Ann Atzberger and others, Maybe I did not explain the original question well. The real case was: I had 10 million lymph node cells and I mixed it really well by pipetting and vortexing. Then, TWO samples, each with 1/5 million cells, were aliquoted into TWO tubes and run separately. One gave me 0.04% and the other 0.15%. If I assume the best mixing was achieved (a plot of fluorescence over time shows whether positive events are well distributed) and dead/debris was minimal (my cells were 99% live), I am still bound to be worried by something else, the systematic error of sampling. This is just like epidemiological survey: I can only survey a certain number of people (cells) to represent the total, all people (all 10 million cells). It is a statistical problem to determine how big that number would be so as to gain certain (but never 100% unless the total were surveyed) confidence on the representativeness of my results. Dr. James Leary's paper (Cytometry 27: 233-238, 1997) addressed this issue and there is a link to a lecture note by Terry Hoy, http://www.uwcm.ac.uk/uwcm/hg/hoy/rare.html <http://www.uwcm.ac.uk/uwcm/hg/hoy/rare.html> , provided by Jeff Barry. Thank you all for replying to my message. Hai Qi Department of Pathology, UTMB Keiller Bldg., Rm. 1.104 301 Univ. Blvd. Galveston, TX 77555-0609 Tel: 409-7728163 Fax: 409-7476869
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