The results of votes are usually quoted excluding those that could have voted but didn't. A common justification of this is that if people don't vote then their views cannot be registered. Some go further and say that views of non-voters do not matter.
Whatever you think about that, excluding non-voters in calculating vote shares is mathematically equivalent to assuming they would vote in the same proportions as voters. For example, if 30% of the electorate voted for the blue party, and 20% voted for the red party then the headline result would be 60%/40% to the blues. But if we supposed that the 50% that didn't vote also split 30%/20% then we would still arrive at the 60%/40% result.
Excluding non-voters can be useful for comparisons, or else if you subscribe to the belief that non-voters do not matter. But in either case you are throwing away some valuable information. Indeed, turnout is mentioned at sub-headline level in media reports precisely because it gives an indication of the strength of the result, and sometimes even as a measure of democracy's health.
The tables below offer an alternative way of viewing recent referendums and elections by quoting various vote shares as a percentage of the whole electorate, i.e. the number of people registered to vote. The idea is that instead of just comparing winner and loser percentages, as is the norm, especially in a referendum, we can assess the strength of the result's mandate amongst the various votes.
The data is from the Electoral Commission. The 2016 EU referendum and 2015 General Election percentages are for the whole UK though breakdowns for Scotland and full details are in this spreadsheet.
The Winning votes table is topped by the No vote in the 2014 independence referendum with 46.7% of the electorate. The next strongest vote is 9.3 percentage points behind this, being for Leave in the EU referendum. In contrast, both the Scottish and the UK governments hold power on the basis of votes from just under a quarter of the electorate.
The Losing votes table is topped by the non-Conservative vote in the 2015 General Election at 41.7% (the total of all votes cast for parties other than the Conservatives). It's striking that this is about 17 percentage points higher than the vote share that gave the Conservatives their majority government under the First Past the Post system. The same is also true for the SNP minority government under Scotland's PR system though the gap is smaller at 6pp.
The Did not vote table is topped by the 44% of Scotland's electorate that didn't vote in the 2016 election. One way to visualise the figures in this table is to think of picking 6 people at random from the population. On average, 1 didn't vote in the 2014 referendum, 2 didn't vote in the 2016 referendum or 2015 General Election and 3 didn't vote in the 2016 Scottish Parliament election. (Strictly, you'd need to pick 6 people from the electorate of each vote.)
The last table shows the Top 5 largest vote shares from the other three tables. Even the top result of this table — the 2014 No vote — fell short of 50%. In other words, none of our recent referendums and elections has given a majority mandate on anything from the electorate. Given the high hopes following the high turnout in 2014, it's sobering that second place in this table is the 44% of the electorate that didn't vote in the 2016 Scottish Parliament election.
It is a fact that more people did not vote than voted for the parties of government in both Scotland and the UK. This in itself is telling us something of concern about our democracy. What we cannot tell from these numbers is the enthusiasm with which voters cast their votes. Anecdotal evidence suggests that a substantial number of people feel they are voting for the least bad choice, rather than one they believe to be good.
The longer term evidence points to declining voter turnout, and declining vote share going to the traditionally dominant parties. Not only that but similar changes in voting behaviour are becoming apparent across Europe, the US and elsewhere. It seems that representative, electoral democracy is being changed by globalisation and relationships are being strained between the media, traditional politicians and the people they wish to represent.