How can someone be a “don’t know” and simultaneously a “Labour or LD voter”?
They categorise people based on their last vote.
Traditionally between elections the rate of 'don't know' is much higher amongst women than men.
This is significant in polling: most polls automatically discount don't knows.
Pollsters try and weight accordingly for sex, but this is a tricky art. It's only in the run up to a GE that more serious attention is paid to the don't knows and pinning down exactly how they will vote (we aren't at that stage yet BTW)
Thus much of the art of trying to work out accurate polling is working out what the don't knows will do.
This is really important to understand when you read polls. Generally speaking women are more likely to lean left, so polls with high don't know rates run the risk of overstating the right wing vote.
Given how things stand, this is a particularly important factor right now.
It's to be expected that women are going to be particularly targeted at the next GE.
This is both to attract their vote and to try and demoralise women from voting at all. (this is also why the trans issue is a really big issue - to attract voters and to depress voters).
I've not checked in a while but the Don't Know Rate amongst women had been running at around 15% to 20% of all women since the last GE. For men it was around the 10% mark.
It's something I have not thought to have a look at in a while, so I might take a look this week if I get a chance to see if there is something going on with the figures.
I also note that one criticism of a recent Lord Ashcrofts poll was how it wasn't following polling basics in weighting, and that men were significantly over represented which would naturally give a much more right wing distortion than the population as a whole.
Just something to keep in mind when you see polling.