Dear Rob,

Thank you for your letter of 17th March outlining the findings and recommendations of your recent ‘Review of ONS’s Treatment of Seasonality in quarterly GDP’ and for your team’s engagement with ours throughout the review period.

We welcome the review’s conclusion that ‘ONS has followed internationally recognised best practice in its approach to seasonal adjustment and current statistical tests show no evidence of significant residual seasonality in quarterly or monthly GDP’. We also recognise the need to give this topic continued focus and are appreciative of OSR’s suggestions for how we can further strengthen our approach. In particular, the review provided ONS with three high level recommendations, each of which we support and, as you note in your letter, we are already progressing important activity against each of them.

In relation to the first recommendation, we have commissioned work that will focus on the detection and assessment of residual seasonality that may develop. This will also include external expert input and reflect the experiences of other countries. We intend to publish this technical report in May 2026.

The second recommendation relates to a further increase of transparency on this complex topic area. We will build on our recent approach of publishing our annual residual seasonality assessment of the aggregate GDP estimates. As part of our regular quality assurance we already test for residual seasonality before publication, and where needed, make updates to our seasonal adjustment parameters before we publish. Our seasonal adjustment factors change dynamically as part of our production of economic data. Separately, we have recently announced in our recent GDP release, that for future releases of monthly and quarterly GDP releases we will, in response to user requests, regularly publish our non-seasonally adjusted data as a new dataset, significantly enhancing our transparency.

In the context of the third recommendation which relates to resourcing and planning on this topic, we have already focused on increasing the capability and capacity of the expert and dedicated Time Series Analysis branch in recent periods, alongside learning from external expertise as well as drawing on in-house expertise from across the organisation. The branch has strengthened their approach in response to increased user demand and has been making steady progress in a programme of continuous improvement to ensure quality of the service and bring sustainability, including improving guidance and documentation, reviewing risk, and automating manual processes where possible.

We will provide regular updates on progress against the recommendations to our users and yourselves as part of our quarterly reporting alongside our wider delivery plans for economic statistics.

Yours Sincerely,

Sarah Henry, Director of Methodology

Liz McKeown, Director of Economic Statistics, Production and Analysis