Perhaps Medical Exam Received is blank for one of the records in your found set. That would cause this calculation to evaluate as a very negative number and it would then skew your average in the negative direction by a large value.
... and I'm guessing that if more than 1 record is blank it would really mess it up eh?
Can I get around this by adding a "if this is not blank" if staement to my days calc?
Dates in FileMaker record the number of days since December 31, year 0000 and a blank date field in this calcuation evaluates as zero.
What value is valid if a date field is blank? Do you want that to be null, 1, 0 or....? The answer to that question should guide you in how to modify the calculation so that it checks for empty date fields.
Another alternative might be to perform a find that omits all records where this calculation evaluates to a number less than zero so that those records are excluded from the summary calculation.