Arriving as the new Office for National Statistics permanent secretary in August, Darren Tierney says two things struck him.
The first was the sheer amount of data and statistics the organisation publishes: “I think it's [around] 750 different outputs that we produce and, as a consumer of ONS products, I thought I knew quite a lot about what they produced, but just the range of outputs was quite surprising to me.”
The second was the “inspiring” level of expertise: “I always think the phrase ‘world class’ is overused and been abused in Whitehall – I think we overclaim frankly quite a lot on how world class we are. But in the ONS there are some genuine world class experts on statistical methodologies; on for example the production of national accounts; on productivity – where genuinely the world looks to the ONS for leadership on some of those methods and statistical production.”
Tierney was appointed to the role after the Cabinet Office launched a rapid recruitment campaign in July only open to applicants at perm sec or DG level. The campaign followed Sir Robert Devereux’s damning review on failings at the ONS, published in June, which included a recommendation to split the role of national statistician and perm sec.
Devereux called for the government to appoint someone to the perm sec role with “a track record of leading an operational business” and of “turning round such an organisation”.
Tierney, who moved to the ONS from his role as director general, propriety and ethics at the Cabinet Office – a role made famous by one Sue Gray – says he thinks his experiences at the former Department for International Trade and the Ministry of Justice leading on “big transformation programmes and helping to move organisations from state A to state B” got him the job.
“That's what I'm really good at,” he says.
“I have no background in statistics, other than being an avid consumer of ONS statistics as a policy wonk over the years,” he adds with a smile. “But given the way in which Robert Devereux suggested the split should happen – so you have the expertise in the national statistician and then someone who can run the [ONS] as the permanent secretary – I thought I had something to offer an organisation that needed some help.”
At DIT, he was a DG responsible for export and investment services in the UK and overseas, leading two-and-a-half thousand people. In that role, he redesigned the system after ministers asked for a change of focus. Before joining DIT, he spent nine years at the MoJ. In his last role there, as interim DG, he was responsible for large scale prison reform policy and implementation.
Tierney says he’s “really enjoyed” his first few months in the job and describes the ONS as “an amazing institution with a really rich history – and a really proud future I’m sure.
“The here and now needs a little bit of attention,” he adds, with classic Whitehall understatement given the year the stats department has just had. “So that's what we're spending our time on.”
Public Administration and Constitutional Affairs chair Simon Hoare this summer described the ONS as having “gone to hell in a handcart” under the leadership of Sir Ian Diamond, who was its national statistician and perm sec for five years until May this year. Diamond, meanwhile, told MPs that he hadn’t “slept a lot for a long time” over the criticism he has faced.
One of the things Tierney has been working on is improving the ONS’s culture. Devereux’s review found “deep-seated” cultural issues including a reluctance at senior levels to hear and act on difficult news.
Devereux “was right”, Tierney says. “I think that culture was definitely very apparent in the organisation and, when I talked to quite a few stakeholders before I started, that sense of a slightly closed ONS was quite real. So we've deliberately gone about trying to change that in a few ways, which I think people have started noticing.”
Tierney says this has included being “uber transparent with what's going on in the ONS”. He points to recent blogs written by James Benford, who joined the ONS from the Bank of England in August as its new DG for economic statistics, which explain recent errors and clarifications.
“We've been totally open about why something has happened and what we're going to do to get on top of it,” Tierney says.
He says the ONS has also “brought the outside in” by putting the Bank of England, HM Treasury, Office for Budget Responsibility, Cabinet Office and other stakeholders on the oversight group for the ONS’s economic statistics plan, “which is a way of us getting maximum transparency with people on what's going on”.
Tierney says these changes “will really help shift that culture of defensiveness that I think did exist in the organisation”.
He has also driven the creation of a new mission statement to give clarity on what the ONS is “for” as an organisation, and a new leadership statement which is “about how we hold our senior leadership to account for how they behave in the ONS”.
Tierney says the ONS will tie its feedback and reward and recognition systems to that leadership statement “so that we're really clear that we want people to not just sign up to words on a page, but to actually behave differently in the ONS. And I think we're starting to see some of that shift happening already”.
Another shift Tierney is driving at the ONS is greater prioritisation of the work it focuses on. Earlier this month, the ONS set an aim of cutting the number of outputs it produces annually by 10% in 2026.
“One of the most consistent bits of feedback I had when I arrived was: ‘The organisation is just doing too much’,” Tierney says.
