This example shows the interpolation of a series of values measured at irregular points in time. The data are hypothetical. Assume that a series of randomly timed quality control inspections are made and defect rates for a process are measured. The problem is to produce two reports: estimates of monthly average defect rates for the months within the period covered by the samples, and a plot of the interpolated defect rate curve over time.
The following statements read and print the input data, as shown in Output 15.3.1.
data samples; input date : date9. defects @@; label defects = "Defects per 1000 Units"; format date date9.; datalines; 13jan1992 55 27jan1992 73 19feb1992 84 8mar1992 69 ... more lines ...
title "Sampled Defect Rates"; proc print data=samples; run;
To compute the monthly estimates, use PROC EXPAND with the TO=MONTH option and specify OBSERVED=(BEGINNING,AVERAGE). The following statements interpolate the monthly estimates.
proc expand data=samples out=monthly to=month plots=(input output); id date; convert defects / observed=(beginning,average); run;
The following PROC PRINT step prints the results, as shown in Output 15.3.2.
title "Estimated Monthly Average Defect Rates"; proc print data=monthly; run;
The plots produced by PROC EXPAND are shown in Output 15.3.3.