How to calculate difficulty metric?

How to calculate difficulty metric?

pI have several projects which all have different tasks. For (some of)
these tasks I do have a difficulty rating, ranging from type 1 (the
easiest) to type 6 (the hardest). I do now want to calculate a project
difficulty metric./p pNow, I want to calculate how hard a project is
depending on how difficult its tasks are. But I have absolutely no idea
how I would do this. I think easier tasks are most important. So if one
project has 50% level 1 tasks and 50% level 3 tasks and another one has
100% level 2 tasks, I think project 1 still would be easier, because it's
more likely that I know at least 50% of this one./p pSo my questions
are:/p ol liIs there any algorithm for this? Has this problem a name?/li
liIs there even ONE correct answer? While experimenting with the data I
made up a bunch of calculations which somehow represented difficulty.
However, I can't justify why/how this calculation would really be a
meaningful indicator for difficulty./li liHow to deal with tasks that are
not rated from 1-6? It doesn't necessarily mean that they are harder than
other tasks./li /ol pI have looked around and found stuff like a
href=http://www.evanmiller.org/how-not-to-sort-by-average-rating.html
rel=nofollowhttp://www.evanmiller.org/how-not-to-sort-by-average-rating.html/a,
but it doesn't do quite what I want. Maybe I'm also just missing the
correct keyword, as I have very limited mathematical background. Would be
kind if someone could answer some of my questions or give me some
pointers./p pThanks./p