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# Please explain this maths formula to me! Tweet

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1. Please explain this maths formula to me!

I can't translate this to straightforward arithmetic
2. Re: Please explain this maths formula to me!
Could we have some context/background.
3. Re: Please explain this maths formula to me!
I got this formula from a paper. Here is the snippet.

What I understood of it is that the first rank in each result set will have a score of 1, and the lowest will have a score of 0.

Rank and num_of_retrived_docs can be anything numerically valued? There are lots of ranks and one num_of_retrived docs?

Not sure what else? Thanks.
4. Re: Please explain this maths formula to me!
I partially agree.

The hightest rank_sim will be 1, but the lowest won't quite reach zero.

Using their example for retrieving the top-ranked 1000 documents.

Then your formula becomes:

So if your document is ranked number 1 out of 1000, the Rank_Sim value is

And if it is ranked 10, as per the example, then the Rank_Sim value is

The overall effect of the Rank_Sim function is, given n top ranked documents, then to scale the ranking so that instead of running from 1 up to n, it runs from 1 down to 1/n, linearly.

Edit: See attached for a plot of Rank_Sim against rank, for n=100
Attached Thumbnails

Last edited by ghostwalker; 17-07-2012 at 20:49.
5. Re: Please explain this maths formula to me!
Sorry I still don't understand.

My maths is awful.

I'm trying to figure out how to convert that into something I can work out on the computer with basic arithmetic operators.

Do I want to use brackets for the divide by bit?

Many thanks.
6. Re: Please explain this maths formula to me!
Thanks just got it!
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