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Re: st: Re: single sample pre/post comparison of proportions

From   Svend Juul <SJ@SOCI.AU.DK>
To   <>
Subject   Re: st: Re: single sample pre/post comparison of proportions
Date   Thu, 11 Jun 2009 08:53:50 +0200

Michael asked and Joseph responded (not shown) - and Michael then wrote:
The suggestion of a one-sample test restricted to pre-intervention ADOPT=NO crowd makes sense. I think you are also sneakily suggesting that the most obvious null hypothesis -- "H0: p = 0" is not a good choice; there would probably be some adoption even in the absence of the intervention, and the intervention probably cannot be called a success unless the proportion of adopters exceeds a minimum cost/benefit threshold. Instead, I could choose, e.g., "H0: p < .25" (a one-tailed test). That seems reasonable. 
I wonder whether a P-value related to a somewhat arbitrary null hypothesis is useful. I think the following is more informative:
Assume that you had 90 participants, 40 of whom already had the good habit, leaving 50 "at risk" for improvement. 20 (40%) of these improved. The 95% CI for this estimate is 26%-55%:
. cii 50 20 , binomial
                                                         -- Binomial Exact --
    Variable |        Obs        Mean    Std. Err.       [95% Conf. Interval]
             |         50          .4     .069282        .2640784     .548206
Hope this helps

Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Bartholins Allé 2
DK-8000 Aarhus C,  Denmark 
Phone:   +45 8693 7796 
Mobile:  +45 2634 7796

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