ONS decides to continue with inaccurate RPI calculation

Statistics agency chooses consistency over accuracy.

Against expectations and the recommendations of a number of influential economists, as well as its own consumer prices advisory committee, the ONS has decided not to change how the RPI – one of the two key indices measuring the rate of price inflation – is calculated.

The pressure to examine the index comes from the longstanding difference between the RPI and CPI measures of inflation. Except for the brief period in the midst of the recession, when both indices were recording deflation, RPI has consistently shown levels of inflation higher than CPI. Here's the chart for the last three years, for instance:


Some of that difference is due to the fact that the two indices measure subtly different things – RPI includes a broader measure of housing costs, for instance, and it ignores very high and low income households. But the ONS has known for a while that there is also a discrepancy caused by the different formulae used to calculate them.

The ONS began a consultation into whether and how it should eliminate this "formula effect", and has concluded that:

Use of the arithmetic formulation (known as the ‘Carli’ index formula) in the RPI is the primary source of the formula effect difference between the RPI and the CPI… This formulation does not meet current international standards.

So what's it going to do about it? Well, nothing:

The National Statistician also noted that there is significant value to users in maintaining the continuity of the existing RPI’s long time series without major change, so that it may continue to be used for long-term indexation and for index-linked gilts and bonds in accordance with user expectations.

Therefore, while the arithmetic formulation would not be chosen were ONS constructing a new price index, the National Statistician recommended that the formulae used at the elementary aggregate level in the RPI should remain unchanged.

The ONS will, however, develop a new measure of inflation, called RPIJ, which will use a different, better, mathematical formula.

The consumer prices advisory committee, a body which meets around five times a year to advise the ONS on measures of inflation, accepted that the ONS has a responsibility to make sure that there is a level of continuity in the RPI calculations which would be destroyed if there were a change to the formula. But, given the ONS also has a responsibility to compile those statistics "in line with best practice", CPAC concluded that not changing RPI would be "unsuitable".

On the other side, arguing for no change, were 332 of the 406 replies to the public consultation. The ONS said:

The large majority of responses did not address methodological issues but identified the impact that the changes implied… would have for them.

The competing requirements present a tricky path for the ONS to follow, but it does feel like it has picked the wrong option at this point. The job of the statistics agency is surely to produce accurate statistics, rather than statistics which are continually inaccurate in known ways. The fact that RPI is used to decide the value of, amongst other things, index-linked bonds is a reason for it to be correct, not for it to be artificially inflated.

As it stands, the ONS has decided to continue publishing a "measure" of inflation which has an accepted and understood upward bias of 1 per cent a year. It has done this, not because of any real statistical reasoning, but because greater accuracy would be bad for a majority of stakeholders. That seems like a bizarre abdication of its duty.

Photograph: Getty Images

Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter.

Photo: Getty Images
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Britain's shrinking democracy

10 million people - more than voted for Labour in May - will be excluded from the new electoral roll.

Despite all the warnings the government is determined to press ahead with its decision to close the existing electoral roll on December 1. This red letter day in British politics is no cause for celebration. As the Smith Institute’s latest report on the switch to the new system of voter registration shows, we are about to dramatically shrink our democracy.  As many as 10 million people are likely to vanish from the electoral register for ever – equal to 20 per cent of the total electorate and greater than Labour’s entire vote in the 2015 general election. 

Anyone who has not transferred over to the new individual electoral registration system by next Tuesday will be “dropped off” the register. The independent Electoral Commission, mindful of how the loss of voters will play out in forthcoming elections, say they need at least another year to ensure the new accuracy and completeness of the registers.

Nearly half a million voters (mostly the young and those in private rented homes) will disappear from the London register. According to a recent HeraldScotland survey around 100,000 residents in Glasgow may also be left off the new system. The picture is likely to be much the same in other cities, especially in places where there’s greater mobility and concentrations of students.

These depleted registers across the UK will impact more on marginal Labour seats, especially  where turnout is already low. Conversely, they will benefit Tories in future local, Euro and general elections. As the Smith Institute report observers, Conservative voters tend to be older, home owners and less transient – and therefore more likely to appear on the electoral register.

The government continues to ignore the prospect of skewed election results owing to an incomplete electoral registers. The attitude of some Tory MPs hardly helping. For example, Eleanor Laing MP (the former shadow minister for justice) told the BBC that “if a young person cannot organize the filling in of a form that registers them to vote, they don’t deserve the right to vote”.  Leaving aside such glib remarks, what we do know is the new registers will tend to favour MPs whose support is found in more affluent rural and semi-rural areas which have stable populations.  

Even more worrying, the forthcoming changes to MPs constituencies (under the Boundary Review) will be based on the new electoral register. The new parliamentary constituencies will be based not on the voting population, but on an inaccurate and incomplete register. As Institute’s report argues, these changes are likely to unjustly benefit UKIP and the Conservative party.

That’s not to say that the voter registration system doesn’t need reforming.  It clearly does. Indeed, every evidence-based analysis of electoral registers over the last 20 years shows that both accuracy and completeness are declining – the two features of any electoral register that make it credible or not. But, the job must be done properly.  Casually leaving 10m voters off the electoral resister hardly suggests every effort has been made.

The legitimacy of our democratic system rests on ensuring that everyone can exercise their right to vote. This is a task which shouldn’t brook complacency or compromise.  We should be aiming for maximum voter registration, not settling for a system where one in five drop off the register.