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From |
Ronan Conroy <rconroy@rcsi.ie> |

To |
"statalist hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |

Subject |
Re: st: ratios first or last? |

Date |
Mon, 24 Nov 2003 10:06:54 -0000 |

on 23/11/2003 01:27, David Airey at david.airey@vanderbilt.edu wrote: > 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. The method of taking an average at each time point gives each time point equal weighting in the final calculation. Summing all time points allows time points to affect the sum in proportion to the absolute measured value. It is equivalent to an area under the curve where the time points are equally spaced. Still, the way you describe it, there is no stated rationale for either method. Summarising measurements really does require a knowledge of what construct is being measured by the data. In other words, it is the scientific meaning of the data that dictate the appropriate method of summarising the data. So, in the end, it's a science question, rather than a statistical one in the narrow sense. Ronan M Conroy (rconroy@rcsi.ie) Lecturer in Biostatistics Royal College of Surgeons Dublin 2, Ireland +353 1 402 2431 (fax 2764) -------------------- Ugh - what's that bitter taste in my coffee? www.maketradefair.org -------------------------------------------------------------------------------------------------------------------- This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the originator of the message. This footer also confirms that this email message has been scanned for the presence of computer viruses. Any views expressed in this message are those of the individual sender, except where the sender specifies and with authority, states them to be the views of The Royal College Of Surgeons in Ireland. -------------------------------------------------------------------------------------------------------------------- * * 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/

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