An year-on-year (YoY) year-on-year comparison is made between the results for a period and the same period in the previous year. This means that it is not usually distorted by an seasonal effects.

Using year on year comparisons is not an infallible way of eliminating seasonal effects because some seasonal effects are not firmly fixed to calendar dates. For example Easter has an effect on retail sales, but Easter days changes from year to year. Some companies provide estimates of the impact of changes in the date of Easter, but this, like all such non-standard estimates, needs to be regarded with a little scepticism.

Changes in the weather are even worse: no fixed dates, unpredictable, and not purely seasonal.

If the structure of a company's businesses has changed (e.g. though acquisitions), then it may be preferable to do year on year comparisons on pro-forma and organic numbers.