The prioritisation effort began with looking for resources that could be “repurposed” to work on the ONS’s recovery plans to urgently improve the quality of its key economic outputs and the surveys that underpin them.
Officials doing data analysis work on the integrated data service and a team working on cross-cutting analysis were moved over to work on the recovery plans, which Tierney says has “freed up quite a lot of money and some headcount".
“We needed 150 posts put into the economic statistics plan,” he says. About 100 of those are expected to be filled by the end of the year, and the ONS has plans to fill the remaining 50 early next year.
The department then looked at outputs, with the aim of “decompressing” the organisation to “improve the bandwidth” of senior leaders and the “small and precious corporate resource we have on things like methodologists, digital teams, and some of our publication teams”.
First it carried out a survey with its users in other government departments and then ranked the outputs based on the responses.
“That's a bit of a blunt tool because it doesn't take into account everything you'd want to take into account,” he says. “So we then did a bit of work internally to look at: What do our users think? What do our stakeholders think? How important are these outputs to our economic statistics? And indeed, quite a lot of them that you wouldn't necessarily think of do feed directly into the national accounts.”
The ONS is now consulting on these plans. For some of the outputs, the consultation will last a few weeks, while others will take longer, Tierney says, as some “will take a bit more time to work through. And they're not all set in stone,” he adds. “These are all ideas that we think are worth considering. And then we'll listen to people's feedback and work out what’s the best thing to do for us and for our users.”
Legacy IT is another issue Tierney is keen to get on top of in the coming years.
“Every department, every large organisation, has lots of legacy IT. Some of ours are quite old. And what I'm keen to put in place is a plan for how we address each of those on a systematic basis over the next three to five years.”
The ONS relies on other departments for much of its data, and recently had to make corrections to two of its outputs due errors in statistics provided by HM Revenue and Customs.
What role does Tierney see the ONS playing in helping other departments to feed in accurate data? And whose responsibility is it to ensure there is accurate data across government?
Tierney says this is “a really tricky issue".
“Obviously we have a role in making sure that our statistics are as trustworthy as we can make them, but we rely on other government departments when they export data to us to have made sure that that data is accurate, and because they're the subject matter experts, we can't do all the quality assurance on their behalf,” he says.
“The thing that I'm heartened by is that it genuinely seems to be a collective endeavor across government and it will be an ongoing thing. All the departments, including HMRC, see this as a joint thing for us to fix with them. We're not just pointing fingers at each other. There's also a role here for the national statistician in making sure that the quality and the processes across government are up to scratch.”
Another issue Tierney has picked up is the ongoing industrial action by unions over changes to the ONS’s hybrid working policy. He says he is “really keen to reset the relationship” with the PCS and Prospect unions and “the tone of the discussions with them has been really warm.
“I’m confident we’ll reach a sensible conclusion with them, hopefully quite soon, which will help us break the deadlock. At the end of the day, what I said to them – and I really mean – is that we come at these things from different angles, but we want the same thing. We all want staff from the ONS to be well treated, and to want to work in the ONS and give of their best”.
Returning to the issue of stats quality, does he have a target of what success will look like and how he’ll measure that?
“I don't think we will ever declare success because production of stats is an ongoing thing and there will be ongoing errors and needing to clarify things,” he says. “That's just a normal part of the statistical production process.”
But he says there are “a couple of totemic things that we need to get right. One is getting on top of the quality in some of the economic outputs,” he says. “I think once we have landed the reforms that we're making to the Labour Force Survey and the Transformed Labour Force Survey, that'll be an important moment.”
Tierney says there is already evidence of improvement. Producer prices statistics were recently reinstated, having been suspended because of quality issues, and are back with “better quality than it was before”. And he says the ONS’s field force team, which carries out the surveys, is now back at pre-pandemic levels of strength, with 1,000 staff on the team, while LFS response rates are also back to the pre-pandemic rate.
“So there are a whole load of things that we're doing that I think send the right signal and which are real measures of success,” he says. “But there will be lots of those as we go through the plans in the coming couple of years. So it'll be all of those things combined that will equal success.”
As our time draws to close, CSW asks if there is anything else he wants to add.
“The recovery is underway,” Tierney says. “No one is complacent here. We've a long way to go. But we've definitely started with purpose and with some focus and that's what we'll continue doing. I think 2025 was a really difficult year for ONS for all the reasons that were set out in the Devereux report. But I think we're ending it in better shape and I'm confident 2026 will be a year of recovery.”