Change over time is the most-read and least-reliable number in job posting data. Here is how to tell a real movement from an artefact.
Four false alarms
- A new source. Adding a large employer or a board to an index looks exactly like a hiring surge. Any provider that changes coverage without saying so will produce spurious momentum.
- Seasonality. January and September are structurally busy; late December is structurally dead. Comparing to the previous month rather than the same month last year turns the calendar into a finding.
- A single large employer. One retailer opening seasonal roles can move a national count for a category by double digits.
- Republishing. A mass refresh of old listings creates a wave of new postings without a single new role.
Three checks that take a minute
- Look at the age distribution. A genuine surge is made of young listings. A refresh artefact is made of listings whose content is not new.
- Look at the employer concentration. If the top employer accounts for most of the change, it is one company's decision, not a market.
- Compare year on year. It removes seasonality at the cost of sensitivity, which is the right trade for most questions.
What movement is worth acting on
Sustained, broad and young: a change that holds for more than one period, is spread across employers, and shows up in fresh listings rather than recycled ones. Everything else is worth watching and not worth a decision.
We show the thirty-day change alongside the age distribution and the employer concentration on every cohort page, so these three checks are on the same screen rather than requiring three queries.