This equation means that in a particular learning or extinction situation, the probability of tendency to respond is the product of motivational level and strength of habit. For measuring motivational or drive state level, hours of deprivation is used as parameter. Hence, a rat that has learned a reinforced PR rewarded response, will be more likely to perform the response if it is hungry for 22 hours compared to 3 hours. In order to predict habit strength, the number of previously experienced reinforced trials is an important factor. Assuming this, a law of habit formation was framed which is expressed as follows.