Example 14.3 Interpolating Irregular Observations

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 14.3.1.

   data samples;
     input date : date9. defects @@;
     label defects = "Defects per 1000 units";
     format date date9.;
   datalines;
   
   ... 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 14.3.2.

   title "Estimated Monthly Average Defect Rates";
   proc print data=monthly;
   run;

The plots produced by PROC EXPAND are shown in Output 14.3.3.

Output 14.3.3 Interpolated Defects Rate Curve