Have always liked this app because its simple and tracks everything I need (PMS, period, fertility, etc). I like that it uses an average cycle and that you can change it, however, your left and right ovaries can have different cycles. My cycle is consistently 28 days one month, then 26 days the next. So I have to adjust my calendar every month. For this reason the average is not really useful for me. Wish there was an every-other-month average!