Ignoring external factors (e.g., holidays)?

External factors can confound A/B test results by affecting variants differently:

Holiday effects: Recipient behavior changes dramatically around holidays. Tests spanning these periods mix normal and abnormal behavior.

News and events: Major news, weather events, or industry announcements can shift attention and behavior unpredictably.

Seasonality: Time of year affects engagement patterns. Tests comparing different seasonal periods are not comparable.

Day-of-week variation: Short tests may be skewed by which days they include.

Mitigation strategies:

Avoid testing during known unusual periods. Run tests long enough to average out day-of-week effects. Note external events that occurred during tests. Compare to historical baselines to detect anomalies.

Context matters. A test result is only valid for the conditions under which it was run.