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+include <mach.h>
+include <ctype.h>
+include <error.h>
+include "pdm.h"
+
+# PDM_STATISTICS -- Calculate the sum of squares and the variance of the data.
+
+procedure pdm_statistics (pdmp)
+
+pointer	pdmp			# pointer to PDM data structure
+
+int	npt, i, j
+double	var, sumx2, sum
+
+begin
+	npt = PDM_NPT(pdmp)
+
+	# Calculate the sum of squares and the variance of the data.
+	sumx2 = 0.0
+	sum = 0.0
+	j = 0
+
+	do i = 1, npt {
+	    if (PDM_INUSE(pdmp,i) == 1) {
+	        sumx2 = sumx2 + PDM_DY(pdmp,i)**2	    # Sum of squares.
+	        sum = sum + PDM_DY(pdmp,i)
+		j = j + 1
+	    }
+	}
+
+	if (j != 1)
+	    var = (sumx2 - sum**2/double(j))/double(j - 1)    # Variance.
+
+	# Put these two values in the data structure.
+	PDM_SUMSQ(pdmp) = sumx2
+	PDM_DVAR(pdmp) = var
+end