AS a non-SNP member, I don’t have a vote in the party’s depute leadership contest. Nor would I express an opinion on the relative merits of the three candidates. But for what it’s worth, as an outsider I’m happy to offer this piece of advice to all contenders.

Get two figures engraved on your consciousness, then consider their implications.

The first is 1.6 million. That’s the difference between the respective turnouts in the 2014 referendum and the 2016 Scottish Parliament.

The second is 600,000. That’s the difference between the Yes vote in 2014 and the SNP vote in 2016. It reflects the fact that more than a third of Yes voters did not vote for the SNP. For me, these figures spell out three lessons that need to be taken on board by both the SNP leadership and the party’s activist base.

The first may seem pretty obvious but it needs to be spelled out over and over again. The SNP is not the independence movement. It is by far the largest chunk, in terms of both activists and voters. But at least one-third of Yes voters – and possibility an even bigger proportion of potential Yes voters – have no strong affinity with the SNP.

For SNP activists that means recognising the vital importance of broad alliances. It means resisting the temptation to denounce anyone who criticises the Scottish Government or deviates from SNP policy as some kind of traitor to the Yes movement.

It means trying to rebuild the spirit of 2014, when organisations such as Labour for Independence and the Radical Independence Campaign were treated as valuable allies rather than enemies within.

I’ve been outside party politics for quite a few years for many reasons. And I find it tiresome when SNP activists tell me that if I want to express an opinion – for example on the timing of the referendum – I should either join the party or shut up.

Many of us worked long and hard to help get us to that 45 per cent. And a lot of us were not SNP members, then or now.

At a time when the Scottish Government is being pounded daily by the media after 11 years in power, we need more than just the SNP to keep the flame of independence burning brightly.

So, I would suggest that the new deputy leader of the SNP, whoever that may be, should spend time rebuilding alliances and discouraging political sectarianism.

The second reason these figures are important is because they throw up some fundamental questions about ideological orientation. Political scientists have long recognised the concept of differential turnout. In a normal party-political-based election, older and more affluent people turn out to vote in vastly greater numbers than those who are younger and poorer.

On Thursday the Tories won a council by-election in Highland Perthshire – an area with an exceptionally high proportion of well-to-do retired voters. What was more remarkable than the result was the 56 per cent turnout. Across the poorest parts of the West of Scotland, a 30 per cent turnout in a council election would be regarded as respectable.

The reason the Yes movement came from nowhere to shake the establishment to its foundations in 2014 was that 85 per cent turnout. People who had never voted in their lives turned out in colossal numbers to back independence. For the first time, they believed that their vote really could make a difference. The Yes movement offered hope and inspiration.

These are the people who will decide in the future whether Scotland surges forward to independence or sticks with the United Kingdom. But if we are to galvanise them, and drive even deeper into that territory, the independence movement needs to offer more than just a safe pair of hands.

The third reason why these figures are important is because they have a strong bearing on the timing of the next independence referendum. I am not desperately impatient – and I don’t buy into the idea that we must have a referendum before the UK leaves the European Union.

Let’s get real. We’re not going to have an independent Scotland before March 21, 2019. More realistic than racing to beat an impossible deadline is that we hold a referendum sometime before the next Scottish election in 2021. Some people have said to me that if we can’t win the next election then we can’t win a referendum. I disagree.

In fact, I’d put it the other way around. If we can’t win a referendum before 2021, then it’s unlikely we’ll win a pro-independence majority in the next Scottish Parliament.

Why not? Because the figures don’t stack up. The 2016 election was touch and go, and if anything, those 600,000 Yes voters who failed to vote SNP last time round will be even less motivated to turn out to the polling stations in 2021 if nothing much has changed before then.

Maybe Brexit will go catastrophically wrong and the Tories will be left high and dry. But the transitional period means that’s less likely to become apparent before the next Scottish Parliament election.

So back to that differential turnout. A low poll in 2021 will work against the SNP and other pro-independence parties, while a high turnout in an independence referendum in 2019 or 2020 will have the opposite effect.

I suspect the main reason why Ruth Davidson, Richard Leonard and Willie Rennie are so hostile to a referendum during this parliamentary term is because they fear defeat. They rate their chances of ousting the SNP from power at Holyrood higher than their chances of winning a second No vote.

And that’s exactly why we should, before the end of this year, set a date – and then focus on uniting the disparate independence movement into a respectful coalition of different strands, all focused on the single goal of taking political power into our own hands.