Part 1: Experiment Driven Development

In this series of three we’ll go into Experiment Driven Development and A/B testing. The opposite of developing features based on anecdotes heard in stories from the CEO’s next-door neighbor, EDD seeks proof and is iterative. EDD can be defined as fact-based development: development based on evidence gathered from the field, not intuition.In EDD every new feature or process implemented is validated through a formal experiment design process, which looks to test a hypothesis that describes the status quo of the feature. Example hypotheses could range from "Making a button bigger does not impact clicks" all the way to "Making a web app responsive does not increase visitation" compared. In statistical terms, the base statement is referred to as the null hypothesis (H₀), the status quo, and then an alternative hypothesis(H₁) is proposed. The null hypothesis will usually state that the change introduced by the experiment will not affect the current behavior while the alternative supports that there is in fact a change.The alternative hypothesis is a prediction of what is expected to happen before running the experiment. It can be a bold statement, not an open question and it should have three parts:

1. The variable (if we add/change/remove...): the change that the experiment will measure against the current state of the feature/process.
2. The desired result (then we expected to see...): what we expect to see after the change is introduced, a qualitative difference between the current state and new state.

The rationale behind the prediction (because we have seen that...): prior knowledge that has led you to come up with the current hypothesis (from prior observation).For example, one can define the pair of hypotheses for a new registration form in a website as:

• H₀: Changing the registration form from multiple to single page will not impact the current user registration rate.
• H₁: Changing the registration from multiple to single page will increase the current user registration rate by 5% because we have previously seen that there is a 5% abandon rate on the multi page form format.

EDD is based on A/B Testing, which is a randomized experiment method to compare two variants of a single variable. In this case, a baseline metric is compared thanks to the definition control (status quo) and treatment (new feature) groups in order to determine if the variation has a significant impact or not. Ideally, most decisions to release a feature would be based on the results given by A/B Tests. At Sama, we want to find viable ideas or fail fast. Instead of developing a monolithic solution and pushing a release, we iterate through experiments, evaluating how features perform and, most importantly, if and how customers use them.Next up: A/B Testing and A/B Testing with Python.