ONS reports that ONS actually quite good at estimating GDP

Estimates rarely change that much, but have got worse since the crash.

A new study by the ONS (pdf) on the practice of revising quarterly GDP reveals that, despite some high-profile revisions, they are actually doing pretty well.

The current practice for GDP figures is to release a preliminary estimate 25 days after the quarter ends, then update it for a second estimate a month later, and release a final estimate shortly before the next quarter ends – before starting the whole process again. Even after the "final" estimate is made, there's the possibility of still more releases:

As further data become available there are potential revisions to the quarterly GDP figures in subsequent QNA releases, as well as in the annual national accounts Blue Book publication.

The Blue Book process enables annual data to be balanced at a much more detailed level and is also the opportunity for major methodological changes to be introduced.

Whenever the preliminary estimate delivers surprising news – as it did with the most recent results – there is always a rush to point out that these estimates are usually revised (as indeed they are). But the overall picture remains remarkably accurate. The following chart compares the GDP growth given in the initial estimate to the final estimate given five years later.

 

There are some pretty large changes month-to-month, but only one revision in the last 20 years which changed the big picture in any substantial way (the near-collapse in growth in early 1998 actually appears not to have occurred).

Even when the whole period is covered, the authors conclude:

In broad terms the picture of growth in GDP over the period from 1961 to 2012 quarter two is similar, irrespective of the maturity of data, although there are some exceptions. In particular the differences caused by revisions in the late 1980s were previously discussed in Brown et al, where a period of consistent upward revisions led to the Pickford Review (1989). The review implemented a number of methodological changes including the publication of a single measure of GDP.

But what about the most recent period of recession and recovery?Has the ONS got worse at making those initial estimates due to the changed economic circumstances?

Maybe.

The below chart shows the absolute revisions to GDP estimates in the first two years. The Pickford review of 1989, which changed the methodology substantially, clearly worked, reducing the mean revision from a change of at least 0.5 points to one of barely 0.2 points. But while there was a period of unprecedented accuracy in the late 1990s and early 2000s, things have got marginally worse recently – even while staying significantly better than they had been two decades before.

 

As a result, the review concludes that:

There is some evidence that in the latest periods, the size of revision has increased [although not significantly]. . .

It is possible that the assumptions and methods underpinning the early estimates of GDP may not be as robust in periods of greater volatility or at turning points in the economy as they were during the long period of stability from 1992 to 2007.

The most important thing to note, however, is that there is no significant trend for the direction of the revision. Although it's usually positive, it hasn't been recently:

 

So if you are hoping that a GDP estimate will be revised, be careful what you wish for – it may not go the way you hope.

GDP percentage growth, quarter on same quarter 1 year ago - estimates.

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

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The government must demand that Iran release Nazanin Zaghari-Ratcliffe

Iran's imprisonment of my constituent breaches the Convention on the Rights of the Child.

I grew up with a very paranoid mother. She had tragically lost members of her family as a teenager and, as a result, she is extremely fearful when it came to her children. I used to laugh at her growing up – I indulged it but often scoffed at her constant need to hear from us.

A few days ago, I was in Parliament as normal. My husband, his parents and our baby daughter were all in Parliament. This rare occasion had come about due to my mother in law’s birthday – I thought it would be a treat for her to lunch in the Mother of Parliaments!

The division bells rang half way through our meal and I left them to vote, grabbing my phone of the table. “See you in ten minutes!” I told them. I didn’t see them for more than five hours.

The minute the doors bolted and the Deputy Speaker announced that we were indefinitely being kept safe in the chamber, all I could think about was my daughter. In my heart of hearts, I knew she was safe. She was surrounded by people who loved her and would protect her even more ferociously than I ever could.

But try explaining that to a paranoid mother. Those five hours felt like an eternity. In my head, I imagined she was crying for me and that I couldn’t be there for her while the building we were in was under attack. In reality, I later found out she had been happily singing Twinkle Twinkle little star and showing off her latest crawl.

That sense of helplessness and desperate impatience is hard to describe. I counted down the minutes until I could see her, as my imagination ran away with me. In those 5 hours, I started thinking more and more about my constituent Nazanin Zaghari-Ratcliffe.

Here I was, temporarily locked in the Parliamentary chamber, surrounded by friends and colleagues and door keepers who were doing all they could to keep me safe. I knew I was going to be let out eventually and that I would be reunited with my daughter and husband within hours.

Nazanin has been detained in the notorious Evin prison in Iran for nearly a year. She only gets an occasional supervised visit with her two-year-old daughter Gabriella. She’s missed Christmas with Gabriella, she missed Gabriella’s second birthday and no doubt she will be missing Mother’s Day with Gabriella.

But it’s not just the big occasions, it’s the everyday developments when Gabriella learns a new song, discovers a new story, makes a new friend. Those are the important milestones that my mother never missed with me and the ones I want to make sure I don’t miss with my daughter.

Unfortunately, Nazanin is just one of many examples to choose from. Globally there are more than half a million women in prison serving a sentence following conviction, or are awaiting trial. Many of these women are mothers who have been separated from their children for years.

In 2010, the United Nations General Assembly unanimously adopted the Bangkok Rules - the first international instrument to explicitly address the different needs that female prisoners have. It was also the first instrument to outline safeguards for the children of imprisoned mothers.

The Bangkok Rules apply to all women prisoners throughout all stages of the criminal justice system, including before sentencing and after release. However, Nazanin’s case has seen a wilful flouting of the rules at each and every stage.

Rule 23 states that ‘Disciplinary sanctions for women prisoners shall not include a prohibition of family contact, especially with children’. Tell that to her daughter, Gabriella, who has barely seen her mother for the best part of a year.

Rule 26 adds that women prisoners’ contact with their families shall be facilitated by all reasonable means, especially for those detained in prisons located far from their homes. Tell that to her husband, Richard, who in almost a year has only spoken to his wife via a few calls monitored by the Iranian Revolutionary Guard.

Iran has ratified the Convention on the Rights of the Child and supported the Bangkok Rules, yet it is breaching both with its treatment of Nazanin. It is therefore incumbent upon our government to take the formal step of calling for Nazanin's release - it is staggering they have not yet done so.

As I pass the window displays in shops for Mother’s Day, most of the cards have messages centred around ‘making your mother happy’. If there’s one mother I’d like to make happy this year, it’s Nazanin Zaghari-Ratcliffe.

Tulip Siddiq is Labour MP for Hampstead and Kilburn