Fisher's Exact Test

[ReptileJK]

Hello,

I'm currently a doctoral candidate in wildlife ecology. I'm analyzing habitat selection data for bullsnakes collected over three years.

Initially, I ran a chi-squared test to determine if observed habitat use differed from expected values. However, I have several expected values below 5, and several observed cells with zeros. I was told that perhaps a Fishers exact test would be the best way to analyze my data.

However, I can't conform to a 2X2 contigency table. I have to analyze habitat use data for males and females in 14 different habitat types...and repeat these analyses for 2003, 2004, 2005.

At any length, I'm having trouble determining: (a) if this will work for me and (b) if there is a statistical software package available that can analyze it.

Thank you for your time,
Josh

[JohnM]

Even though your data violates some of the criteria for using chi-square, and since it sounds like you cannot combine categories or whittle(sp?) it down to a 2x2 table, then I would just run a chi-square anyway....and see how it goes.

I don't think you have many other options....

[ReptileJK]

Thanks, John.

In the chi-squared analysis, what should I do in habitat types where I have no observations (ie, zero in the observed cells)? Eliminate them for that analysis?

I'm assuming I can run a G-stat test (logliklihood analysis) to "correct" the fact that several of my expected values are below 5.

Thanks,
Josh

[ReptileJK]

Hi John,

I know you must be ridiculously busy, but I was just curious if you had gotten a chance to think about my previous question?

Thanks alot for your time,
Josh

[JohnM]

I would keep them in the analysis. Just because your sample is a 0, doesn't necessarily mean that it will always be 0.

[ReptileJK]

I would keep them in the analysis. Just because your sample is a 0, doesn't necessarily mean that it will always be 0.

Thanks for the reply, John.

Just curious: isn't it a prerequisite for chi-squared tests that observed values can't be zero...in other words, no "observed" cells can have a value of zero? Thought I read that somewhere, but could be completely wrong.

Thanks...I should be just about done pestering you.

Josh

[JohnM]

Not that I'm aware of....

[ReptileJK]

Thanks, John!

Josh