Dennis Roberts wrote:
> sorry ... i can't agree with this ...
>
> it could be that in the "serious" cases ... there is a unidentifiable gene
> factor that INTERACTS with the treatment ... that is not available in the
> "mild" cases group (that's why you have serious and mild cases) ... so, it
> is not the treatment that is doing this ... it is the presence or lack of
> presence of the gene factor
>
> in the above ... you are trying to identify ... IF there is an effect, WHAT
> it is due to and, the design tendered above will not do that
On reflection, my example was very poorly chosen as random
allocation to groups here is important as a control. My example
conflated random sampling with random allocation to groups.
I still think that there might be cases where a pattern observed in
a convenience sample might be more informative than one from a
random sample.
I think its a moot point because I'm not convinced that random
sampling is possible in most cases. Even if you know what population
you want to sample, I'm not sure an adequate sample strategy could
be devised to sample it.
Thom
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