Statistical Analysis Advice Required

Forensic Entomologist

New Member
Hello,

I posted a topic yesterday, and I assumed I had the issue "solved" so to speak. However, after much deliberation last night I think I may be back to square one, plus I have a few other queries I would would be ever so grateful for help with.

I want to establish whether covering affects the abundance of insects onto a corpse. I was using a Chi-Squared test, but as the majority of my data values were below five, it was decided that I would use Fisher's exact test. When I performed this test I came up with a p-value of 1.000, which would suggest that there was no affect by covering; but to look at the data it would be obvious that there is a difference in abundance.

An example of my data set is attached as an excel spreadsheet. I believe the issue to be the number of data points with zero. Essentially, I would like to know where to go with this data; i.e. what tests can I do?

I also have to investigate the correlation between temperature and rainfall on overall abundance; can I do both factors at the same time?

I also have access to Excel with XLStat and Minitab.

Sorry I had to post a separate thread, and thank you for any help; i'm a complete statistics novice.

Dason

Ambassador to the humans
What program did you use to do the test? In R I get a p-value of .000001

Forensic Entomologist

New Member
What program did you use to do the test? In R I get a p-value of .000001
Hello, I used EXLStat to get the two tailed result of 1.000!

gianmarco

TS Contributor
A significant result is returned by:
a) XLStat (chi-square test; chi-square with Monte Carlo simulation)
b) PAST (Monte Carlo p)

Regards
Gm

Forensic Entomologist

New Member
Within your XLStat sheet the fisher's exact is also 1.000; however you have not highlighted this as where you obtained your p-value.

I'm not sure what the Monte Carlo simulation is? Does it allow for expected frequencies less than five?

Thanks.

gianmarco

TS Contributor
I was just stressing the results similar to that obtained by Dason.
As for Montecarlo simulation applied to chi-square and its context of use, this is an issue that I am trying to investigate...
I would like to have feedbacks on this by more experienced people here.

Gm

Forensic Entomologist

New Member
Right, ok. I'm afraid I'm completely out of my depth with this level of statistics. I really only want to know whether the covering type is significant on the abundance of arthropods.

FM

Dason

Ambassador to the humans
I was just stressing the results similar to that obtained by Dason.
As for Montecarlo simulation applied to chi-square and its context of use, this is an issue that I am trying to investigate...
I would like to have feedbacks on this by more experienced people here.

Gm
I'm guessing the Monte Carlo simulation you're talking about is a just a randomization approach to get an estimate of the sampling distribution under the null hypothesis? I like that approach especially for low cell counts and/or low expected counts.

Forensic Entomologist

New Member
I'm guessing the Monte Carlo simulation you're talking about is a just a randomization approach to get an estimate of the sampling distribution under the null hypothesis? I like that approach especially for low cell counts and/or low expected counts.
This is all getting a bit double Dutch to me...

Dason

Ambassador to the humans
Well I know zero Dutch so it's impressive that I'm speaking Dutch twice.

Forensic Entomologist

New Member
Well I know zero Dutch so it's impressive that I'm speaking Dutch twice.
Haha!

Well, i'm thoroughly confused! I thought it would have been simple to establish the effect of covering.

Is there another test I could perhaps use, instead of Fisher's exact or chi-squared?!

Thanks.

gianmarco

TS Contributor
Thanks Dason,
I was guessing something similar, but I lack the appropriate stat terminology.
I would like to learn more about its context of use and, in particular, if it can be used when chi-square "assumptions" are not met (as in the case object of this thread).
Any bibliographical reference is welcome.

Thanks
Regards
Gm

Forensic Entomologist

New Member
If anyone has any suggestions I'd be ever so grateful!

I want confirmation that Fisher's exact test returning a p-value of 1.00, if that's correct then I'll stick to that test.

Thanks.

Dason

Ambassador to the humans
The results I returned and partially confirmed by gianmarco are pval < .05. I get something like .00001 actually but I don't know what GM ended up getting (he only said significant). Is there a reason you want a p-value of 1?

Forensic Entomologist

New Member
Hello,

There is no reason why I want a p-value of 1.000, it's just what I keep obtaining using XLStat for Fisher's exact. gianmarco used a monte carlo simulation, which I assume isn't the same thing, although I have no idea as i've never heard of it before.

I investigated using R, but alas, have no idea how to use it i'm afraid.

Dason

Ambassador to the humans
I've never used XLStat but I think something must be going wrong. Regardless of significance - with the data that you have you shouldn't be getting a p-value of 1.

Forensic Entomologist

New Member
Hmm, that's what I assumed.

Any tips for using R, or any other "easy to use" statistical programmes?

gianmarco

TS Contributor
hmmmm....this discussion is getting more and more intricaded by the minute....
I do not know if something is wrong with the way XLStat is running the test. So, I would leave is aside for the time being.
Secondly, the chi-square test and the same test performed using the Montecarlo simulation is pointing to a significant result, or (to keep with the chi square terminology) to a significant dependence between rows and columns, that is between the two categorical variables under analysis.

This significant result should be (if I am not mistaken) compatible with the one Dason got using (again, if I am not mistaken) Fisher test with R.

So, if chi-square with Montecarlo simulation can be' used in this context (table with sparse values), I think that your issue should be solved: you are in the position to test a significante association between rows and columns.
If this is true, you could report the Xlstat output for the chi-square with Montecarlo simul, or the Past output.

I hope this helps
Regards
Gm

I look forward to hear about the use (and validity) of montecarlo simulation in case for chi square in case of tables with sparse values.