  Online communities provide a fertile ground for analyzing people's behavior
and improving our understanding of social processes. Because both people and
communities change over time, we argue that analyses of these communities that
take time into account will lead to deeper and more accurate results. Using
Reddit as an example, we study the evolution of users based on comment and
submission data from 2007 to 2014. Even using one of the simplest temporal
differences between users---yearly cohorts---we find wide differences in
people's behavior, including comment activity, effort, and survival. Further,
not accounting for time can lead us to misinterpret important phenomena. For
instance, we observe that average comment length decreases over any fixed
period of time, but comment length in each cohort of users steadily increases
during the same period after an abrupt initial drop, an example of Simpson's
Paradox. Dividing cohorts into sub-cohorts based on the survival time in the
community provides further insights; in particular, longer-lived users start at
a higher activity level and make more and shorter comments than those who leave
earlier. These findings both give more insight into user evolution in Reddit in
particular, and raise a number of interesting questions around studying online
behavior going forward.
