Contingency tables sometimes have cells with frequency counts of 0. These may cause problems or require special treatment. There are two kinds of zeros:
Structural zeros: These are cells for which a nonzero count is impossible because of the nature of the phenomenon or the design of the study. The classic example is a cross-tabulation of sex by type of surgery in which structural zeros occur for male hysterectomies and female vasectomies.
Random zeros: In these cells, nonzero counts are possible (at least as far we know), but a zero occurs because of random variation. Random zeros are especially likely to arise when the sample is small and the contingency table has many cells.
Structural zeros are easily accommodated ...