Take a look at the "Applicant Rating" form that comes with InfoPath.  That
form uses radio buttons to do the ratings and the calculations are done in
script (the script.js) file.  That should give you all of the answers you
are looking for.

--josh bertsch

also need to know how to calculate the average of the rating scores.

You could use a score within a limited range (1 - 10, 1 - 100) and then use
Conditional Formatting, Data Validation or Rules to alert the user that they
are outside of the allowable range. Set this in a Repeating section (with
"Allow user to insert or delete..." turned off).

Another approach is to use a drop down list box. The display value can be A,
B, C , D, F with the value equal to 1, 2, 3, 4, 5.

Use another field (outside of the repeating section) to show the average.
Merge the fomrs and there is your average score.

also need to know how to calculate the average of the rating scores.