A categorization of a particular semiconductor wafer based on void size is obtained from sigma data and T0.1% failure data that has been obtained from wafers subjected to isothermal testing. The sigma data and the T0.1% failure data for the particular wafer is compared to stored data corresponding to ranges for sigma and T0.1% data for each of a plurality of void categories, and the particular wafer is categorized based on the stored data. The T0.1% failure data is computed based on a T50% failure data and the sigma value, so that small sample sizes can be utilized to obtain the stored data.