Calculating the mean score for scoring categories for each respondent

I am helping a postgraduate student with their research survey. They have a couple of large matrix table questions where the statements fall into various categories - the number of questions under each category varies. For a couple of the categories there is just one statement, for others up to 6 or 7 statements fall under that category. I have set up scoring categories so that the appropriate statements are scored together and this is displaying fine in the data when a survey is completed.
However, the student needs to get a mean score (and standard deviation) for each of the categories for each respondent - so for example if the score for one respondent for category A was 24 and there were 5 statements in that category the mean of 4.8 would be recorded in the data file, if the score for category B was also 24 but there were only 4 statements then the mean of 6 would be recorded.
Is there a way to get these calculations recorded in the data file?
At the moment to test the calculations needed I created a block where the means and standard deviations are shown using piped text and this shows what we need. However, this is no good as we would not want those shown to respondents and the data is not recorded in the data file. I can't see how to translate those calculations into recorded data.
The link to a test copy of the survey is here: https://staffordshire.qualtrics.com/jfe/form/SV_etgIgzUIQgwwaVg
Hope this makes sense and someone can help
Many thanks
Julie