QUOTE (Die Kotzé's @ Jul 19 2007, 11:38 AM)

These panels represent people from all over the spectrum. I'm sure Markinor in South Africa does the same.
As big as it is in SA, they are one of the worst in terms of statistical validity.
QUOTE (Dedré @ Jul 20 2007, 12:38 AM)

Yes, you guys are right. Polls are widely used, and can be above board. But their sampling technique used can be dodgy.
Can be? - very often is - totally agree with you
QUOTE (ckotze @ Jul 20 2007, 12:55 PM)

I get the impression that many of these companies do Market Research, rather than true surveys.
Quite right - however proper market research calls for proper statistical techniques - thus I would not call it Market Research but rather a uncontrolled, non-random surveys, pretty much as worthless as the paper written on. What is the purpose of any such research is you have absolutely no measure as to the validity thereof.
The above may well be valid for a pilot study - but often not designed for the formal study
In some cases the choices is limited, e.g. mobile operator phone testings. - that is normally done through a pre-selected panel, whose profiles represents the distribution of their subscriber population. When done as a properly controlled (i.e. with a control group) experiment that could be equally valid. Having said all, one does get the odd case where the possibility of a proper statistically valid research is not possible - then one has to make to with the best possible closest option(s).