Between "I" and "R", the transition rate is assumed to be proportional to the number of infectious individuals which is γ"I". This is equivalent to assuming that the probability of an infectious individual recovering in any time interval "dt" is simply γ"dt". If an individual is infectious for an average time period "D", then γ = 1/"D". This is also equivalent to the assumption that the length of time spent by an individual in the infectious state is a random variable with an exponential distribution. The "classical" SIR model may be modified by using more complex and realistic distributions for the I-R transition rate (e.g. the Erlang distribution).