How Long an Experiment Should Run?

Understanding the Basics: Statistical Power vs. Confidence Level

Before diving into the specifics of experiment duration, it’s essential to understand two key statistical concepts: statistical power and confidence level. While these terms may sound technical, they can be explained in simple terms.

Statistical Power: Statistical power measures the likelihood that your test will detect a real difference between your variants if one truly exists. Think of it as the sensitivity of your experiment. If the statistical power is high, your test is more likely to identify a genuine effect. Typically, researchers aim for a statistical power of 80%, meaning there’s an 80% chance of correctly identifying a difference if it’s there.

Confidence Level: The confidence level reflects how certain you are that your results are accurate and not due to random chance. For example, a 95% confidence level means that if you ran the experiment 100 times, you’d expect the same result 95 times. While 80% confidence is good enough, 95% is ideal for ensuring more reliable results. This level of certainty is crucial when making data-driven decisions based on your A/B tests.

Factors That Determine the Duration of an A/B Experiment

1. Sample Size: The number of participants in your experiment is a critical factor in determining how long it should run. A larger sample size generally leads to more reliable results and requires shorter experiment durations. However, if your website has low traffic, it may take longer to gather enough data to reach statistical significance. Ideally, your site should have at least 1,000 transactions per month to run an effective A/B test. If your traffic is lower, you may need to extend the experiment duration to achieve meaningful results.
2. Expected Effect Size: The expected effect size refers to the magnitude of the difference you anticipate between the control and variant groups in your A/B test. For example, if you expect a new design to increase conversion rates by 5%, that 5% is your expected effect size. Smaller expected effects (like a 1% increase) require larger sample sizes and longer durations to detect, while larger expected effects (like a 10% increase) can be detected more quickly with a smaller sample size. Understanding the effect size helps in determining how long the test should run to produce reliable results.
3. Traffic Consistency: Consistent traffic ensures that data is collected steadily over time. If your website experiences significant fluctuations in traffic, such as during seasonal changes or sales events, it might be necessary to run the experiment longer to account for these variations and gather a representative sample. It’s often advisable to wait out these periods to ensure that your results are not skewed by external factors like a sale, where users may be purchasing more due to discounts rather than the changes you’re testing.

How to Determine the Optimal Experiment Duration

1. Use a Sample Size Calculator: Before starting your experiment, use an A/B test sample size calculator. These calculators take into account your desired statistical power, confidence level, sample size, and expected effect size to estimate the necessary sample size and experiment duration.
2. Avoid Stopping the Experiment Too Early: It can be tempting to stop an experiment as soon as you see significant results. However, doing so increases the risk of making decisions based on incomplete data. It’s essential to allow the experiment to run until the required sample size is reached and the confidence level has stabilized to ensure the results are reliable. However, if the experiment is significantly underperforming in the first week, it can be stopped earlier to save time and resources.
3. Run the Experiment for at Least Two Full Business Cycles: To account for any potential variations in user behavior (such as weekend vs. weekday traffic), it’s advisable to run your experiment for at least two full business cycles. This means running the experiment for a minimum of two weeks. In some cases, one week might be enough, but two weeks generally provide a more accurate picture of user behavior.
4. Monitor the Statistical Significance Over Time: Keep an eye on how the statistical significance and confidence level evolve throughout the experiment. If the results stabilize and remain consistent over time, it could be a sign that the experiment has run long enough.

Bottom line

Determining the optimal duration for an A/B experiment requires careful consideration of several factors, including sample size, expected effect size, traffic consistency, statistical power, and confidence level. By understanding these concepts, you can make informed decisions about when to start and stop your experiments. While 80% confidence is good enough, aiming for a 95% confidence level ensures more reliable results, leading to more accurate and impactful conclusions.

A well-designed A/B test, backed by solid research and sufficient duration, will help you make data-driven decisions that contribute to long-term success. While the methods discussed provide a strong foundation, always remain adaptable, as each experiment may present unique challenges and opportunities.