How do I format while trying to Average?

Q: How do I format while trying to Average?

A: format(AVG(if([this]='criteria',[this],0)),num))

where:
 criteria = criteria set by author of report and 
 num = integer indicating scale of the decimal number

Why do we do this?

 To display the result of average with specified number of digits after decimal, above syntax can be used to apply the formatting.

Where can this be applied?

Calculated Field section in reports.