It's what leaders do rather than who they are that shifts votes

So Jim Murphy, one of the few on the unionist side who came out of the referendum looking better than he went into it, is in the running for leadership of the Scottish Labour party.

Aside from historically having more factions than members one of Scottish Labour's biggest problems since devolution and the creation of the Scottish Parliament has been that all of their 'big beasts' have tended to stand for election in Westminster rather than Holyrood leaving the SNP to mop the floor with a subs bench of Labour talent.

One of the more compelling arguments I heard from Yes supporters in the referendum campaign was that separation from the UK would prevent the brain drain of many of the brightest Scottish minds moving South (although the rest of the UK would still be stuck with the brain drain of everyone moving to London).

Jim Murphy's decision may have more to do with the fact that he backed the losing Miliband brother in the 2010 Labour leadership contest but the fact that a senior Labour figure wants to become First Minister of Scotland rather than be in the cabinet, the first since Donald Dewar made that decision in 1999, may reverse this effect a little.

There are polls that say otherwise though and that Labour would do as badly, if not worse, under Mr Murphy than they have done under Johann Lamont and Iain Gray before her.

I'm very sceptical of alternative leader polls and Anthony Wells has a good rundown of the faults of some Lib-Dem polling done after the European elections. These were the polls that were part of Lord Oakeshott's abortive coup against Nick Clegg that backfired so spectacularly.

Essentially the issue is that most alternative leaders aren't very well known to the public who can't know how they would actually react if that person took over. So many people still think David Miliband is leader of the Labour party that any polling on Ed vs. his brother is pointless. With Lord Oakeshott's polls very few people had heard of any of the contenders apart from Vince Cable. Political junkies know who Tim Farron is but ordinary voters don't unless they live in his constituency.

For some better known politicians (Boris Johnson springs to mind) voters might have an idea whether they'd be more likely to vote for that party and this may be the case with Jim Murphy but mostly it's still to be taken with a pinch of salt.

The real reason why we can't know the impact of a different party leader is that party leaders themselves change the context of polling by their actions. Your opinion of Ed Miliband comes from what they do, how they respond to the media and events, how people and commentators and other politicians react to that.

Changing party leader is a major change to the political narrative. Look at this chart, again from UK Polling Report. Even without the annotations you'd be able to tell when David Cameron replaced Michael Howard and when Gordon Brown replaced Tony Blair. Similarly, look at the Labour line when the financial crisis hits in 2008 and the bank bailouts began.

We overestimate what politicians and political strategy can do all the time but in this case, what they do matters more to voters than showing their name in a poll question.