Stata The Stata listserver
[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: Configuring Data to percentage ratios


From   Joseph Coveney <[email protected]>
To   Statalist <[email protected]>
Subject   Re: st: Configuring Data to percentage ratios
Date   Sat, 04 Dec 2004 14:01:05 +0900

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?

-------------------------------------------------------------------------------

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


*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



© Copyright 1996–2024 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index