Analysis of the variation the value of the exponent : .

is the number of individuals analyzed for a given role class and language category that have at least five time points (for consistency with the minimum number of points of the correlation analysis; see Methods). For each individual, four statistics concerning are computed: the minimum (), the mean (), the maximum () and the standard deviation () are calculated over all his/her transcripts. The mean plus/minus standard deviation of these four statistics is shown for each role class and language category (when , a standard deviation of is assumed).