A/B Testing

Challenges of A/B testing

While A/B testing is a valuable tool for optimizing website or application performance, it also presents a number of challenges that must be addressed in order to achieve accurate and meaningful results. Some of these challenges include ensuring consistent assignment of users to test and control groups, involving developers in the testing process, collecting and analyzing event data, and dealing with statistical challenges such as small sample sizes or low conversion rates.

Implementation Challenges

Consistent Assignment

Ensuring that the appropriate features are shown to a given user and that the user continues to be exposed to that experience over time is one challenge of A/B testing. In most cases, simply randomizing each request to one of many potential experiences will just not cut it.

For example, imagine you are testing a new feature for a social media platform that allows users to post long-form content. You want to determine if the new feature leads to increased user engagement and retention, so you randomly assign users to either the control group (no new feature) or the treatment group (new feature).

However, if users see different versions of the new feature at different times, then the results of the experiment may be difficult to interpret, as the user may be influenced by other factors, such as changes in their behavior, preferences, or external circumstances, that are unrelated to the specific variations being tested. In this case, you would want to make sure that each user is consistently assigned to either the control or treatment group throughout the duration of the experiment.

A/B testing platforms need to be able to accommodate the assignment needs of an application, whether than means random assignment, consistent assignment by session, or even consistent assignment based on a unique user identifier (e.g., user ID or email address).

Developer Involvement

Another challenge of A/B testing is that developer involvement is typically required to ensure that the appropriate features are rendered for a user given the parameters of a defined experiment. Smaller companies may be limited in the amount of developer expertise they have available, and it can be difficult to balance ease-of-use with robust feature creation and management tools.

One solution to this challenge is What You See Is What You Get (WYSIWYG) experimentation platforms. These tools allow you to insert text or images, or drag and drop elements onto a web page, with little or no developer expertise. The software automatically generates the code, and you can inject the code directly into your application as a script tag.

Although this is certainly a straightforward solution, it only works for client-side rendered applications, and it also limits your A/B testing to visual features. In addition, there is no guarantee that the automatically-generated code will be error-free. Companies may want to test a range of features that may or may not result in a visual change for the user, so there are trade-offs to consider between ease-of-use and more control, including the ability to conduct more complex A/B tests.

Collecting Event Data

In order to evaluate the results of an A/B test, you need to collect some form of event data, which is information collected about user actions or behavior during an A/B testing experiment.

Examples of event data that may be collected during an A/B testing experiment include:

  • Clicks: the number of times a user clicks on a particular button or link
  • Conversions: the number of users who complete a desired action, such as making a purchase or filling out a form
  • Engagement: the amount of time a user spends on a particular page or feature
  • Bounce rate: the percentage of users who leave a page without taking any action

An analytics platform is a tool or service that is used to collect, store, and analyze event data. By integrating an A/B testing tool with an analytics platform, businesses can collect and analyze event data about user behavior for both the test and control groups, and use this information to make informed decisions about which variations to implement.

As you will see in the next section, many A/B testing platforms, including Firebase, GrowthBook, and Unleash, rely on data already being collected through analytics services such as Google Analytics.

However, this can pose a challenge to companies that are just getting started in A/B testing, as they may not have a preferred analytics platform. They may not even be collecting their own event data, or they may not be collecting it in a way that it is amenable to A/B testing (at a minimum, data with a user or session identifier, the variant presented, and a timestamp).

An option for companies that either do not not collect their own event data or do not have a suitable analytics platform is to use an A/B testing tool that also collects, stores, and analyzes event data. However, there are trade-offs to consider, as the company would need to determine whether they retain full ownership and control of their event data and any user data that may be associated with those events. In addition, additional developer time and expertise would be needed to ensure that event data is properly sent to the A/B testing platform.

Statistical Challenges

In addition to engineering challenges, there are also a number of statistical challenges related to A/B testing.

Extracting the Impact of a Single Variation

One statistical challenge related to A/B testing is determining which specific variant (or variants) is truly influencing user behavior. If a user is presented with multiple variations in a website or application at the same time, the variations may be interdependent or have additive effects. For example, if a website has variations in its headline, subheading, and call-to-action button, it may be difficult to determine whether a change in conversion rate was due to the headline, subheading, or call-to-action button, or some combination of these elements.

Two common solutions to testing multiple variations in a single website or application are multivariate A/B testing and sequential A/B testing.

  • Multivariate A/B testing is one solution, where multiple variations of different elements on a web page, such as headlines, images, and call-to-action buttons, are tested simultaneously to determine which combination of elements is most effective in achieving a specific goal.
  • Sequential A/B testing, on the other hand, involves testing one element at a time, and then using the results of each test to inform subsequent tests.

The multivariate approach can be more efficient than sequential testing because it enables businesses to test a large number of variations in a single experiment, which can save time and resources. Multivariate testing can also help businesses identify interactions between different elements on a page, which can be valuable for understanding the complex relationships between different parts of a website or application.

One of the main challenges of multivariate testing is the need for a large sample size to achieve statistical significance. Testing multiple variations simultaneously can quickly multiply the number of possible combinations, requiring a larger sample size to ensure that each variation is tested with a sufficient number of users. As a result, multivariate testing can be more resource-intensive and time-consuming than testing of a single variation at a time.

Another challenge is the complexity of analyzing the data. With sequential A/B testing, it is relatively straightforward to determine which variation is most effective by comparing key metrics such as the conversion rates of the control group and treatment group. In multivariate testing, however, there are many different combinations of variations to consider, making it more difficult to isolate the impact of a single variation. As a result, multivariate testing often requires more complex statistical analysis to tease out the effect of each variation.

The sequential approach can be more precise than multivariate testing because it enables businesses to isolate the effects of each element on user behavior. Additionally, sequential testing can be more efficient in terms of sample size, because fewer users are needed for each individual test.

Power to Detect Small Effects

Another major challenge in A/B testing is the need to have enough data to accurately measure the impact of changes to your website or application. If you are testing a large number of variants or the effect of each variant on user behavior is small, it could require a lengthy experiment to generate enough data to see a statistically significant impact. This can be especially difficult for websites or applications with low traffic, where it may take a very long time to gather enough data to make an informed decision.

A sample size that is too small can lead to unreliable results, as it may not accurately represent the behavior of your user population as a whole. Additionally, if the users in your test group are not representative of your overall user base, you may not be able to generalize the results to your entire user population. Ensuring that your A/B tests have enough statistical power to accurately measure the impact of changes to your website or application is critical to making informed decisions about the user experience.

Some A/B testing platforms limit the display of experiment results to those where results are statistically significant. This is a reasonable choice, as it makes sense to ensure that results are meaningful before implementing a change to your application. However, it it is limiting for those applications where the user base or the expected effect size is small, where statistical significance may not be reached.

A/B Testing Landscape