How to standardize 3 data sets in SPSS


I'm trying to standardize 3 data sets in SPSS to increase the power of my sample to 120 subjects in total.

The 3 data sets were all conducted using the same procedure but at different times and the number varies between each of the data sets (Set 1: 50 subjects, Set 2: 34 subjects, Set 3: 36 subjects).

Right now the only way I can see to do this is to first calculate the combined mean (comb. m) of all 3 sets using this formula:

comb. m = Σi ni mi / Σ ni

(Where x is the value for each data point, ni is the data set size of set i, and mi is the mean of set i)

and the combined standard deviation (comb. SD) using this formula:

comb. SD = √ [Σi ((x – mi)2) / ((Σi ni) – 3)]

And then calculating the z-score for each data point using the formula:

z = (x - comb. m) / comb. SD

The 2 questions I have are:
1. Is this method appropriate for standardizing across 3 data sets?
2. How could I do this in SPSS?

Please let me know if anything's unclear.
Many thanks in advance.
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