# What is the minimum detectable effect (MDE)

> The minimum detectable effect (MDE) is the smallest lift an A/B test can reliably detect with the traffic you have. You choose it before launching the test, and the smaller it is, the more visitors you will need.

- Source: https://bixlow.com/en/glossary/minimum-detectable-effect
- Language: en
- Updated: 2026-11-02
- Publisher: Bixlow

## What it means in practice

Say your store converts at 2%. Setting a 10% MDE means: I want my test to be able to see whether the variant lifts conversion to 2.2% or more. If the real lift is smaller, say 3%, the test will probably miss it, even though it exists.

That is why the MDE is not what you expect to win, but the bar below which you accept being blind. It is a business decision: which improvement is worth detecting.


## Relative or absolute

It can be expressed in two ways, and mixing them up is a very common mistake with calculators.

- Relative: a percentage of your current conversion. From 2% to 2.2% is a 10% relative MDE.
- Absolute: the difference in points. From 2% to 2.2% is an absolute MDE of 0.2 points.
- An absolute MDE of 1 point on a 2% baseline is a 50% relative lift, something almost no change achieves. Always check what the calculator is asking for.


## Why small effects are so expensive

Visitors per version for a 2% baseline conversion rate, at 95% confidence and 80% power.

| Relative MDE | Variant conversion | Visitors per version |
| --- | --- | --- |
| 5% | 2.1% | 315,210 |
| 10% | 2.2% | 80,683 |
| 20% | 2.4% | 21,109 |
| 30% | 2.6% | 9,798 |

_Calculated with the classic two-proportion formula. Halving the MDE roughly quadruples the sample._


> **Rule of thumb.** If the page you want to test gets 30,000 visitors a month, a 10% MDE on a 2% conversion rate takes more than five months. That is the moment to test bolder changes (a redesigned product page, not a button colour) or to measure a more frequent conversion, like add to cart.


## How to choose your MDE

Start with the business: which improvement is worth the effort. If 5% more sales means 200 euros a month, it may not be worth six months of testing. If it means 20,000, it is.

Then look at traffic. Work out how long the MDE you would like takes and, if the answer is absurd, raise the MDE or change what you test. A three-week test that concludes beats a six-month one nobody finishes.


## How Bixlow handles it

The MDE is a planning concept: it helps you decide before you start whether a test is viable. Bixlow does not show an MDE in its results. What it shows is the probability that each version is the best, the estimated lift with its credible interval and whether the minimum sample and duration have been met.


## FAQ

### Is the MDE the lift I expect to get?

No. It is the smallest lift your test will be able to see. You can expect 20% and set a 10% MDE to leave some margin.

### Which MDE is usual?

Online stores commonly plan with a relative MDE between 10% and 20%. Below 5% you need an enormous amount of traffic.

### What happens if the real lift is smaller than the MDE?

The test will probably end without a clear winner. That does not mean the change doesn't work, only that your test lacked the resolution to see it.

### Can I change the MDE halfway through a test?

It makes no sense: the MDE is for planning the duration. If halfway through you see you won't get there, it is better to stop and rethink the test.

## Further reading

- [What is sample size](https://bixlow.com/en/glossary/sample-size)
- [Sample size calculator](https://bixlow.com/en/tools/sample-size-calculator)
- [A/B test duration calculator](https://bixlow.com/en/tools/ab-test-duration-calculator)
- [Bixlow's statistics](https://bixlow.com/en/platform/statistics)