhelming, justifies the district court’s decision to deny Barker’s mo*1411tion for a new trial.4 See United States v. Pimentel, 654 F.2d 538, 545 (9th Cir.1981) (In applying abuse-of-discretion standard, a motion for a new trial is granted only in exceptional circumstances in which the evidence weighs heavily against the verdict).
II. Offsetting Undercharges
Barker also contends that the district court erred in denying his proposed instruction that required the jury to determine whether the claim contained undercharges that would offset any overcharges.5 A defendant is entitled to an instruction concerning his theory of the case if it is supported by law and has some foundation in the evidence. United States v. Echeverry, 759 F.2d 1451, 1455 (9th Cir.1985). Here, we conclude that Barker’s proposed instruction is not supported by law.6
Section 287 provides for the punishment of any person who presents a claim to the United States “knowing such claim to be false, fictitious, or fraudulent.” Barker draws from this language a requirement that the total amount claimed, after other corrections and adjustments, must itself be fraudulently overstated. We disagree. Section 287 contains no requirement that the intent, purpose, or effect of the false claim must be to cause the government a loss.7 “[T]he purpose of 18 U.S.C. § 287 would be frustrated if criminal prosecutions were limited merely to those instances ‘where the defendant is motivated solely by an intent to cheat the government or to gain an unjust benefit.’ ” United States v. Milton, 602 F.2d 231, 234 (9th Cir.1979) (quoting United States v. Maher, 582 F.2d 842, 847 (4th Cir.1978), cert. denied, 439 U.S. 1115, 99 S.Ct. 1019, 59 L.Ed.2d 73 (1990)).
The instructions that were given to the jury made it clear that Barker could not be convicted unless he had made the false or fraudulent statements with the knowledge of their falsity or fraudulency, and that he had made them willfully with a bad purpose. When a claim is made to the government, the government must be able to verify the claim by scrutinizing the information that the claimant has supplied. Willfully false statements obviously frustrate that function. Such an intentional impairment of a legitimate governmental function satisfies any requirement of fraud in section 287. Maher, 582 F.2d at 848.
“The plain purpose of section 287 is to assure the integrity of claims and vouchers submitted to the government.” Id. at 847-48. That purpose would not be served by permitting a claimant who had knowingly and willfully submitted a false item in a claim to escape all consequences if he is able to demonstrate an unrelated, previous*1412ly unstated, and possibly quite inadvertent offsetting undercharge. See id. We conclude, therefore, that the district court did not err in refusing to give Barker’s requested instruction.
III. Competency of an Expert
Barker further contends that a government witness, Steven Rowe, was not qualified to testify as an expert witness under Federal Rule of Evidence 702.8 Whether to admit expert testimony in a criminal trial is a decision left to the discretion of the trial judge, and we will not overturn that decision in the absence of an abuse of such discretion. See United States v. Marabelles, 724 F.2d 1374, 1381 (9th Cir.1984).
Here, Rowe was a civil engineer working for the Corps of Engineers as a claims analyst. He had analyzed claims and reviewed contractor’s documents for approximately seven years. His testimony may well have assisted the jury in understanding the evidence or determining a fact in issue. See United States v. Binder, 769 F.2d 595, 602 (9th Cir.1985). It was not an abuse of discretion for the trial judge to admit Rowe’s testimony.
IV. The Right to Petition for the Redress of Grievances
Finally, Barker contends that the government violated his First Amendment right to petition for redress of grievances. Barker argues that the First Amendment right will be chilled because anyone who files a claim against the government does so knowingly, and if subsequently a trier of fact determines the claim to be false, that person will have violated section 287. This is an improper construction of the law. For a claimant to be in violation of section 287, the trier of fact must determine that the claimant knew he was filing a false claim, not that he merely knew he filed a claim. There is simply no constitutional right to file a false claim.9 This rule poses no conflict with the constitutional right legitimately to petition the government for the redress of grievances.10 Barker’s claims, filed with the knowledge that they were false, are not protected by the First Amendment. See Bill Johnson’s Restaurants v. NLRB, 461 U.S. 731, 743, 103 S.Ct. 2161, 2170, 76 L.Ed.2d 277 (1983) (baseless litigation is not immunized by the First Amendment right to petition).
