John McCloskey wrote:
I am trying to figure out how configure raw data to be expressed as ratios of
controls. I want to do this because my experiments utilize fluorescense and the
signal intensity can vary from time to time.
To be specific, I entered raw data for my controls and three concentrations of
drug. Sample size is 4 for each group. For the control group my mean is 254, SD
2.88, and the SEM is 1.66. For one concentration of drug, the mean value is
278, SD 5.5, and the SEM is 3.18.
I imagine the ratio of drug to control is 1.32 but how do I express the SEM or
SD? Variance divided by the mean of the control or mean of the drug?
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Pharmacology journals, at least in the past, allowed you to divide individual
fluorescence values in each of the drug groups by the mean of the control group
values as the divisor, and then calculate SEs or SDs from the quotients. This
treats the control group's mean as if it were fixed, known. It gives an SE
that is symmetric about the ratio of the means.
You might want to take a look at Fieller's theorem for an alternative. (Google
should bring something up.) Perhaps, jackknifed or bootstrapped confidence
intervals would be more accessible. Using -regress, nocons- followed by -nlcom-
might be doable with large sample sizes.
Joseph Coveney
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