While I was fairly sure that the numbers were correct, I did know that I hadn't taken one potential systematic problem in to account: users that had signed up and produced only empty changesets. Now it could be argued that such users at least tried to contribute and should be counted, but opinions may differ on that. It was clear that any effect on current trends would be minimal, all modern editors will typically only allow you to save if you have actually changed something, implying that an empty changeset is something that can only happen in an error situation (for example an editing conflict, or a crash of the application).
Here are the corrected graphs:
The effect of removing the empty changesets lowers the overall number by roughly 25'000 users, however as expected does not effect the general trend. End of June 2014 we had an accumulated total of 421'701 contributors, increasing at a rate of around 8'000 per month.
The following graphs compared to the previous ones show that most of the effect of the change is in 2009, which as we know is the year changesets were introduced in, so some issues there are not quite unexpected.
In mid-2014 we have already had more than 90'000 active contributors, we can expect a substantial increase in the year total if the trend continues.
Back to the empty changesets, when and how much of an effect did removing them from the numbers have:
and the same relative to the new contributors per month:
(note: the above is the difference between the old numbers with zero edit changesets and the new ones, in some cases this simply caused users to be counted one month later, which explains the single negative month)
As already mentioned all changesets before May 2009 were generated when changests were introduced. I haven't been able to determine what caused the large numbers from May to November 2009, however the changesets in question do not have a created_by tag and I suspect that they were created after the fact, by some kind of mechanical process. Nobody that I asked can remember, maybe a reader can shed some light on this.
After November 2009 most of the empty changesets were created by Potlatch 1 and a well founded suspicion is that they were caused by P1 live mode. This continues up to April 2011 when Potlatch 2 was made the default editor and the absolute numbers have remained stable in the 100-200 users per month range since then.
Now it is important to remind ourselves what we are looking at: the numbers are new users that signed up, tried to edit, failed and never had a successful edit after that.
In other words, users that wanted to participate that we lost (the total number of empty changesets is far higher, however for now I'll assume that regular contributors are more tolerant of things going wrong than first-timers).
Done deeds are done deeds and there is nothing we can do about users that we have already lost, however what we can do is try and improve the situation going forward.
I've produced some numbers on which editors the users were using and nearly all (yes including JOSM) turn up, however the majority of the users effected are using iD (which is not a surprise given that it is the default editor) and, big surprise, "Go Map!" at nearly the same level. Given that "Go Map!" has a far lower user base than iD (editor stats), this indicates that there may be a real problem with the app this has in the mean time been resolved. Naturally given that iD is what is usually used by potential new contributors, further investigation is warranted too.