Change the value in the summary bar above each group to “average” instead of “sum”.

Thanks @kuovonne , How can i create a table and show an average of each group? 

You will need a linked record table based on what you want to average on.  Say by month for example.  So all of the January records would link to a January row in your linked. Table.  Then your rollup will use average(values). Do this for every month and you will have a table of averages. 

Kuovonne wasn't suggesting that you create a table and show an average of each group. They were suggesting that you click at the top of your grouped field where it says sum above the column you highlighted and change that from Sum to Average. 

If you want a separate field in that same view, I'm sure you could use a formula to get the average either using a basic formula field or using a roll-up type formula. 

You could also create a separate table, but I'd need to know your expected output and what you group and filter by to get you the output you need.