Richard Exell draws attention to figures from the US's BLS, which show that since the early 90s, manufacturing productivity growth in the UK has far outstripped real wage growth. This strikes me as odd. It poses the question: if UK manufacturers have been increasing the rate of exploitation, how come their rate of profit is so low? ONS data show that manufacturers' net return on capital was 4.7% in 2012, way down from 15.8% in 1997, when the ONS's present records began.

One reason is that the capital-output ratio has increased. Another is that manufacturing prices have not risen as much as consumer prices; since 1996 (when ONS records began) producer prices have risen 31.6% whilst the CPI has risen 43.4%. This has driven a wedge between the real wage as experienced by manufacturing employers, and that as experienced by workers.

In fact, there is a different set of data which paints a very different story from the BLS, shown in my chart.

This shows labour productivity for the whole economy - defined as GDP per employee* - and real wages**, as deflated by the GDP deflator at basic prices; this is the real wage as experienced by employers, rather than employees. The data starts in 1992 because that's when ONS data on employees begins.

This shows a cyclical pattern in the rate of exploitation. Productivity rose faster than real wages in the early 90s, more slowly in the late 90s, faster in the early 00s but has since fallen. The rate of exploitation is about the same now as in the early 90s. This is consistent with the trends in wage and profit shares during this time.

This is a very different picture from the BLS's numbers, not least because productivity in the whole economy has grown more slowly than it has in manufacturing - by 37.5% between 1992 and 2011 in my data against 78.7% in the BLS's.

This poses the question: if the rate of exploitation hasn't increased much lately, why are wages squeezed?

Two reasons. One is that productivity has fallen since late 2007, so the pie is smaller for both capital and labour. Another is that, thanks in part to rising VAT, the prices experienced by workers have risen faster than the GDP deflator - by 17.8 per cent between 2007Q4 and 2012Q4 against 9.3% for the deflator.

I do not say all this to deny that workers are exploited. They are; anyone who thinks you have to believe in the labour theory of value in order to claim this should be hit repeatedly with a copy of Roemer's Free to Lose. But we should distinguish between this fact about the general nature of capitalism and the cyclical problem of depressed real wages, which has little to do with an increased degree of exploitation.

* ONS code identifiers CGCE divided by MGRN

** This is defined as total employees compensation divided by the number of employees: identifiers DTWM and MGRN, divided by CGBV