The Sinn Féin and SDLP political pathologists have some interesting data to dissect as a matter of urgency to try to determine why the nationalist vote was significantly down.

Normally, Northern Ireland election post mortems focus on why apathetic unionists failed to turn out and why nationalists were so keen to get to the polling stations.

But not this time.

The DUP won 29.2 per cent of the vote compared to 24 per cent for Sinn Féin. Where DUP leader Arlene Foster will be happier is that she held her 38 seats, and her party just dropped its vote by 0.8 per cent, whereas Deputy First Minister Martin McGuinness saw Sinn Féin lose a seat and its vote drop by 2.9 per cent.

The UUP took 12.6 per cent of the vote, the SDLP 12 per cent and Alliance 7 per cent. The UUP vote was down 0.7 per cent, the SDLP down 2.2 per cent and Alliance down 0.7 per cent.

Overall, the Sinn Féin and SDLP vote was down by 5.1 per cent.

In this election, 703,744 people voted in the 18 six-seater constituencies. The turnout was 54.91 per cent, which was down slightly down on the 2011 Assembly turnout of 55.64 per cent when 674,103 people voted.

Some of the drop can be explained by the 8,299 first preferences People Before Profit candidate, now MLA, Gerry Carroll won in West Belfast – much of which would have come from the former Sinn Féin share of the West Belfast republican vote and a smaller portion from the SDLP share.

But that’s only a minor part of the explanation.

The electorate in 2011 was 1,210,009. This time it increased to 1,281,595.

This time, with a higher electorate, Sinn Féin won 166,785 compared to the 178,224 in took in 2011.

This time, the SDLP won 83,364, whereas five years ago it took 94,286 votes.

And, in 2011, Sinn Féin and the SDLP won 272,510 votes between them, while this time they won 250,149 – a drop of 22,361 votes.

Compare this with the fact that the DUP increased its total from 198,436 votes in 2011 to 202,567 votes in this election.

Part of the reason for the strong DUP showing was the fact that unionists warmed to the personality of Arlene Foster and also that many seemed to have been persuaded by the argument that a failure to vote for the DUP could result in Mr McGuinness, rather than her, as First Minister.

It is more difficult to explain why more nationalists stayed away this time.

Part of it may be to do with a perception among some nationalists that Sinn Féin, in endorsing the Fresh Start Agreement, signed up to British government welfare cuts. It could also have to do with a more general disenchantment with the stop-start-stalled Stormont politics of recent years. It could be younger people of a Catholic background not being so caught up with Orange-Green politics.

Whatever the reason, Sinn Féin and SDLP strategists must turn their minds to this issue or possibly face further decline.

The calibre of candidates also may be a factor.

For instance, consider West Tyrone, where the SDLP should have been under great pressure due to a rift within the party. Two SDLP councillors – Dr Josephine Deehan and Patsy Kelly – split from the party to run against the chosen candidate Daniel McCrossan. Dr Deehan and Mr Kelly polled a respectable 2,139 votes between them, yet Mr McCrossan was easily elected with 4,287 first preferences.

His success was down to the fact that 27-year Mr McCrossan, an honours law graduate from Liverpool John Moores University, was an able and articulate candidate who impressed the voters of West Tyrone.