Systems and methods for determining if a wearable photoplethysmography device is correctly positioned in operating to medical signs of a user by using a classifier to determine if a signal is valid or invalid. In some embodiments, in using the classifier to determine in a signal is valid or invalid, a lean method of linear computational complexity and minimal memory complexity is provided for determining at the wearable photoplethysmography device if it is correctly positioned. In some embodiments, in using the classifier minimal computational complexity is used in determining at the wearable photoplethysmography device if it is correctly positioned.