‘Zero Hedge’ is in a flap about poll ‘oversampling’ here http://www.zerohedge.com/news/2016-10-23/new-podesta-email-exposes-dem-playbook-rigging-polls-through-oversamples
It even includes a hack email from John Podesta which discusses ways of ensuring that the Democrats own polling over samples minority groups. Again, gasp!
Except. Well over sampling a smaller demographic group is the right thing to do. When I say ‘right’, I don’t mean for opinion polls but for collecting statistics on a population in general.
Say you have a representative sample of a population consisting of a thousand people. Now, of that thousand people you are particularly interested in a sub group that represents 1% of the population. If your sample is exactly proportionate, then it should have 10 people belonging to that sub group. Unfortunately 10 is a shitty sample, if you are unlucky to get 2 odd people with unusual views they then form 20% of your sub-sample.
Sample size is a dark art but the easiest issue to understand is it that magnitude matters. A good sample size is less about the proportion of the whole population in your sample and more about the raw number of people. More is better, but ‘more’ is subject to diminishing returns.
Over sampling means you can get a better picture of the sub group. However, because you end up with more of group X than you should have, their response are then weighted proportionally when looked at the statistics overall.
Are polls manipulated! Well, if by ‘manipulated’ you mean ‘use statistics’ then yes.