Refills are super expensive (by 30 months) compared to simple ovulation tests (25 to 5 months) and accuracy of 4 to 6 days around ovulation leaves to be desired. I regret having purchased the device. I cycle from 21 to 28 days but the unit still wonder strips 20 because I have a late ovulation (19th day). Also I have never had a period of 4a6 days around ovulation but high fertility period 10 days before and 2 days maximum during ovulation. 10 days before it does not mean anything and so I have to wait for the day of ovulation to hope to conceive.