When the mapping from rank to selection probabilities is linear, only limited selection pressure can be applied. This arises from the assumption that, on average, an individual of median fitness should have one chance to be reproduced, which in turn imposes a maximum value of s = 2. (Since the scaling is linear, letting s>2 would require the worst to have a negative selection probability if the probabilities are to sum to unity.) If a higher selection pressure is required, i.e., more emphasis on selecting individuals of above-average fitness, an exponential ranking scheme is often used, of the form: