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Test of agreement for presence/absence by month

fishguy

Hi,

I collected data on the monthly occurrence of a parasite and the monthly occurrence of a histological lesion in the tissues where the parasite infects. The parasite is strongly seasonal - peak prevalence and intensities occur in March and April. The lesions are more spread out throughout the year but appear to be more common in those months. So for each (lesion or parasite), I can construct a contingency table (2x12) to demonstrate the months where the prevalence of lesions and parasites are highest. What I would like to do is test how closely the parasites and lesions agree. Is there a test of agreement to compare monthly lesion and parasite prevalence?

I can disregard month and construct a contingency table of parasite and lesion. The results show a Kappa of 0.0248 with a P = 0.5804. Also, Bowker's test for symmetry (greater than 2x2 contingency table) indicates symmetry of disagreement chisq =22.6, P <0.001. I would take these results that there is no strong measure of agreement between the lesion and parasite overall. However, this doesn't account for month.

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Mark_Bailey

Agreement is for Y and X with the same scale. Your description mentions counts (lesions, parasites) but not the scales used for either one.

Association is more general. You could stratefy your contingency table by Month. One of the CMH tests should cover your hypothesis.

fishguy

Hi,

I recorded parasite and lesion presence or absence for each fish. The
monthly proportion of fish with each ranges from low teens to 100% so If I
understand correctly, they are both on the same scale

What are CMH tests and how do I stratify by month.

Thanks!
Mark

fishguy

Ok, I think it is the Cochran Mantel Haenzsel test you're referring too. I found an example and understand how to run it now. So I chose the presence/absence of lesion and parasite as the X and Y and then in the pop-up window chose month as the grouping variable. Is the default that the null hypothesis is no correlation between parasite and lesion?

Thanks!

Mark_Bailey

You are way ahead of me! Yes, the null is no association. You test the overall association (unstratified) first with the Contingency platform, then select CMH from the red triangle for stratification by Month.