In a study of water quality trends, EPA compiled stream biological monitoring data in the...

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In a study of water quality trends, EPA compiled stream biological monitoring data in the Mid-Atlantic region before and after 1997. They are interested in whether there was a shift in biological conditions in streams in the area. The indicator they used is EPT taxa richness (number of taxa belong to three genera of ies commonly known as mayy, stoney, and caddisy). As distributions of count variables are typically skewed, log transformation was used. The log mean and standard deviation of EPT taxa richness before 1997 are 2.2 and 6.9 (n = 355), and are 1.8 and 5.4 after 1997 (n = 280). Is the di_erence in log mean statistically signi_cant? How do you report the di_erence in terms of EPT taxa richness?

Monica · Accepted Answer

Solution 
Given:
In a study of water quality trends, EPA complied stream biological monitoring data in the Mid – Atlantic region before and after 1997. 
The mean and the standard deviation of EPT tax richness before 1997 is 2.2 and 6.9, the sample size is equal to 355 
The mean and the standard deviation of EPT richness after 1997 is 1.8 and 5.4, the sample size is equal to 280
This 
Testing for equality of the lognormal mean (the population mean of logs) will be the same as testing for equality of the lognormal standard deviation.
So, under that assumption,

In a study of water quality trends, EPA compiled stream biological monitoring data in the Mid-Atlantic region before and after 1997. They are interested in whether there was a shift in biological...

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