The second clinical study on Natural Cycles showed that 7 women out of 100 got pregnant in 1 year (equivalent to a typical use Pearl Index of 7.0) when using Natural Cycles. Whereas only 0.5 women out of 100 got pregnant in a year due to the algorithm falsely attributing a green day (equivalent to a method failure rate of 0.5). This study was performed on more than 4000 women using the application in real life.
The typical use Pearl Index refers to how many women get pregnant using Natural Cycles in general, also taking into account incidents of the method used on red days failing (e.g. condom breaks) or human error (having unprotected sex on a red day).
Whereas method failure rate refers to the app attributing a false green day within the fertile window.
Be sure to refer to the instructions for use to ensure you are using Natural Cycles correctly.