Missing values can be identified in Stata by using either the mdesc or missings commands, which do not include zeros as missing values. When using the pwcorr command, correlations are only calculated for observations with non-missing values. The number of missing values and proportion of missingness can be obtained through the mdesc command. Stata also provides ways to denote extended missing values. To identify missing values in survey data, the summarize command can be used after importing the data. Missing values in survey data may indicate a skipped question, an unanswered question, or actual missing data.