What does "Seasonality" mean?

Definition of Seasonality in the context of A/B testing (online controlled experiments).

What is Seasonality?

Seasonality is a generic term describing any kind of cyclical fluctuation in time-series data. It could be daily, weekly, monthly, seasonal or yearly.

A typical example of seasonality are the peaks in demand for vacations and holiday trips during summer. Any kind of goods and services that are only sought after around particular dates such as ones specific to a certain holiday are also example of seasonality. The weekly decrease in traffic to b2b websites during the weekend is an example of weekly seasonality.

Seasonality and time-related factors in general are a major treat to generalizability. Establishing the seasonality of the website a conversion rate optimization expert is working on is an important first step in determining how best to acquire a representative sample that will ensure good predictive value of any A/B test performed on the website, app or other software. A simple way to account for seasonality is to make sure the test duration covers one full seasonal cycle. More advanced methods wherein the result will be adjusted based on observed correlation with seasonality can be applied with varying degrees of success.

Like this glossary entry? For an in-depth and comprehensive reading on A/B testing stats, check out the book "Statistical Methods in Online A/B Testing" by the author of this glossary, Georgi Georgiev.

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