Retention Rates - counting how many new members in a year compared to previous year

Hi,

There's a lot of material on this but I can't quite get the answer. I can make calculations to determine number of members for each [Department] for each [Year]. I can also determine the number of new members. What I can't do yet is calculate the retention rates.

This is what I need to do:

- I need a line graph

- I need Year on the X axis

- Percentage on the Y axis

- I need to show a line of the retention rates of Field Sales, as well as a line for the retention rates on Inside Sales

I essentially need to know;

How do I calculate how many members (in Sales department) returned from the previous year, and then divide that by the total number of member of the Sales department from the previous year.

Note: I don't seem to be able to create a package workbook. I assume it's because I'm using the public version? Is there anyway I can share what I have? I have attached an excel spreadsheet of my data.

Is anyone kind enough to help me out?

Regards,

James