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Introduction to Categorical Data Analysis Procedures |

As indicated above, the CATMOD procedure assumes that the data are from
a stratified simple random sample, so it uses the product
multinomial distribution. If the data are not from such a sample, then
in many cases it is still possible to use PROC CATMOD by arguing that each
row of the contingency table *does* represent a simple random
sample from some hypothetical population. The extent to which the
inferences are generalizable depends on the extent to which the
hypothetical population is perceived to resemble the target population.

Similarly, the Cochran-Mantel-Haenszel statistics use the multiple hypergeometric distribution, which requires fixed row and column marginal totals in each contingency table. If the sampling process does not yield a table with fixed margins, then it is usually possible to fix the margins through conditioning arguments similar to the ones used by Fisher when he developed the Exact Test for 2 ×2 tables. In other words, if you want fixed marginal totals, you can generally make your analysis conditional on those observed totals.

For more information on sampling models for categorical data, see Bishop, Fienberg, and Holland (1975, Chapter 13).

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