How to calculate the overall mean of several Likert scale data?


Hi, I have a set of questionnaire as follow:

Perceived Usefulness Rating(5-point Likert Scale)

PU01
PU02
PU03
...
I used SPSS to compute the mean of these data. The output as follow:

Descriptive Statistic N Minimum Maximum Mean Standard Deviation

PU01
PU02
PU03
Valid(listwise)

The result show that mean is compute for each PU01, PU02, PU03. This is not the result that I desired.

The question is how can I calculate the mean of the PU01, PU02, PU03 under a category which is Perceived Usefulness?
*English is not my native language.

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