What is the burden of proving that underpayment was not a result of negligence?

Because the assessment of the penalty is presumptively correct, the Norgaards have the burden of proving that their underpayment was not the result of negligence. Allen v. Commissioner, 925 F.2d 348, 353 (9th Cir.1991). Negligence, under 26 U.S.C. 6653, is defined as "the lack of due care or the failure to do what a reasonable and prudent person would do under similar circumstances." Id. The Norgaards urge that they were not negligent because they employed a logical accounting system to substantiate their gambling losses. We agree that their system was logical.