Yay, data! Thanks for putting this together, Jon.

On Tue, Oct 13, 2015 at 10:14 PM, Jon Katz <jkatz@wikimedia.org> wrote:

I don't know if we can explain all of our traffic decreases to the drop in session length, but it is certainly a big factor. Basically 60% of our pageviews (internal) shrink by 33% on mobile. So all else being equal, if we transfer all our traffic to mobile we lose 33% of our pageviews. Right now we're at 50%. This assumes that there is no change in numbers of sessions...on which we have no data right now.

We would only lose 20% (33% of 60%). But that is a completely unjustified assumption.

(Also, there is another unjustified assumption that every mobile user is an ex-desktop user, ie. that this really is a shift in the habit of an existing userbase, not Wikipedia becoming less attractive to the old userbase and attractive to a new one. While it is not strictly relevant to this analysis whether the user disappearing on desktop and the one appearing on mobile is the same one, it is very relevant to how we interpret it.)

We don't have to guess the session numbers, though - your analysis assumes that non-internal pageviews start a session and internal ones continue it, in which case the number of sessions is simply the number of non-internal pageviews. That is, we get

desktop: 510M pageviews / 335M sessions -> 275M pageviews / 175M sessions

mobile: 150M pageviews / 100M sessions -> 230M pageviews / 175M sessions

total: 660M pageviews / 435M sessions -> 505M pageviews / 350M sessions

between the two ends of the graph (numbers are vague; I just looked at the image and guessed averages). That's a 20% decrease in sessions vs. the 25% decrease in page views, so it shouldn't cause much change in how worried we are about the drop.