Calculating average while excluding specific fields
I'm working on a customer survey database/layout. The possible scores for each question are 1 - 5, however, if a question is not relevant to a specific customer then the surveyer enters a 0 for that question.
I would like to calculate an average score, taking into account that questions with a score of 0 should not be included in the average (including the question in the denominator will screw up the average). What calculation logic would I use to exclude the '0's from the count?