I was ovulating late as noted by positive opk's. This would cause the predictor to change when it thought I'd ovulate and get my next period. However, my period would come earlier than the adjusted prediction and the next month's predicted ovulation would go back to earlier in the month than what was happening. I determined through my own research that my luteal phase was short and found natural progesterone cream was the answer to my problem. The ovulation predictor didn't note any of this nor suggest a short luteal phase. It just kept recalculating an earlier than correct ovulation date since my cycle was short. Dissatisfied after this happened the first cycle I discovered when I was ovulating; however, kept logging data in case I needed it for reference during a physician visit.