Re: st: Question regarding postgr3

[Marcello Pagano <pagano@hsph.harvard.edu>]

Flavours aside, Maarten, I disagree that this gives the average over 
cohorts. It is not an average. It is the probability of an `average' person.

a) This is exactly my point.  Taking average heights yields the average 
of a physical property.  That no one exists who has that height is not 
important.  If, on the other hand, you wish to construct such a person, 
for whatever reason, and I cannot think of one, fine.  It is not 
necessary since you have the average height. Life, of course, gets silly 
when we think of a person with average binary covariates: half-dead, etc...

b)  Why it would "be a better description of the typical predicted 
probability" goes back to why an average is a good predictor of a 
variable.  Same context. You want a "typical" probability.  Each person 
has a probability fitted to her/him. This is just like any other 
characteristic (height, weight etc...). Take the average of these, like 
any good statistician, to describe the typical, however you take averages.

m.p.



Maarten Buis wrote:

> Marcello:
> A reasonable argument can be made for Stijn's position, if the 
> mean changes over cohorts, e.g. the proportion of mothers that 
> are working. It would show the change over cohorts, including 
> the change in the distribution of working status of the 
> mothers. In this sense this approach has a clear "population 
> average flavour". 
> There are however clearly some issues with this approach:
> a) It is true that the person with average values on the 
> explanatory variables cannot exist, but we almost never think 
> the "average person" is a real individual. This is just a 
> construct that helps us summarize what we see. 
> b) It is true that the predicted probability for an 
> individual with average values on the explanatory variables is 
> different from the average predicted probability, but I don't 
> see why one would be a better description of the typical 
> predicted probability than the other. 
> Maarten
>
> -----------------------------------------
> Maarten L. Buis
> Department of Social Research Methodology 
> Vrije Universiteit Amsterdam 
> Boelelaan 1081 
> 1081 HV Amsterdam 
> The Netherlands
>
> visiting address:
> Buitenveldertselaan 3 (Metropolitan), room Z434 
> +31 20 5986715
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>   
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