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st: ratios first or last?

From   David Airey <>
Subject   st: ratios first or last?
Date   Sat, 22 Nov 2003 19:27:00 -0600

Recently I was surprised to find a difference between two methods of calculating a ratio during an experiment. Each animal has two measures taken repeatedly over time. The ratio is of the two measures. I could take the ratio at each time point, and then average the ratios to get my animal ratio. Alternatively, I could average each of the two measures and then form a ratio of the two averages, again getting my animal ratio. The second method consistently gets a higher ratio than the first method. Why would this occur? The second method is standard in my literature base.


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