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(N(N-1)) / (sum of n(n-1)) ?!?!?!?

For my biology coursework at the end of last year (you know that annoying bit at the end of year 12 when you have to go back after exams for a few weeks and start a2 stuff) we had to do a load of statistical stuff.

I finished most of the coursework then/ over the summer holidays but then forgot about it, until now, when the teacher wants to mark it over the easter holidays before it's submitted. I had the above formula down as being for the simpsons index, but it's not and the teacher doesn't know where its from . . . The teacher I had last year isn't here this year so I can't ask her.

I had a list of plants and how many there were of each species in a given area. N is the total of all the induviduals of all species and n is the total for each species.

What is it?

I dunno how clear that was so I'll give a random example using random plants and numbers (that are way simplified compared to the amount of data I collected on the day), say that in a given area I had;

4 poppies
2 daisies
4 dandelions

N would be 10 so N(N-1) would be 90

and this would be divided by the sum of 4(4-1) + 2(2-1) + 4(4-1) which is . . . 50

so you end up with 90/50 for the data I've given which is 1.8 but I now have no idea what that number means . . . in my actual data I ended up with 4.1 and 2.5 !!!

I had that down as the simpsons index, but it turns out that that isn't the simpsons index, but me and my teacher and now perplexed as to what I worked out using that formula . . . which I may add that I went through with the teacher so it msut be an actual formula.

It's probably some kind of meaure of biodiversity, I've done species richness and evenness aswell . . .

Reply 1

mathz

looks like you have simpsons index but upside down

http://en.wikipedia.org/wiki/Diversity_index#Simpson_index

you need the formula under simpsons index for small data sets l=[sum ni(ni-1)]/N(N-1)

Reply 2

Clare~Bear

So I don't have to start from complete scratch to get the SI. I did it step by step so ended up with;

total 1878 1945
N(N-1) 3525006 3781080
n(n-1) 858251 1511050
SI 4.107197079 2.50228649

And I divided N(N-1) by n(n-1) but it should be the other way round toget the SI???