The relative error in the value of the stock size calculated by Virtual Population Analysis (Cohort Analysis) that results from assuming that catches are evenly distributed throughout the year when, in fact, they are not is examined using both analytic methods and simulation. A measure of the intra-year distribution of catches called the balance is defined, and a relationship between the relative error in the calculated values of stock size, the balance, the natural mortality rate, and the rate of exploitation is derived.