Newbie statistician looking for help...
I'm trying to determine whether birds in two different area have different diets, based on 8 categories of food items. Running a chi-squared test in R (using
chisq.test()) gets me a beautiful, tiny p-value, indicating diet is indeed different between these areas. But I'd like to know how, exactly, they're different. I pulled the standardized residuals from the test using
stdres which gets me this:
corvid grouse thrush mammal squirrel unknown bird hare tz 2.065822 0.05288435 -2.659504 2.464809 0.3361617 -1.785988 0.4370721 0.8356943 cs -2.065822 -0.05288435 2.659504 -2.464809 -0.3361617 1.785988 -0.4370721 -0.8356943
First of all, is this the correct way to pinpoint which categories differ between the groups? And, in very plain and applied language, how do I interpret these residuals? Is it accurate, for example, to say that birds in the tz area consume many more corvids than expected? Can I safely say there is no significant difference in the amount of squirrels consumed between the two area? There doesn't seem to be a lot of material about using residuals with chi-squared tests and what's out there uses more technical language like which category "contributes most" which isn't super helpful when I'm trying to make a concrete ecological statement.
Thanks in advance!
Edit to clarify what is being counted: my birds are carnivorous raptors, so each count represents a prey item (like one squirrel or one thrush) eaten by some bird in that zone. Each bird can belong to only one zone, but each bird can consume any number of different prey items. The original dataset looks like this:
corvid grouse thrush mammal squirrel unknown bird hare tz 12 6 36 50 248 36 21 2 cs 0 2 24 7 84 20 6 0
p.s. I know the counts for some categories are a bit small, this is just a preliminary look and more data are coming :)