The Mirror, the i and lots of local papers are today all running stories about their "big Brexit survey" which purports to show a majority of people want to remain in the single market. (1/...)
It claims relevance due to the unusual large sample size - 200,000+. As you'll hopefully know, a big sample size alone does not make a poll accurate or meaningful. What determines whether a poll is worth paying attention to or not is whether it is *representative* of the public
The classic illustration of this is the Literary Digest poll - a magazine that used to do polls of literally millions of Americans at Presidential elections, and got it catastrophically wrong in 1936 when George Gallup's representative poll got it right.
That said, you don't need to go back to 1936 to get a good example, someone demonstrates it most days by doing twitter polls with "RT for bigger sample" at the bottom. Encouraging more of your mates who share your views to answer does not make a poll more accurate.
Anyway, back to the i/Mirror "survey". As far as I can tell, this was conducted by putting up questions on the websites of the national and local newspapers they own
People were prevented from voting more than once, but that only addresses one source of potential bias in a sample. You're still only going to get the sort of people who view newspaper websites, which will have its own skews.
A proper poll needs to collect demographics like age, gender, class, education (and for subjects like this, things like past vote & EU ref vote), allowing the sample to be weighted to ensure the correct proportions, to make it representative of the British public.
Polls from members of the BPC need to report all these things (if you look at the tables on pollsters websites, all those demographics will be listed - across the tops of the tables, or down at the end of the surveys).
Most people don't look at that stuff of course, but enough people do to check these things and hold pollsters to account.
Without that data, without being able to see if the sample is representative, or heavily skewed by demographics or attitude, a survey shouldn't be taken as a meaningful measure of what the wider public think.
We know, for example, that young people are more pro-EU than older people, that graduates are more pro-EU than non-graduates. If samples have the wrong balance of age groups or education, they will be misleading.
We know how the country actually voted in 2016, so if samples have significantly too many or too few people who voted for Brexit back in 2016 they will be misleading.
These are the things that determine if a poll means anything. Not whether it is "big".
