HELP -standard error calculation for MPN???

Annette Nygaard Jensen anj at pop.bio.aau.dk
Fri Oct 31 19:52:17 EST 1997

I estimated bacterial density in dilution series, using the 
computerprogram by M.A. Hurley & M.E.Roscoe (journal of applied 
bacteriology 1983, vol 55,p 159-164)
For comparing 2 MPN estimates they conduct a t-test referring to an 
article by Cochran, W.G.(Estimation of Bacterial Densities by Means of 
the 'Most Probable Number'. 1950. Biometrics 6: 105-116). 
The nominator for the t-value is Standard Error S.E. calculated by a very 
simple way with dilution factor and number of samples per dilution beeing 
the only variable > constant S.E. However, the S.E. given by the 
computerprogramme by Hurley and Roscoe is not a constant eventhoug my 
sample number and dilution factor are the same??. 

Furthermore according to the article - for estimatiion of the 95% 
confidence limits they use the value 1.96 from the normal distribution??
Shouldn't they use the 95% value from the t distibution for the same 
reason they use a t-test???

Please reply if you posses any information that will add to my poor 
understanding of statistics 
Thank You very much

Annette Jensen	e-mail:  anj at pop.bio.aau.dk

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