Is David Herdson right about Labour and the Lib Dems?

Over on Political Betting, David Herdson has several times made comments such as this:

Since the start of March, there have been sixty national opinion polls published and they have shown a remarkable consistency in the total share identified for Labour plus Lib Dems. When one party rises, the other tends to fall.

I’ve been intrigued by this because David certainly knows his stuff, but that’s not been my impression of the data – which has been that changes in Lib Dem support are more equally balanced between Labour and Tory than the much more lopsided position David suggests.

So I’ve done some number crunching, looking at this question from two angles:

a) How do the changes in party support between polls from the same pollster compare?

I’ve looked at the changes in vote share between consecutive polls from the same pollster for all such pairs of polls since the start of March.

There is a correlation of -0.46 between the Labour and Liberal Democrat changes (i.e. for a 1 point increase in Labour’s share there is typically a 0.46 fall in the Lib Dem share and vice vera).

However, there is also a very similar correlation between the Conservative and Liberal Democrat changes: -0.33. That’s slightly less than the Labour / Lib Dem correlation, but not by much.

Both of these correlations are though higher than that between Conservative and Labour, which comes in at -0.16, suggesting that it is votes flowing to and from the Liberal Democrats which is the most important factor.

b) How do the actual levels of support in the polls compare?

A different way of making the comparison is to look at overall levels of support rather than changes in it. However, this gives a similar picture with the highest correlation being between Labour and Liberal Democrat (-0.27), ahead of Lib Dem/Conservative (-0.14) and with Labour/Conservative the least correlated (-0.04).

So the verdict? It’s true that the Liberal Democrat share of the vote is more closely related to the Labour share than the Conservative share, but the different is not that great. There’s more to what is going on than simply shifts to and from Labour.

Update: as Mike Smithson pointed out, I wrongly attributed the PB story to him initially. Apologies Mike.