Analysis
Comparable period
Also called matched period.
Comparable period is a comparison window picked to match the other one in weekday mix and calendar events. The match is what makes the difference readable.
How it is measured
Count the weekdays, holidays, promotions, and school breaks in each window. Two windows of the same length but with a different number of Mondays are not equal if Monday is your best day.
Document the match: the dates, the length, and the known differences. If a match is not possible, use a longer window or adjust, and say that you did.
Worked example
A ski-rental shop wants to judge January. The prior four weeks included Christmas week and New Year's, which are unusual. Comparing January against December shows a fall of 38 percent.
Matching against the same four weeks in the previous January, which also had one holiday week, shows a fall of 6 percent. The shop reads that as a mild weather effect instead of a collapse.
How it differs
A comparable period is chosen by calendar shape. Year over year is one way to find one, but it only works if the dates line up. The period-over-period default picks the last window of the same length without checking shape.
Common errors
Defaulting to 'previous period' without looking. Ignoring a holiday that moves each year. Comparing a window with five weekends to one with four. Not noting a campaign. Using a window too short to smooth.
In practice
Before sharing a comparison, list the weekdays and special dates in both windows. If they differ, redo the comparison on a matched pair. Put the dates beside the percentage.