In the Lee approximation, it is assumed that each internal link between stages is already occupied by a call with a certain probability "p", and that this is completely independent between different links. This overestimates the blocking probability, particularly for small "r". The probability that a given internal link is busy is "p" = "uq"/"m", where "q" is the probability that an ingress or egress link is busy. Conversely, the probability that a link is free is 1−"p". The probability that the path connecting an ingress switch to an egress switch via a particular middle stage switch is free is the probability that both links are free, (1−"p")2. Hence the probability of it being unavailable is 1−(1−"p")2 = 2"p"−"p"2. The probability of blocking, or the probability that no such path is free, is then [1−(1−"p")2]"m".