In Kim Klein's article on stopping a subtle decline in donations, she mentions the importance knowing your donor retention and attrition rates. You can figure donor retention (how many you have kept) and atttrition (how many you lost) easily using Fundraiser.
Both are calculated by taking all the donors you had in the previous calendar year and comparing which of those donors gave in the most current complete calendar year. In other words, which of the people who gave in 2014 also gave in 2015? Expressing that number as a percentage of loss gives you your attrition rate; expressed as a percentage of renewal gives you your retention rate.
Example: To figure retention and attrition rate in FundRaiser:
Step 1: create a grouping of the number of donors ins 2014. For the example, let's say that the number of donors in that grouping is 5198.
Step 2: Modify that grouping to be all donors who gave in both 2014 and again in 2015. Let's say that the number of donors in that grouping is 3250.
Step 3: To figure retention rate divide the number you got in step 2 by the number you got in step 1. The answer expressed as a percentage in your retention rate.
3250 / 5198 = 62% retention rate
Step 4: To figure attrition rate, subtract the percentage retention rate from 100%.
100% - 62%= 37% attrition rate