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The Household, Individual, and Community files contain largely unclean data. Most cleaning decisions are therefore left up to the individual researcher. The one systematic change in the data sets that was implemented post-data entry was the conversion of missing values to SAS special missing values. The following chart displays the pattern of these changes

Missing Values SAS Special Missings Meaning
96/996/9996/99996 .a not applicable
97/997/9997/99997 .b doesn’t know
98/998/9998/99998 .c refuses to answer
99/999/9999/99999 .d did not answer
out of range .z