IT’S sometimes said that Westminster general elections are just 650 entirely separate constituency battles (or 59 in Scotland) that happen to simultaneously take place on the same day. With Holyrood elections it’s not quite as simple, because in addition to the 73 constituency seats there are also 8 electoral regions, each returning a further 7 MSPs. And, crucially, the constituency results directly affect the distribution of regional seats, with parties being compensated with regional MSPs if they fail to win as many constituency seats as their share of the vote should entitle them to.

The key to whether the SNP win an overall majority in May’s election, or indeed whether the pro-independence parties in combination win a majority, does not therefore necessarily lie in closely-fought constituency contests. In some cases, a constituency seat will spectacularly change hands, causing no end of excitement on the TV results programme, but will make absolutely no difference to the final result. The losing party will simply be awarded an extra list seat to make up for it, and the winning party will be given one list seat fewer.

On other occasions, though, a constituency gain or hold will be just as crucial as it would be in a Westminster election. For example, if the SNP hadn’t won Cunninghame North in 2007 by just 48 votes, they wouldn’t have replaced Labour as the largest single party in the Scottish Parliament. Alex Salmond might never have become First Minister and we might never have had an independence referendum. By contrast, Ruth Davidson’s gain in Edinburgh Central, and Iain Gray’s hold in East Lothian, were decisive in preventing the SNP from winning an overall majority in 2016.

It’s not always possible to tell in advance whether the outcome in a particular constituency will be important or a sideshow – although sometimes there are enough pointers to make an educated guess. Between now and polling day, I’ll be profiling all 73 constituencies, identifying the ones that are most likely to change hands, and attempting to separate out the noisy irrelevances from the battles that could actually determine whether Scotland becomes an independent country.

And on Sundays I’ll be profiling the eight electoral regions where the 56 list seats will be decided – and getting to grips with the much-misunderstood formula that determines how that happens.