clumping vs. random

Hello list,

It seems this should be quite a basic question, but I have no idea how to
approach it.  For 100s of sites, I have an environmental variable that has
occurences over a 20 year period.  I am looking for a way to create an
index between 0 and 1 that represents the degree of clumpiness vs.
randomness over time.  Where 0 is totally clumped and 1 is totally spaced
out evenly.

So for example:

Site     Year1   Year2  Year3   Year4......Year19   Year20    Total   Index
1         Yes     Yes    Yes     Yes        No        No      10       0
2         Yes     No     Yes     No        Yes        No      10       1

Yes and No can be converted to 1 and 0.  I am not sure what you call this
sort of index, technically, so I am not sure how to search for an
appropriate method.  I am, obviously, looking for a way to do this in R.

Thanks for any suggestions!
Kind Regards,
Kristen