The metric average replies per thread is an indicator of an active discussion. It is proposed by Angeletou et al. (2011), Hacker et al. (2015) and Viol et al. (2016). It is of global scope as it is calculated for the entire network. The average replies per thread $at$ metric is calculated as an average for the network. The median is used instead of the mean, as some threads can have an unusual high count of responses.
1. select all posts grouped by threads