Markets do not move because economic data is good or bad. They move because it is better or worse than expected. A weak number that was forecast to be even weaker can lift shares, while a decent figure that misses lofty hopes can send them lower. Economic surprise indices turn that idea into a single running score.
What a surprise index measures
An economic surprise index tracks how actual economic releases compare with the consensus forecasts made by economists beforehand. When data comes in above expectations, the index rises. When it falls short, the index drops. A reading above zero means data has generally been beating forecasts recently. Below zero means it has been disappointing.
The best-known versions are the Citi Economic Surprise Indices, which cover the US, the eurozone, the UK, China and other economies. Other banks and data providers produce their own versions using similar methods.
How it is built
Each release is scored by how far it missed or beat the forecast, usually adjusted for how volatile that figure normally is. More market-moving releases, such as jobs or inflation data, tend to get more weight. Older surprises fade out over time, so the index reflects the past few months rather than the long term.
That last point matters. Surprise indices tend to swing back towards zero on their own, because economists adjust their forecasts. If data keeps beating estimates, forecasters raise their numbers, making future beats harder.
What it tells traders
The index is a useful gauge of momentum in the economic story. A rising US surprise index can support the dollar and bond yields, because stronger-than-expected data makes central bank rate cuts less likely. A falling index can weigh on yields and support expectations of easier policy.
Comparing countries can be revealing too. If US data is surprising positively while eurozone data disappoints, that gap can help explain why the dollar is strengthening against the euro. Our guide to how interest rate expectations move FX shows the link.
For shares, the picture is more mixed. Good economic surprises can lift earnings expectations, but if they also push up bond yields, the overall effect can be negative for highly valued growth stocks.
Surprise is not strength
The most common misreading is to treat a high surprise index as a sign the economy is booming. It only shows that data is beating forecasts. An economy can be slowing while the index rises, if economists had become too gloomy. Likewise, a strong economy can produce a negative reading if expectations ran ahead of reality.
That is why traders look at surprise indices alongside the actual level of activity, such as GDP growth, purchasing managers’ indices and the jobs market. Our guide to the US GDP advance estimate covers one of those headline measures.
Using it in practice
Some traders use extremes as a contrarian signal. When the index is very high, expectations are likely to catch up, making disappointments more probable. When it is very low, the bar for good news is lower. This works as a general tendency rather than a reliable timing tool.
It can also help with preparing for data days. If the index has been falling for weeks, markets may already be braced for weak numbers, and a decent figure could have an outsized positive effect.
The bottom line
An economic surprise index scores data against expectations, not against some absolute standard. It shows the direction of surprises and helps explain moves in yields, currencies and shares, but it naturally reverts as forecasters adjust. Read it as a measure of expectations, not of economic health. If you want to see how well you understand the gap between data and expectations, take the free Samuel & Co trader assessment.
