Of the many museum facts I've read recently, my favourite is that £1 in every £1,000 in the UK economy can be directly related to museum and galleries. I have no idea if that is high or low, let alone good or bad, but I find it strangely compelling.

This information appears in a new report, Museums and Galleries in Britain: Economic, Social and Creative Impacts by Tony Travers of the London School of Economics (LSE).

It was commissioned by the Museums, Libraries and Archives Council (MLA) and the National Museum Directors' Conference (NMDC) and published as a contribution to the government's Comprehensive Spending Review that concludes later this year.

The report contains some fascinating information; however it is skewed by spin. Too often, research and evaluation into museums is weakened by the pressure to give a positive message to funders. And he who pays the piper calls the tune - researchers commissioned by the cultural sector can perhaps sometimes be too willing to dance to the sector's music.

At the heart of this report are newly researched statistics on the finances and performance of 22 large museum organisations: eight of the nine lead hubs and the larger nationals. The most striking thing is the difference in scale between large regional museums and national museums.

For example, the biggest English regional service appears to be Tyne and Wear, which has 219 full-time staff. This is considerably more than, say, Sheffield with just 71, but small compared with the Imperial War Museum with 556 or, largest of all, the British Museum with 947.

One or two notable regionals are omitted, such as museums in Glasgow and Manchester, but their inclusion would not alter this national/regional contrast significantly. Between them, the large regionals in the report spent a total of £58m in 2005/06, less than the Victoria & Albert Museum alone (£67m) and this even after the introduction of Renaissance funding.

Few regional museums raise more than £150,000 a year in donations and sponsorship, whereas several nationals regularly exceed £5m a year. There's a similar picture with trading income. In 2005/06 the total for regionals was about £2.2m; nationals generated more than £100m.

The report shows that there are limits to self-generated income for all UK museums, particularly those with free admission. Profits from trading will never make more than a 'marginal contribution to core income'. Donations and sponsorship are volatile and are not going to generate more than 10 to 15 per cent of museum income, even in the capital cities; elsewhere the potential is far less.

The report also begins to construct a plausible argument that expenditure by museums is falling behind growth in the UK economy, and this threatens the ability of museums to meet the rising expectations of audiences.

In addition, museums are expected to be business-like, but are not allowed to borrow money for investment. The overall message is convincing: museums will always be reliant on public funding for the majority of their income.

There are many more interesting facts in this report, including a fair bit on visitor numbers, some international comparisons and information about the volume of loans and publications. In addition to the statistics, the report tries to present the 'wider benefits' of museums and galleries, particularly 'social and creative impacts'.

This is much less successful, as it is attempted through a combination of good news case studies of one-off education and inclusion projects and the simple assertion of museums' value to creativity. This material is quite different to the hard statistics at the core of the report and perhaps comes less from the LSE economists than the report's MLA- and NMDC-appointed steering group.

The report says museums are popular and efficient (but government does not appear to reward improved productivity) and there's less from the lottery, tight controls on local-authority capital spending and limited capacity for self-generated income. In places the report is gloomy: if the government doesn't come up with enough cash, museums will suffer.

It's a shame that there isn't more genuinely independent, warts-and-all research, evaluation and analysis, which would help effective decision-making by museum managers and policy makers.

Instead, Travers's report gives a taste of what to expect as the next bit of 'evidence' of museum impact. It notes: 'The capacity of cultural institutions to provide people with opportunities to develop in ways that enhance their happiness - a key government concern - is not yet researched.'

I can see the report now. Commissioned from a leading academic by a consortium of bodies funded by the culture department, it will show that museums and galleries do boost individual and national happiness and are doing so with greater efficiency than ever. However, they could create far more happiness if only they had just a little more government funding.

Maurice Davies is the deputy director of the Museums Association