Analysis
Year over year
Also called YoY.
Year over year is this period compared with the same period a year earlier. It removes most seasonal shape from the comparison.
How it is measured
Align the dates so the weekdays match, as far as you can. Compute the change as new minus old, divided by old. A moving holiday or leap day can shift the match and needs a note.
Check that the base was normal. If last year's window had an outage or a viral spike, the comparison inherits that. Show both raw values and the date range for each.
Worked example
A camping-gear store compares October to last October and sees orders up 31 percent: 4,720 against 3,600. October of last year was affected by a two-week stock shortage on tents.
Against October two years ago, which had normal stock, growth is 12 percent. The store reports both numbers and explains the shortage, so the board does not plan hiring on a flattering base.
How it differs
Year over year compares to the same time last year. Period over period compares to the window just before. The first controls for the season; the second shows momentum.
Common errors
Using a distorted base year. Ignoring a site migration that changed tracking. Comparing a growing business's partial year. Forgetting leap years or moving holidays. Quoting the percent without the raw numbers.
In practice
For any seasonal business, use year over year as the main comparison. Check the base period for anomalies and note any in the report.