Crosstab average of averages

[rbrule]

I have a crosstab query that shows the average product errors for each product each month of the year. Some products are not produced in certain months which leave null fields. The months with data show percentages of errors from 0.00% to 100.00%. I would like to have a summary column that shows the average errors for the year based on only the months with data. For example some products have activity for four months and some have activity for six months. I guess I need an expression for the calculated field that says "sum Jan through Dec divided by count of fields with data".