They way most successful internet sites deal with this is a combination of help to beginners, welcome emails, newsletters and social networking. I suspect that a lot of the churn from OSM is because there is very little of these activities.

On the issue of retention: I think it’s pretty obvious that almost everyone (Flash haters apart) starts with Potlatch. As you say, “it’s the editor on the OSM home page which requires the least effort to access”.

So, inevitably, its retention figure is going to be much lower than JOSM’s. Whether or not someone is a “JOSM user” or a “Potlatch user” (maybe there’s a gene or something), they will have started with Potlatch, even if just for two edits.

One detail: Potlatch’s live mode begins a changeset immediately you enter it (whereas save mode begins a changeset when you save it). And, of course, for several months after 0.6 was introduced, live mode was all that was available in Potlatch. So I suspect the Nb-Nc discrepancy in Potlatch’s case is largely due to that.

It will be interesting to distinguish between overall retention (in OSM) and retention as users of the particular tool.

Of course with some tools, like Mapzen POI collector, I would expect many user to use multiple tools concurrently. Collecting POI in the Mapzen POI Collector and then doing edits/roads etc with a web editor.

Nice analysis….. j