CONCLUSION
The district court’s denial of Barker’s motion for acquittal and motion for a new trial was proper; the evidence was sufficient to support the jury’s verdict. The jury instructions correctly interpreted section 287 to prohibit a petitioner from knowingly submitting any false item in a claim submitted to the government for monetary reimbursement. Such a construction of the statute does not run afoul of the First Amendment. Finally, the government’s witness had sufficient experience and expertise for the district court, in its discretion, to allow him to testify as an expert.
AFFIRMED.
. 18 U.S.C. § 287 provides:
“Whoever makes or presents to any person or officer in the civil, military, or naval service of the United States, or to any department or agency thereof, any claim upon or against the United States, or any department or agency thereof, knowing such claim to be false, fictitious, or fraudulent, shall be fined not more than $10,000 or imprisoned not more than five years, or both."
. Count 33(b) charged the billing of labor time not actually worked. Counts 48(b) and 57(b) charged the billing of Barker’s salary both directly and in overhead.
. Barker contends that the court erred in admitting the evidence of this witness because it was not corroborated and it was vital to the Government’s case. Because the jury could have based its verdict on the other evidence presented, we need not address the issue of whether the admission must be corroborated. See Smith v. U.S., 348 U.S. 147, 155, 75 S.Ct. 194, 198-99, 99 L.Ed. 192 (1954).
. Because the trial judge at one point opined that he probably would not have convicted Barker if he had been the trier of fact, Barker contends that the verdict must have been against the great weight of the evidence. We disagree. The two positions are quite distinct, and the trial judge clearly, stated that he could not say that the verdict was against the weight of evidence, or that the jury had been wrong.
. Barker’s Proposed Instruction No. 16 stated:
"In determining whether a claim is false you should consider the entire claim. If there are any undercharges that offset any overcharges then, in that event the claim can not be false, fictitious or fraudulent.”
. Our cases have not been consistent regarding the proper standard for reviewing a district court's denial of a proposed jury instruction on the defendant’s theory of the case. See United States v. Sotelo-Murillo, 887 F.2d 176, 179-80 (9th Cir.1989). With regard to the determination whether the proposed instruction is supported by law, review de novo appears to be more appropriate than review for abuse of discretion. See id. We need not decide that question here, however, for our conclusion would be the same under either standard.
.A rough parallel may be drawn between section 287 and 26 U.S.C. § 7207, which prohibits submission of tax returns or statements known by the submitter to be fraudulent or false as to any material matter. Unlike some other sections of the Internal Revenue Code, section 7207 contains no requirement of intent to evade taxes. "Conduct could therefore violate § 7207 ... where the false statement, though material, does not constitute an attempt to evade or defeat taxation because it does not have the requisite effect of reducing the stated tax liability. This may be the case, for example, where a taxpayer understates his gross receipts and he offsets this by also understating his deductible expenses.” Sansone v. United States, 380 U.S. 343, 352, 85 S.Ct. 1004, 1010, 13 L.Ed.2d 882 (1965).
. Federal Rules of Evidence 702 permits a witness who is "qualified as an expert by knowledge, skill, experience, training, or education ...” to testify as to his opinion.
. The constitutionality of statutes criminally forbidding false statements has been upheld on numerous occasions. See, e.g., United States v. Des Jardins, 772 F.2d 578, 580 (9th Cir.1985) (upholding 18 U.S.C. § 1001); United States v. Staats, 49 U.S. (8 How.) 41, 12 L.Ed. 979 (1850) (upholding 18 U.S.C. § 289).
. United States v. Hylton, 710 F.2d 1106 (8th Cir.1983), relied on by Barker, is not to the contrary. In that case, Hylton had filed factually accurate criminal complaints against federal agents, and the government retaliated by charging her with the crimes of corruptly endeavoring to impede IRS agents in the exercise of their duties and of obstruction of justice. In holding that the government’s retaliatory action violated Hylton’s First Amendment rights, the court stated that "were it demonstrated that Hylton's complaints were frivolous and based upon contrived allegations, a totally different result might follow.” Id. at 1112.