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# Help needed with this formula watch

1. This is the Formula, Shannon's diversity index H= −∑ (ρilnρi)

I have worked out the (ρilnρi) part of the formula I just dont know what the −∑ symbols are asking me to do to the formula now, does anyone know please?

(by the way the answer from the (pilnpi) section is a negative figure)

Sorry for this post, its just im terrible at maths and would appreciate any replies , thanks !

Luke
2. anyone ?
3. (Original post by CHLUNI)
This is the Formula, Shannon's diversity index H= −∑ (ρilnρi)

I have worked out the (ρilnρi) part of the formula I just dont know what the −∑ symbols are asking me to do to the formula now, does anyone know please?

(by the way the answer from the (pilnpi) section is a negative figure)

Sorry for this post, its just im terrible at maths and would appreciate any replies , thanks !

Luke
The sigma part just refers to summing the various pilnpi associated with each event. As you say these will give you a negative value when all summed, and then take the negative of the sum to find the diversity (or uncertainty, as I learnt it).
4. So how do you take the negative of the sum ?

By doing minus 1 of the answer to turn it into a postive ?

( sorry for sounding like a complete smack tard but I'm soooooooooo bad at maths )
5. (Original post by CHLUNI)
So how do you take the negative of the sum ?

By doing minus 1 of the answer to turn it into a postive ?

( sorry for sounding like a complete smack tard but I'm soooooooooo bad at maths )

Or equivalently, sum -(pi ln pi) for each i; that way you'd be summing positive numbers from the off.
6. so -(-myanswer) ........ right ?

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