Definition
Multivariate testing (MVT) is a statistical method that tests multiple email elements at the same time to determine the best-performing combination. Unlike A/B testing (which tests one variable), multivariate testing examines how different variables interact with each other.
For example, an A/B test might compare subject line A vs subject line B. A multivariate test might compare subject line A with hero image X, subject line A with hero image Y, subject line B with hero image X, and subject line B with hero image Y — all four combinations simultaneously.
A/B vs Multivariate Testing
| Factor | A/B Testing | Multivariate Testing |
|---|---|---|
| Variables tested | 1 per test | 2+ simultaneously |
| Combinations | 2 | N variables × M variations |
| Sample size needed | Smaller | Significantly larger |
| Statistical complexity | Simple | Complex (interaction effects) |
| Time to result | Faster | Slower |
| Insight depth | Which version wins | Which elements drive which outcomes |
| Best for | Quick optimisation | Deep optimisation with large lists |
When to Use Multivariate Testing
- You have a large, active subscriber list (10,000+ engaged subscribers per cell)
- You have tested individual elements and now want to optimise combinations
- You suspect interaction effects between elements (subject line affects button response)
- You are optimising a high-value campaign that will be reused
- You have the statistical expertise to analyse interaction effects
Common Variables to Test
| Variable | Example Variations |
|---|---|
| Subject line | Question vs statement, personalization vs generic |
| Preheader text | Summary vs CTA, short vs long |
| Hero image | Product vs lifestyle, static vs animated |
| Body copy | Long-form vs short, benefit vs feature |
| CTA button | Colour, size, text, placement |
| Offer | Percentage off vs fixed amount vs free shipping |
| Social proof | Testimonial vs review count vs trust badges |
Implementation Steps
- Identify the page goal (click, conversion)
- Select 2-3 variables to test
- Create variations for each variable
- Determine required sample size (use a power analysis calculator)
- Set up the test in your ESP or testing platform
- Run the test until statistical significance is reached
- Analyse results including interaction effects
- Implement winning combination
Related Glossary Terms
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Email Marketing Best Practices
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Email Campaign Optimisation
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Frequently Asked Questions
In practice, limit to 2-3 variables with 2-3 variations each (4-27 total combinations). Beyond that, sample size requirements become impractical — you need roughly 1,000 engaged subscribers per combination for statistical significance.
You need a minimum of 10,000 engaged subscribers (not total list size) for meaningful multivariate testing. For every combination you test, you need enough engaged subscribers to reach statistical significance. With smaller lists, stick to sequential A/B testing.
Run until you reach statistical significance (95% confidence minimum, 99% preferred). Typical multivariate tests run 2-4 weeks. Unlike A/B tests, you cannot stop a multivariate test early based on interim results — this invalidates the statistical analysis.