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

You're running an A/B test and the subject line wins big. Then holiday season hits and the test spanned Thanksgiving week, Black Friday, and New Year's. Now you're wondering if that winner actually works on a regular Tuesday in March.

External events mess with A/B tests because they affect behavior unpredictably. Holiday shopping makes everything look good. Major news or weather events shift attention away from email. Seasonal promotions pump up engagement metrics. Even day-of-week variations can skew a short test if you happen to catch the busiest or slowest days.

And Here's the practical problem: you can't always avoid testing during unusual periods. Instead, run tests long enough to average out day-of-week swings (at least 2 full weeks is safer than a few days). Note any external events that happened during your test window. Compare your test results to your historical baseline to spot anomalies. If something looks weird, ask whether external factors caused the spike or your variant actually performed better.

Sample size matters too. Bigger samples are more resistant to random swings from external noise. When you've got enough addresses in each variant, a holiday bump affects them equally and doesn't distort the comparison. Next step: review your last three test results and flag any that overlapped with holidays, major sales events, or company announcements. For tests you're about to launch, check your calendar first. If you can't avoid a known event, just plan to run longer so the external factor doesn't dominate the data.