CRE                     A Model with Correlated Random Effects in
                        Poisson and Probit Equations
CRE_SS                  A Sample Selection Model with Correlated Random
                        Effects
PLN_RE                  A Poisson Lognormal Model with Random Effects
PanelCount              Random Effects and Sample Selection Models for
                        Panel Counting Data
PoissonRE               A Poisson Model with Random Effects
ProbitRE                A Probit Model with Random Effects
rt                      Number of words in quoted retweets
