There are two major issues, when weighting. Is the sample frame a random sample, or , were certain groups over sampled, in order to get enough respondents in each group, so that they can be compared?
If you take a random sample, people who refuse to be interviewed can skew the results. This is only a problem, if the people who refuse are significantly different from the people who agreed to be surveyed.
For example, if young people refuse at a higher rate than older people, the younger respondents need to be weighted up to compensate. If younger and older people refuse at the same rate, you don't need to weight the results.
These decisions are generally based on known demographic data. It's not a perfect system, but it does a pretty good job. The key is to reduce the refusal rate, by putting your best interviewers on 'refusal conversions', etc., and get the refusal rates down.
If you over sample certain groups, when looking at the sample universe in total, the groups need to be weighted back to the population. If you randomly survey a population that would result in a very small sample size of Group A and a large sample size of Group B, you have to over sample Group A, in order to get a large enough sample of Group A to be able to compare it to Group B. When looking at the total, you weight Group A down and Group A up.
If these latest polls appear to be over sampling Republicans, there could be an innocent explanation.
Say that you take a random sample. You then ask party affiliation and how likely the respondent is to vote. If you are surveying likely voters, respondents not likely to vote are not eligible.
More Republicans than Democrats could get into this survey, based on how likely these groups are to vote. I wouldn't find this odd, during and immediately after the Republican convention.
That being said, the people that pay for the poll have control over everything. So, keep that in mind, when looking at the results. Pollsters are independent in the same way that 'independent auditors' that audit the public companies that pay for the audit are independent. Arther Anderson comes to mind.
Polls work. People would stop paying me to conduct them, if they didn't work.
An over sampling of one party over the other, in a poll of likely voters, doesn't raise a red flag for me. A poll paid for by Time or Newsweek is a bigger concern to me.
I watch the reactions of the politicians. Kerry's recent gyrations lead me to believe the President Bush got a pretty big bounce out of the convention. And, that's a good thing.
But there has to be some control number which would point to "over sampling". Is the control number based on previous sampling, polls asking party affiliation?
Bottom line: IF there were a shift in partisan allegiance, how would the pollster allow for it in his "control number"?
Zogby's internet polls are just infuriating. The polling industry really needs to get a handle on these bogus polls.