# Pre-test calculator

URL: https://docs.ablyft.com/guides/results/pre-test-calculator/

> Estimate how long a test needs to run and how large an effect it can detect.



Before you start an experiment, the Pre-Test Calculator helps you answer: *with my traffic, how long must the test run,
and how small a change can it still detect?* The answer is the **Minimum Detectable Effect (MDE)**: the smallest relative
change in conversion rate that your test can reliably detect. A lower MDE needs more participants and a longer runtime.

Open it in your project under **Utilities → Pre-Test Calculator**. The calculator is a planning tool. It does not read data
from your experiments and changes nothing in your project.

## Enter your data

Use the numbers of the page and the metric you want to improve, ideally your primary goal.

**Test page data**

| Field                    | Description                                                         |
| ------------------------ | ------------------------------------------------------------------- |
| **Weekly traffic**       | Total unique visitors or sessions per week.                         |
| **Weekly conversions**   | Number of conversions (for example purchases or sign-ups) per week. |
| **Number of variations** | From 2 to 10, including the control (original). Default: 2.         |

**Calculator settings**

| Field                        | Description                                                                                        |
| ---------------------------- | -------------------------------------------------------------------------------------------------- |
| **Baseline conversion rate** | Calculated automatically as weekly conversions ÷ weekly traffic, in percent. You can overwrite it. |
| **Confidence level**         | From 80% to 99%. It starts with the confidence level of your project.                              |

The results appear as soon as you enter weekly traffic and a baseline conversion rate. They update while you type.

## Read the results

The table shows what happens if you run the test for 1, 2, 3, 4, 6, 8, 10 or 12 weeks:

| Column                           | Meaning                                                                  |
| -------------------------------- | ------------------------------------------------------------------------ |
| **Number of weeks running test** | The runtime of the test.                                                 |
| **Min. Detectable Effect (MDE)** | The smallest relative change that the test can detect after this time.   |
| **Participants / Variation**     | Weekly traffic × weeks, divided by the number of variations, rounded up. |

The rows are shaded from green (short tests) over yellow to orange (long tests).

Example: with a baseline of 5% and an MDE of 10%, the test can detect a change from 5.0% to 5.5% or more. A smaller
change stays hidden in the noise. If your idea is likely to cause a smaller change, you need more weeks, fewer
variations or more traffic.

## How it is calculated

The calculation uses a statistical power of 80% and this formula for the absolute difference δ:

`δ = (Z(confidence) + Z(power)) × √(2 × p × (1 − p) ÷ n)`

where *p* is the baseline conversion rate, *n* the participants per variation, and Z(confidence) the two-sided z-value of
your confidence level. The MDE is δ divided by *p*, shown in percent.

> The calculator expects a baseline conversion rate between 0% and 100% and positive traffic. Otherwise it shows
> "Enter your test data above to see results."

## Tips for planning

* Test for **full weeks** so that every weekday is included.
* If the MDE is higher than the effect you expect, the test will probably end inconclusive. Test a bolder change, a
  page with more traffic or fewer variations.
* Compare the runtime with the **Minimum runtime** and **Minimum conversions** of your project, see
  [Statistical methods](https://docs.ablyft.com/guides/results/statistics-methods/#decision-rules).

## Next steps

* [Create an experiment](https://docs.ablyft.com/guides/experiments/create-experiment/)
* [Interpret results](https://docs.ablyft.com/guides/results/interpret-results/)