title: Power BI: Calculate Retention Rate
body: <p>I have a table showing every donation made to a non-profit. A single row represents a single donation including the donor ID, date, and amount given. </p>
  
  <p>For any date period, I'd like to be able to see how many donors did we have for that date range one year ago, and how many are still active donors in the current period. So, if I look at 2017, it should look at 2016, find how many people donated, then see how many of those people donated in 2017. </p>
  
  <p>I have a calendar table showing all days. But, other than that, I guess I'm not even sure where to start. Can someone help?</p>