Why does the amount of 80% problems keep changing?

[SmileyCraft]

So a few weeks back I noticed that the amount of 80% problems had dropped from 23 to 22. I first thought I probably remembered incorrectly, but ever since then I have been noticing that the amount of 80% problems keeps alternating between 23 and 22. Earlier today there were 23 and now there are 22 again. What is going on?

[euler]

If by 80% you're referring to the difficulty rating, then you will notice (as I make this post) that problem #671 is currently not finalised due to only having 99 solvers; a problem requires 100 solvers to be finalised. It's possible that it has been moving between 75% or 85% as it has been approaching the 100 count. At that stage it will be finalised and the count will be fixed until another unfinalised problem enters the scene.

[SmileyCraft]

Right, that makes sense. Thanks for the response!

[LilStalker]

But the difficulty might still change after those 100 solvers right? I believe that the difficulty of problem 541 went up from 85 to 90 some time after I managed to solve it. I think the difficulty of problem 589 is also slowly increasing

Or are just the first 100 solvers considered as a metric for the problem difficulty? If so, might it be that difficulties change because some people delete or make their accounts private and they are not anymore considered in the top 100 ladder?

[euler]

Yes, since problem #277 the difficulty rating is based on the time taken to solve the problem taken from the fastest solvers tables. It could, in theory, change slightly if one of the members in the fastest solvers changed their account status to private or deleted it and the next fastest person to solve the problem, who would move to the end of the fastest solvers table, had a sufficiently large time gap between the previous quickest solver. However, that is highly unlikely given the theoretical stability of a large data set.