PROC DISTANCE creates a TYPE=DISTANCE or TYPE=SIMILAR data set, depending on the METHOD= option. TYPE=DISTANCE can be used as an input data set to PROC MODECLUS or PROC CLUSTER, but TYPE=SIMILAR cannot be used as an input to any procedures. The proximity measures are stored as a lower triangular matrix or a square matrix in the OUT= data set (depending on the SHAPE= option). See Chapter 33, The DISTANCE Procedure, for details. You can also create a TYPE=DISTANCE data set in a DATA step by reading or computing a lower triangular or symmetric matrix of dissimilarity values, such as a chart of mileage between cities. The number of observations must be equal to the number of variables used in the analysis. This type of data set is used as input by the CLUSTER and MODECLUS procedures. PROC CLUSTER ignores the upper triangular portion of a TYPE=DISTANCE data set and assumes that all main diagonal values are zero, even if they are missing. PROC MODECLUS uses the entire distance matrix and does not require the matrix to be symmetric. See Chapter 30, The CLUSTER Procedure, and Chapter 59, The MODECLUS Procedure, for examples and details.