For national opinion - look at the polls, not the local elections

Want to know what the local elections will tell you about the state of national opinion? Please don't wait up all night. I can tell you now.

The local elections will inform you that national opinion is roughly where the national opinion polls say that it is. Of course. If anyone suggests different tomorrow then you will know that they are misinterpreting the results.

How can I be so confident? It's simple.

Local election results are important in themselves, shifting power in many localities. But while they are touted as a proper test of national opinion they are in fact nothing of the sort.

The one advantage that local voting has over opinion polls is sample size. But a sample of 1,000 is plenty big enough. And opinion poll samples are:

• Properly balanced by area. Local elections only take place in some parts of the country and projecting the results to other areas inevitably involves the use of hunch
• Given a choice between all the parties. A major distortion in tonight's results is that many voters will not be choosing between full slates of candidates. In 40 per cent of seats there will be no Labour candidate. It is not easy to work out what voters in these seats really meant by their votes. Did they mean to vote Lib Dem, say? Or did they only do that because the party of their choice wasn't standing. Analysts may easily overestimate the Lib Dem showing.
• Being asked about the General Election. A straightforward point, this, but local votes are, to some extent at least, cast on local issues.
• A better reflection of who will and won't vote. One of the most difficult elements in opinion polling is adjusting for the likelihood that an individual will actually vote. But difficult though it is, the problem is vast when it comes to local elections. A combination of lack of interest and core vote protest leads to an uneven low turnout - in other words to a lower turnout among the voters of some parties than among others.

The analysts will try and unravel all of this, but why bother? Opinion polls have already done the unravelling. That's what they are there for.