Comparing a metric with the corresponding period a year earlier to assess change while considering seasonal and calendar differences.
A year-over-year comparison measures a period against the corresponding period a year earlier. It is often more informative than comparing adjacent months for a seasonal business. However, matching the calendar period does not remove every seasonal influence or ensure the periods were otherwise comparable.
Check the actual dates, number of days and weekday mix. Leap years, moving holidays, promotions, weather and changes in opening hours can affect the comparison. A partial current month should not be compared with a full prior month without explaining the mismatch. The same event, channel, property and filtering definitions must also be used on both sides.
For a positive prior-year value, percentage change is calculated as (current value minus prior-year value) divided by prior-year value, multiplied by 100. A zero prior-year value makes that percentage undefined; show the absolute change instead. When comparing rates, also show their denominators and distinguish percentage-point movement from relative percentage change. Year-over-year growth describes a difference, not proof that an SEO campaign caused it.
Takeaway: Use corresponding periods as context, then check calendar and measurement differences before drawing conclusions.
Example
A supplier records 150 qualified organic inquiries this October versus 120 last October. The increase is 30 inquiries, or 25%. A trade exhibition held this year but not last year remains a possible explanation to investigate.
What to check
State both date ranges and annotate material changes. Consider a daily or business-day view where useful, while retaining the original totals so the adjustment stays transparent.
For more detail, see Change and compare date ranges.