Formula Help - Average Productivity Per Day

There may be an elegant way to do this using a summary report but leveraging a Formula Query formula field.  If you can use a Formula Query to identify the first survey (lowest Record ID#) done by a particular person on a particular day and give that a value of 1, else 0  then you would be able to summarize on a summary report the number of days that they worked.

Then obviously the summary report can summarize how many reviews they did. Then you can use the  feature in summary reports to do a "Summary Formula" to divide.

That avoids the needs to set up and maintain additional statistical tables to keep track of how many days each individual user worked.  

Back if you want to pursue this concept but need help with the formula query field and we can muddle through that together.