# Chi-squared help (for uni, but A-level people and those good at stats take a look!) watch

1. I'm really stuck on how to find the expected values from a set of data. Basically, it's a test of the 'Idea Free Distribution model' and we had to feed birds to see if the assumptions were followed, so the food should have been eaten in proportion to abundance.

Here are some made up results (I don't want to put the real ones as I'd rather do it myself)

Species Bread Eaten Abundance of species

Mallard 100 600
Coot 75 250
Gull 20 60

Obviously I have more data to work with, but if anyone could go through step by step with these three results on how to get the chi-squared value (it's mainly the expected values I'm struggling with) it would be much appreciated.

EDIT: sorry, dunno how to insert a table, but the first numbers are bread eaten and the second are abundance of species.
2. Anybody?
3. for the expected values, have you tried totalling the amount of bread eaten and working out the amount eaten per bird of all species?

you'd multiply that number by the (abundance of species)/ (total number of birds) for each species to get the amount of bread you'd expect each species to have eaten.

i.e. total eaten (all sp) = 100+75+20 = 195

total birds (all sp) = 600+250+60 = 910

amt eaten per bird (all sp) = 195/910

-----
expected amt eaten per mallard = (195/910) * (600/910)
expected amt eaten all mallards = (195/910) * (600/910) * 600
etc...
4. No, I haven't! For a-levels if I remember rightly we just got told to work out the mean for the expected value for chi-squared, so when I saw this I was like 0.o My maths skills are basically non-existent.

Your method seems logical and it's better than anything I could come up with in a million years, so you know. Thank you so much!!!!!!

For the expected amount eaten by all Mallards, why do you have to *by 600 again at the end?
No, I haven't! For a-levels if I remember rightly we just got told to work out the mean for the expected value for chi-squared, so when I saw this I was like 0.o My maths skills are basically non-existent.

Your method seems logical and it's better than anything I could come up with in a million years, so you know. Thank you so much!!!!!!

For the expected amount eaten by all Mallards, why do you have to *by 600 again at the end?
if you expect each mallard to eat x, 600 mallards will be expected to eat 600 times as much
6. I just realized this and was about to edit before I looked stupid, but nm :')

Thank you very much for your help!
7. I'm not sure this works because when I do it on my own data the total of the expected values are supposed to equal the total of the observed values, but they aren't doing.

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Updated: January 4, 2013
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