Let be the number of observations in a BY group. Let be the number of explanatory variables. The minimum working space (in bytes) needed to process the BY group is

Extra memory is needed for certain TIES= options. Let be the maximum multiplicity of tied times. The TIES=DISCRETE option requires extra memory (in bytes) of

The TIES=EXACT option requires extra memory (in bytes) of

If sufficient space is available, the input data are also kept in memory. Otherwise, the input data are reread from the utility file for each evaluation of the likelihood function and its derivatives, with the resulting execution time substantially increased.