plate count statistics

Veljo Kisand kisand at madli.ut.ee
Sun Dec 3 03:19:25 EST 1995

Charles N Haas (haascn at dunx1.ocs.drexel.edu) wrote:
: Bio Techniques Laboratories (cobactin at halcyon.com) wrote:
: : In recent years I have found two editorials on the "correct" method to 
: : statistically analyze plate count data.  The first one said that the mean 
: : and the std. dev. should be determined from the arithmetic values of the 
: : data.  The second one said the data should be transformed to logs and 
: : then the mean and std. dev. should be calculated and then retransformed 
: : to scientific notation.  Are there any more opinions on how to deal with 
: : plate count data?  If anyone could suggest a text or article that deals 
: : with statistics for the analysis of plate count data, I would appreciate it.
: : Thank you!

: If plate count data are in control, then the counts should be Poisson
: distributed.  To compute the density, sum the total colonies by the total
: volumes plated.

: The log transformation is one way to approximately transform Poisson
: variates into normal variates.  There is a long literature on this, although
: some of it is fairly old.  You do not indicate what your overall objective
: is, so it is hard to go further.  But if this thread continues, I will put
: in a few references of mine that I have written regarding statistical
: analysis of microbial data (mostly in the context of disinfection
: experiments, however it is equally applicable to growth curve
: experiments).

Log transformation is basicly used for lognormal distribution, so one can
use all powerful prametric methods. You should decide in every case which is
better solution. Anyway nonparametrci methods are easily applicable,
sometimes I like that these are more strict...
Veljo Kisand

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