Identifying missing data mechanisms in (2 x 2)-contingency tables

Consider the sample of two binary variables X and Y with some missing structure within X or Y. The knowledge about the corresponding values of the observed covariate allows to play through all possible originally' complete data sets. After defining the notation, including some theoretical work, a test for non MCAR within the complete case table is presented. Simulating all possible tables enables some testing on non MAR. A simulation experiment is used to illustrate this context. -- missing data mechanism : odds-ratio ; simulation experiment ; testing non MAR