To probit or not to probit?

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1 total messages Started by Robyn Lee Mon, 26 Jul 1993 13:59

#1199

Author: Robyn Lee
Date: Mon, 26 Jul 1993 13:59

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To Stat-L members,
      I have been confronted with yet another interesting challenge.  I have
an entemologist that is using pesticides on field strain and susceptible
strain cockroaches to test if there is increased resistance to the pesticides.
Currently, nine batches of ten roaches each are placed in containers with the
pesticide.  They are observed every 15 minutes and the numbers of dead roaches
are counted at each observation time.  When graphing the cumulative number of
dead roaches by observation time, the graph makes a fairly good sigmoid curve.
A probit approach is currently being used, but I know that it is not
appropriate since the counts at the observation times are not independent. So
how should I proceed from here? My entemologist wants to be able to compare
the response from the field strain to the susceptible strain as well as
compare how the field strain may change over time ( more exposure to the
perticide). In addition, how might I calculate whether the nine groups of ten
is an adequate sample size?
Robyn Lee
lee_rb@mricd1.apgea.army.mil

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