Great point, Lodewijk
We should ideally be able to measure the relationship between sitenotice and contributors. And, if we currently don't have enough data for that, design WLM in such a way we will be able to measure that so it can be used for future contests.
There is probably a chain of # banner impressions → # visits to contest page → # users actually paying attention → # users participating
While the absolute number of visits to the contest page coming from sitenotice should be relatively simple to count, the number of people that viewed will depend on the campaign configuration and the presence of other campaigns at the same time (e.g. fundraising), plus on the actual traffic received from that country (probably complicated, but could be calculated) *plus* the actual configuration of the user hiding the banner or the diet restricting the number of times it is shown per device.
Then, we have an additional handicap that our reaction won't be immediate.Users are unlikely to have photos of designated Monuments without an image sitting on their computer ready to be uploaded. If a user is going to actually find a suitable nearby monument in the list, plan a trip with its camera (next weekend, perhaps?), actually go out and shoot it, come back, locally review their photos, select good ones and upload them... It will take several days, so the effect of the impressions won't be clear. Furthermore, repeated impressions may be needed to remind them that they intended to actually tak some photos. Whereas most people will simply think "Oh, nice project" and let other people do that work, without wishing to be bothered any more about WLM.
I don't think we actually have the resources to properly measure this, with all its variables. Perhaps we could recruit some help from the Analytics / Product Analytics teams?
Best regards