What is an A/B Test?. This is the second post in a multi-part… | by Netflix Technology Blog | Sep, 2021

What is an A/B Test?. This is the second post in a multi-part… | by Netflix Technology Blog | Sep, 2021

With many experiments, including the Upside Down box art example, we need to think carefully about what our metrics are telling us. Suppose we look at the click through rate, measuring the fraction of members in each experience that clicked on a title. This metric alone may be a misleading measure of whether this new UI is a success, as members might click on a title in the Upside Down product experience only in order to read it more easily. In this case, we might also want to evaluate what fraction of members subsequently navigate away from that title versus proceeding to play it.

In all cases, we also look at more general metrics that aim to capture the joy and satisfaction that Netflix is delivering to our members. These metrics include measures of member engagement with Netflix: are the ideas we are testing helping our members to choose Netflix as their entertainment destination on any given night?

There’s a lot of statistics involved as well — how large a difference is considered significant? How many members do we need in a test in order to detect an effect of a given magnitude? How do we most efficiently analyze the data? We’ll cover some of those details in subsequent posts, focussing on the high level intuition.

Because we create our control (“A”) and treatment (“B”) groups using random assignment, we can ensure that individuals in the two groups are, on average, balanced on all dimensions that may be meaningful to the test. Random assignment ensures, for example, that the average length of Netflix membership is not markedly different between the control and treatment groups, nor are content preferences, primary language selections, and so forth. The only remaining difference between the groups is the new experience we are testing, ensuring our estimate of the impact of the new experience is not biased in any way.

To understand how important this is, let’s consider another way we could make decisions: we could roll out the new Upside Down box art experience (discussed above) to all Netflix members, and see if there’s a big change in one of our metrics. If there’s a positive change, or no evidence of any meaningful change, we’ll keep the new experience; if there’s evidence of a negative change, we’ll roll back to the prior product experience.

Let’s say we did that (again — this is a hypothetical!), and flipped the switch to the Upside Down experience on the 16th day of a month. How would you act if we gathered the following data?

Figure 3: Hypothetical data for the release of the new Upside Down box art product experience on Day 16.

The data look good: we release a new product experience and member engagement goes way up! But if you had these data, plus the knowledge that Product B flips all the box art in the UI upside down, how confident would you be that the new product experience really is good for our members?

Do we really know that the new product experience is what caused the increase in engagement? What other explanations are possible?

What if you also knew that Netflix released a hit title, like a new season of Stranger Things or Bridgerton, or a hit movie like Army of the Dead, on the same day as the (hypothetical) roll out of the new Upside Down product experience? Now we have more than one possible explanation for the increase in engagement: it could be the new product experience, it could be the hit title that’s all over social media, it could be both. Or it could be something else entirely. The key point is that we don’t know if the new product experience caused the increase in engagement.

What if instead we’d run an A/B test with the Upside Down box art product experience, with one group of members receiving the current product (“A”) and another group the Upside Down product (“B”) over the entire month, and gathered the following data:

Figure 4: Hypothetical data for an A/B test of a new product experience.

In this case, we are led to a different conclusion: the Upside Down product results in generally lower engagement (not surprisingly!), and both groups see an increase in engagement concurrent with the launch of the big title.

A/B tests let us make causal statements. We’ve introduced the Upside Down product experience to Group B only, and because we’ve randomly assigned members to groups A and B, everything else is held constant between the two groups. We can therefore conclude with high probability (more on the details next time) that the Upside Down product caused the reduction in engagement.

This hypothetical example is extreme, but the broad lesson is that there is always something we won’t be able to control. If we roll out an experience to everyone and simply measure a metric before and after the change, there can be relevant differences between the two time periods that prevent us from making a causal conclusion. Maybe it’s a new title that takes off. Maybe it’s a new product partnership that unlocks Netflix for more users to enjoy. There’s always something we won’t know about. Running A/B tests, where possible, allows us to substantiate causality and confidently make changes to the product knowing that our members have voted for them with their actions.

An A/B test starts with an idea — some change we can make to the UI, the personalization systems that help members find content, the signup flow for new members, or any other part of the Netflix experience that we believe will produce a positive result for our members. Some ideas we test are incremental innovations, like ways to improve the text copy that appears in the Netflix product; some are more ambitious, like the test that led to “Top 10” lists that Netflix now shows in the UI.

As with all innovations that are rolled out to Netflix members around the globe, Top 10 started as an idea that was turned into a testable hypothesis. Here, the core idea was that surfacing titles that are popular in each country would benefit our members in two ways. First, by surfacing what’s popular we can help members have shared experiences and connect with one another through conversations about popular titles. Second, we can help members choose some great content to watch by fulfilling the intrinsic human desire to be part of a shared conversation.

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