Seasonally adjusted: what it means

Figures that have had regular calendar patterns removed, such as Christmas shopping or summer jobs, so that underlying movements between months or quarters can be seen.

A seasonally adjusted series has had its regular, predictable calendar effects estimated and removed. Retail sales jump every December and some jobs rise every summer; adjustment strips out these repeated patterns so that a month can be compared fairly with the month before.

Worked example

Imagine hypothetical sales of 100 in November and 130 in December. If December is normally 30% above November, the seasonally adjusted figures would show no change. If December came in at 120, the raw data show a rise but the adjusted data show a fall, because sales were weaker than the usual December boost.

How it trips up claims

• Comparing one raw (non-seasonally adjusted, often marked NSA) month with the previous month can show "rises" or "falls" that are just the season.
• Seasonal factors are re-estimated as new data arrive, so adjusted figures for past months can be revised.
• For data that are not adjusted, compare with the same month a year earlier (year-on-year).

The ONS explains its methods and labels each time series as SA or NSA in the series title.

Also searched as: SA, NSA, seasonal adjustment, what does seasonally adjusted mean, not seasonally adjusted

Official definitions: ONS: Seasonal adjustment · ONS: A guide to labour market statistics

Key figures where it matters

• UK employment rate
• UK GDP growth (quarterly)
• UK unemployment rate
• Average weekly hours worked in the UK
• UK consumer credit growth
• UK current account balance
• UK household saving ratio
• UK job vacancies
• UK monthly GDP growth
• Mortgage approvals for house purchase
• Net additional homes in England
• NHS employment (UK)
• UK redundancy rate
• Retail sales volume growth (Great Britain)
• UK trade balance (goods and services)

Questions that use this term

• What is the UK unemployment rate?
• Is the UK in recession?
• How many people claim benefits in the UK?

Related terms

• Year-on-year change: The change between a figure for one period and the same period twelve months earlier, such as March this year compared with March last year.
• Quarter-on-quarter change: The change from one three-month period to the next, such as April to June compared with January to March. Used for GDP growth.
• Revisions: Changes to previously published figures, made when more complete data, improved methods or updated seasonal factors become available. A normal part of official statistics.
• Rolling average: An average recalculated each period over a fixed window, such as the latest three months or twelve months, to smooth out short-term ups and downs.

More measuring change over time terms: Real terms · Cash terms · Percentage point · Year-on-year change · Quarter-on-quarter change · Rolling average · Index number · Base year · Base effect · Compound annual growth rate · Discontinuity (break in series).