Conclusions

ONS follows international best practice in the methodology it uses to seasonally adjust GDP data. This combines a standardised software-based approach with expert review and judgement. ONS has told us about ongoing improvements to further support its approach.

ONS has increased transparency about its approach by publishing details of its assessment of seasonality in quarterly GDP. This assessment found that there is no statistically significant residual seasonality in quarterly (and monthly) GDP statistics, including recently. There are also several one-off events that might have impacted the timing of economic activity over the period.

Analysis of external Purchasing Managers’ Index (PMI) data from S&P Global provides corroborative evidence of stronger economic growth in the first half of the year, particularly in the first quarter, over the period since 2022. While differences in methodology and coverage mean that PMIs would not be expected to align closely with official GDP estimates, the broadly similar seasonal pattern supports ONS’s conclusions. However, it reinforces the case for continued monitoring and further investigation of the drivers of recent patterns in GDP growth.

While ONS reviews key series and routinely tests for residual seasonality in headline GDP, capacity constraints have limited the consistent and timely application of expert review and judgement to component series.

As emerging seasonal patterns can take several years to detect using standard methods, there remains a risk that early signs of change may not yet be visible in statistical tests.

It is therefore important for producers, users, and commentators to keep an open mind about the possibility of newly evolving trends. To strengthen confidence and manage this risk, ONS should continue increasing transparency around its methods and uncertainties, rebuild and stabilise the specialist team responsible for seasonal adjustment, and seek external assurance, particularly on the detection of emerging seasonal signals. Doing so will ensure that its approach remains robust, timely and benefits from challenge.