Analysis
Period over period
Also called WoW, MoM.
Period over period is this period compared with the previous period of the same length. Week over week and month over month are common versions.
How it is measured
Pick the length, then compute the change as the new value minus the old, divided by the old. The window must be complete for both sides, so exclude a partial current week.
State the dates for each side. A week-over-week gain in the first week of the month may be a billing-cycle shape rather than growth.
Worked example
A job board has 18,400 applications this week against 17,150 last week, up 7.3 percent. This week included a public holiday on Monday that reduced the base of last week's comparison weekend.
A month later, a plain month-over-month comparison of April (30 days) to March (31 days) shows a 3 percent fall that is mostly one fewer day. Using daily averages turns it into a 0.4 percent rise.
How it differs
Period over period looks at the nearest previous window. Year over year looks at the same window last year to avoid seasonal shape. The first shows momentum; the second removes calendar effects.
Common errors
Using a partial current period. Ignoring different day counts. Treating a seasonal rise as growth. Quoting a percentage from a tiny base. Reporting the change without the two raw values.
In practice
Show both values and the dates beside every percentage. Use daily averages when months differ in length. Check a matched period when a holiday intrudes.