Statistical Significance

The probability that observed differences between variants are real rather than random chance.

1 min readLast updated Apr 2026

Reviewed by Golden Digital·Operator-reviewed ecommerce reference

Quick Reference

CategoryConversion Rate Optimization

Related Terms2

The probability that observed differences between variants are real rather than random chance.

Why It Matters

Without statistical significance, you're making decisions based on noise. A variant might appear 5% better, but if that difference isn't significant, it could easily flip to 5% worse with more data. Significance tells you when to trust results.

Practical Example

Scenario

A pet food brand's test shows Variant B converting at 4.2% vs Variant A at 3.8%. Looks like a winner.

Calculation

But with only 500 conversions, the confidence level is 78%—well below the 95% threshold

Result

They continue the test. After 2,000 conversions, the difference shrinks to 4.0% vs 3.9% with 62% confidence. The apparent winner was just random variation.

Pro Tips

1Always aim for 95% confidence minimum (99% for major changes)
2Use a sample size calculator before starting to know how long to run the test
3Don't peek at results and stop early when one variant looks ahead
4Consider both statistical and practical significance—a 0.1% lift may not be worth implementing

Common Mistakes to Avoid

Declaring winners based on early results before reaching significance

Using 90% confidence or lower, which means 1-in-10 chance results are random

Ignoring confidence intervals—a 'winner' with wide intervals may still lose

Frequently asked questions

95% is the industry standard for most tests. Use 99% for major changes (site redesigns, pricing changes) where being wrong is costly. Never go below 90%.

Early leads often reverse as more data comes in. This is 'peeking bias.' Random variation early in a test can create false leads that disappear with proper sample sizes.

Put it to work

Related Terms

A/B Test

A controlled experiment comparing two versions to determine which performs better.

Sample Size

The number of visitors or conversions needed to reach statistical significance.

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