The Greens and the Prime Minister have been contradicting each other this week over whether the manufacturing sector is in crisis.

One of the central areas of disagreement is whether the number of jobs in the sector has increased or declined.

Prime Minister John Key says the numbers are increasing slightly and the Greens say they have dropped by 40,000 in the past four years.

They can't both be right, can they? Yes, actually.

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It depends on what survey they are using and what they use as their starting point.

Mr Key uses the Household Labour Force survey undertaken by Statistics New Zealand, the official measure of employment and unemployment. The figures, for the second quarter of 2012 (Q2) ending in June, showed that 246,500 were employed in the manufacturing sector.

Mr Key said in a speech on Monday that the numbers in the manufacturing sector were between 245,000-255,000 and that "it is slightly increasing".

Mr Key was correct if his point of comparison was two years previously, Q2 2010. Then, the number was 239,500.

So since then the numbers employed in the sector have grown by 7000. But if the starting point was four years back, he would be wrong. The numbers employed in the sector then were 266,500, making the more recent tally 20,000 fewer jobs.

The Greens are correct when they say there are 40,000 fewer jobs in manufacturing than four years ago, or 38,600 more precisely, under the Quarterly Employment Survey, also conducted by Statistics New Zealand.

But if their starting point was the year before, the decline was 1600.

Mr Key said that in the past 12 months the sector's output as measured by GDP grew by 2.5 per cent - and 8 per cent over the past three years.

Greens co-leader Russel Norman says manufacturing in GDP terms has fallen by 9.1 per cent over four years.

Again they are both right. It's just that Dr Norman uses mid 2008 as his starting point, just before the global financial crisis.

The one statistic Mr Key used that was wrong was when he said that exports from the manufacturing sector comprised between 11.7 per cent and 11.9 per cent of GDP in the past four years.

His office said he meant to say that manufacturing exports totalled $11.9 billion in the year to June 2008 and $11.7 billion in the year to June 2012.