Year-over-Year Comparison

Year-over-year (YoY) comparison looks at how a measure changed relative to the same period of the previous year. A common percent-change calculation is (current period value minus prior-year comparable value) divided by prior-year value, multiplied by 100. The periods must refer to genuinely comparable time windows, and a zero prior-year baseline needs special treatment rather than a normal percentage.

A home-cleaning company might compare bookings for September this year with September last year instead of simply comparing September with August. That can reduce confusion from predictable seasonal demand. Yet YoY changes can still reflect changes in advertising budgets, price increases, tracking tools, service territory, or one-off weather events. A 20% rise in revenue does not necessarily mean a 20% rise in customers or profitability.

Analysts should record metric definitions and source systems, check whether both time periods have complete data, and include the raw values next to the percent change. A rolling twelve-month window and a single calendar-month comparison are related but not identical. For a newly launched business without a valid previous-year baseline, a month-over-month or cohort comparison may be more useful. This metric is not a customer attribution model or a general service-page comparison, and dashboards should make that difference obvious.