Moody's decision to cut the government's credit rating because of "continuing weakness in the UK's medium-term growth outlook" will, no doubt, focus attention upon the need to raise our trend growth rate.

Doing so, however, might be very tricky.

To see my point, start from the fact that the economy shrank slightly last year. Does this mean our economy is in trend decline? Probably not. More likely, 2012 was just an unusually bad year. But even if we take many years together, it's quite possible that we'll over-sample bad years (or good) and so get a biased measure of trend growth. To reflect this problem, we should measure the standard error of growth. In its simplest form, we do so simply by dividing the standard deviation of growth rates in our sample by the square root of the number of years*.

My chart does this for each 20-year period since 1831; I'm using Bank of England data, updated by the ONS. The two lines show trend growth plus and minus one standard error. Roughly speaking, we can be two-thirds confident that trend growth in any 20-year period was somewhere between these two lines.

Which brings me to the point. Once we allow for this standard error, trend growth doesn't change much. In fact, except for a period to the mid-30s (which covered the post-war slump and Great Depression), it's quite possible that trend growth has never moved much from around 2%.

In other words, whether we have free markets, post-war social democracy or post-Thatcherite neoliberalism, trend growth might be much the same. Neither Victorian virtues of thrift and hard work (and hypocrisy) nor "dependency culture" much affected growth.

I'm not doing anything odd or original here. I'm just making the same point in time-series form as John Landon-Lane and Peter Robertson did in cross-country form when they concluded:

There are few, if any, feasible policies available that have a significant effect on long run growth rates.

It is possible that this is due to luck. Maybe good microeconomic policies have been swamped by bad macroeconomic times and vice versa. Perhaps the interventionist policies of the post-war era were bad for growth, but this was offset by an unusually favourable macro climate, whilst Thatcherite deregulation might have raised growth were it not that bad macro policy gave us two recessions.

Nevertheless, the data suggest that we should be very sceptical of the chances of improving our trend growth rate. Of course, many of the policies recommended by the LSE's Growth Commission - such as better education and infrastructure - are good ideas.But history suggests they are unlikely to have a noticeable effect upon trend growth.