I read the documentation. I think it says the following: The procedure fits a cubic spline to the data and evaluates the spline for every day. If you use the option observed=(beginning), that is equivalent to using observed=(beginning,beginning).  That means that the output value is the value of the cubic spline on the first day of each month: 01JAN, 01FEB, 01MAR, etc. If you use the option  observed=(beginning,average), the output value is the AVERAGE of the values in the month. Thus the output for January is the average of the cubic spline in January, the output for February is the average of the cubic spline in February, etc. Here is the graph of the cubic interpolation function (which I got by setting TO=DAY and PLOTS=(ALL)). See if this helps you make sense of the outputs in both cases:   ... View more

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