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Data Revisions: Why Last Month's Number Might Change

Economic figures are estimates. The first release is based on incomplete information, and agencies revise it as more data arrives. The revisions can be large enough to change the story.

Employment reports show this clearly. The US payrolls figure for a month is revised in the following two months. A headline jobs gain of 200,000 can later be revised to 150,000 or 250,000. The market often reacts to both the new number and the revision to the previous ones, and sometimes the revision matters more.

GDP undergoes multiple revisions, from advance to second and third estimates, and then annual revisions that can alter figures years back.

Why it matters to you: the number you see on a calendar next to the label "previous" may not be the number originally reported. Comparisons need care.

It also reveals something about reliability. Series with large revisions are noisy, and a single data point should not drive a thesis. Look at trends over several months, and at multiple indicators pointing in the same direction.

Revisions can also move price after the release. A strong headline with downward revisions to previous months may produce a reversal.

Experienced traders read the full report: headline, revisions, and details, and only then form a view of the surprise.

Practical step: for the next payrolls report, write down the headline, the revisions to prior months, and the net effect on the total.

I've seen payrolls beat forecast and the dollar still drop within minutes, purely because the previous two months got revised down. If you only read the headline you'd think the market made no sense.

Easy habit: before the release, write down last month's originally reported number next to the current "previous" figure on your calendar. If they're different, you already know a revision has happened.

The biggest one of the year is easy to miss because it isn't a monthly release: the annual benchmark revision to US payrolls. The BLS publishes a preliminary estimate in late summer and folds the final version into the January report in early February. It can shift the level of employment by several hundred thousand jobs at once, and the market has started treating the preliminary estimate as a real event.

Worth knowing the date when it comes round, because it isn't always flagged as high-impact on retail calendars, yet it changes the backdrop for every payrolls number that follows.