Figure 5.

Pairs categorized into different subsets based on gender, frequency of contact and distance of separation. The coefficients α (basal value of call duration), β (strength of the dependence of the duration of the succeeding call on the logarithm of the inter-call gap) and ${\beta }^{\prime }$ (strength of the dependence, when both the duration and the gap are scaled by their respective averages) resulting from regression fits when pairs in $\mathcal{S}$ (set of regular contacts) are categorized into different subsets. A broad distinction into four groups (as indicated on the top of the columns) is done based on whether for a given pair the distance between the most common locations ($d_{ij}$) is less or greater than $d_c$ (i.e., geographically ‘close’ or ‘distant’, respectively) with $d_c$ being the spatial extension of large cities; and whether average gap (${\langle \tau \rangle }_{ij}$) is less or greater than ${\tau }_c$ (i.e., communication is ‘frequent’ or ‘infrequent’) with ${\tau }_c$ being the most probable inter-call gap. A finer classification is made based on the gender of the individuals as indicated along the horizontal axis. Pairs are chosen irrespective of their age. The dashed line is a guide to the eye. (See Additional file 1 for values without gender classification.)