Young people and the Lib Dems
The same point keeps coming up in comments - if there is some great surge of young voters backing the Liberal Democrats, would it be picked up in the polls?
The short answer is that it should be. Pollsters in the UK do not weight their samples to match the demographic profile of voters, they weight them to match the demographic profiles of UK adults, so most of those young people who wouldn't normally have voted should have been represented in the samples anyway. Only Harris specifically ask respondents if they are registered to vote, so they would not have been filtered out of other companies' samples.
Another point I've seen raised is how well pollsters cope with young people who are predominantly online or have only mobile phones. Firstly, online really isn't a problem, since we have plenty of online pollsters. Secondly, all polls weight by age, so this cannot result in an under-representation of young people. The problem would be if mobile-only young people were significantly different to young people with land lines. If landline penetration continues to fall I suspect that this will eventually be a problem that phone pollsters need to tackle, though there is always the option of including mobile phones in samples.
Perhaps the most common question I've seen is whether a change would upset pollsters "assumptions". Pollsters do not assume particular groups are more or less likely to vote. Instead the majority of pollsters factor in likelihood to vote by asking people how likely they are to vote, and then weighting or filtering appropriately. The correct proportion of young people are already represented in pollsters samples, so if those people told pollsters they had become more likely to vote, it would be picked up.
The bottom line on a lot of questions of whether pollsters would pick up a new trend is that pollsters don't actually make many presumptions up front about how people will behave. With a few minor and well evidenced exceptions (such as Populus and ICM's reallocation of don't knows on the assumption they are likely to vote how they did last time), voting intention figures are based on what people tell pollsters, not the pollsters' preconceived assumptions.
Things can go always go wrong of course, and I expect it's more likely to happen at an election where there has been a large shift in support, but I can't see any particular reason to expect the polls to get it wrong this time.
