If population statistics are wrong, sooner or later almost everything for which councils plan will be wrong too. At one level this is a financial problem. Most central funding is calculated on a per capita basis, so if the figures underestimate the size of population councils will have too little cash to meet demand for services.
But inaccurate information is also a major issue in plan-making and development control, because errors in raw data affect subsequent calculations. Identifying land that has the potential for five years' worth of housing development becomes more difficult if a district's population projections are out by several thousand people.
Councils have two main bones of contention with the Office for National Statistics (ONS). The first goes back to the 2001 census, when several councils were surprised to discover that tens of thousands of people had "disappeared". This resulted in a drop in per capita government funding just when authorities were actually struggling to cope with rising population numbers.
Migration impact calculation issues raised
The second concerns the ONS's measurement of international migration. There is evidence that its figures seriously underestimate the real numbers moving in, notably from the newer EU member states. This issue affects large cities, to which arrivals have traditionally headed, and small rural districts that have no established patterns of migration where people have gone to seek agricultural work.
So far, these issues have been low on most planners' radars, but they are likely to assume greater importance. "Statistics are important in the context of government money. But in terms of projections for housing requirements, I am not sure that errors have much effect," says Planning Officers Society vice-president Phil Kirby.
But Westminster City Council is concerned enough to have convened a meeting this summer attended by delegates from several London boroughs and from Slough, Manchester, Cardiff and Peterborough. These worries have even prompted a thaw in the frosty relations between London mayor Ken Livingstone and the boroughs.
The London Councils federation complains that this month's comprehensive spending review is based on ONS data that indicate a drop of 95,000 in the capital's population. "The sudden reduction flies in the face of all the evidence on the ground," it comments. "It only came about because the ONS decided to change the way in which it calculates international migration. This will result in a 140,000 reduction in London's population projection for 2010 - equivalent to the entire population of Kingston-upon-Thames going missing."
According to London Councils' figures, Brent's population is underestimated by 2,000 and Enfield's by 3,300. Newham says it has 750 more children in its schools than the ONS calculates are resident in the borough. At the other end of the economic scale, the figures omit those who live in London part-time, such as owners of second homes or office employees who use city flats during the working week. The authorities fear a double whammy of catering for a rising population with funding based on the assumption of a falling population.
They are challenging the ONS definition of a migrant as someone who leaves their usual country of residence to reside in another for at least a year. This misses out those who arrive in London, spend a few months there and then move elsewhere in the UK to work. Critics also claim that the ONS methodology is flawed because it only counts people who arrive at main entry points such as Heathrow, Gatwick and Manchester Airports, whereas the majority arrive at smaller airports used by budget carriers.
Westminster published figures last month claiming that at any one time there are 13,000 illegal migrants within its boundary and 11,000 short-term migrants who are not registered in official statistics. It describes the ONS methodology as "not fit for purpose" and says it will be "short-changed" by up to £18 million a year in government funding.
"The government needs to deal with this problem or public support for international migration will evaporate, with serious consequences for community cohesion in London," says London Councils chairman Merrick Cockell. Westminster deputy leader Colin Barrow adds: "There are many benefits in hard-working migrants living and working in our city. But the inability of official statistics to keep track means that a rising number of councils are not receiving the government funding that they need."
Statistics put housing targets at risk
Westminster disputes its official population count of 240,000. Officials believe that the number could grow to 319,000 by 2019. The council's target for 6,800 new homes in all tenures over the next decade could clearly leave a huge gap. "We have huge pressures and a very long waiting list," says housing director Rosemary Westlake. "There is pressure on us to create more homes, but it is difficult to keep up. Not knowing how many people we have in need of housing makes planning extremely difficult."
The council has commissioned research on four migrant groups - Arabs, Poles, Australians and Chinese. "We found living arrangements unknown to the council," explains corporate projects and statistics officer Damian Highwood. "We found a lot of subdivision of private rented property and even illegal subletting of council property. We do not know how many people live in what appear to be commercial premises but turn out to be in residential occupation."
Although Westminster is perhaps an extreme example of a council that is grappling with "non-existent" people on its patch, it is not alone among urban authorities. Slough Borough Council has been running a campaign over its population statistics ever since the 2001 census was published. This showed a 14 per cent increase in the town's population to 120,600, making it the 13th fastest-growing council area.
But subsequent ONS mid-term estimates indicate that Slough has lost 4,000 people since 2001, a decline only topped by three other districts in the country. The council says the official figures are contradicted by more than 9,000 applications for national insurance numbers made in the town in 2005-06, only 150 of which were from British nationals. The rest had arrived from abroad.
The council points to the Audit Commission's Crossing Borders report on responding to the challenge of economic migration. The auditors accept that "the lack of accurate numbers makes it harder for agencies to predict and plan change and to develop business cases for extra funding or redeployment of existing resources". That statement could assume more significance if it turns out that the figures are frequently wrong.
CASE FOR THE DEFENCE
The Office of National Statistics (ONS) insists that it does not underestimate numbers of migrants and uses accurate sampling methods among those who change their country of usual residence for at least a year.
Short-term migration is harder to estimate, the ONS acknowledges, although it hopes to produce national estimates soon. It denies that it fails to cover arrivals at international travel hubs, pointing out that it has officials on duty at 16 airports, the Channel Tunnel and 12 ports for 17 hours a day.
It refutes charges that it ignores data such as national insurance numbers, housing pressures and schools numbers. But it says these sources may not be reliable. National insurance, for example, does not register when someone leaves the UK.
The ONS intends to produce a report by the end of the year on the results of four case studies with local authorities on the potential for using alternative sources and methods of calculating population data.
THE CENSUS PROBLEM
The ten-yearly census and the mid-year estimates compiled by the ONS to fill the gaps in between surveys provide the population counts on which the government bases all kinds of data, from grant distribution to housing requirements.
It is a legal obligation to return census forms but many people do not. The census uses enumerators to call on those who have failed to comply to help them fill in forms where needed. Areas with significant transitory populations or large numbers of non-English speakers suffer most acutely from unreturned forms.
In 2004, a scrutiny report commissioned by Southend-on-Sea Borough Council found that the town's population was 170,000 rather than the figure of 158,000 identified by the ONS. The council says the ONS used contractors rather than directly employed temporary staff and they were less zealous in ensuring maximum form completion.
The council's tax returns indicated that there were 1,034 more households than the 70,987 revealed in the census. In four of its 17 electoral wards more voters returned registration forms than the census suggested were present, while the number of people registered with doctors exceeded the census total by 7,000. Talks between the council and ONS reduced some of the discrepancies.
The 2011 census falls at the end of the period covered by this month's comprehensive spending review. Chancellor Alistair Darling is offering one per cent real growth in local government spending. Every pound will count in a bout of belt-tightening.
The census will also fall at a critical time as the government's drive to get an additional three million homes off the ground picks up speed. With those sorts of pressures on finance and land, there is sure to be increased vigilance over preparations for the 2011 count.