Appears in the journal of Data Mining and Knowledge DiscoveryPDF
The web provides an unprecedented opportunity to evaluate ideas quickly using controlled experiments, also called randomized experiments, A/B tests (and their generalizations), split tests, Control/Treatment tests, MultiVariable Tests (MVT) and parallel flights. Controlled experiments embody the best scientific design for establishing a causal relationship between changes and their influence on user-observable behavior. We provide a practical guide to conducting online experiments, where end-users can help guide the development of features. Our experience indicates that significant learning and return-on-investment (ROI) are seen when development teams listen to their customers, not to the Highest Paid Person’s Opinion (HiPPO). We provide several examples of controlled experiments with surprising results. We review the important ingredients of running controlled experiments, and discuss their limitations (both technical and organizational). We focus on several areas that are critical to experimentation, including statistical power, sample size, and techniques for variance reduction. We describe common architectures for experimentation systems and analyze their advantages and disadvantages. We evaluate randomization and hashing techniques, which we show are not as simple in practice as is often assumed. Controlled experiments typically generate large amounts of data, which can be analyzed using data mining techniques to gain deeper understanding of the factors influencing the outcome of interest, leading to new hypotheses and creating a virtuous cycle of improvements. Organizations that embrace controlled experiments with clear evaluation criteria can evolve their systems with automated optimizations and real-time analyses. Based on our extensive practical experience with multiple systems and organizations, we share key lessons that will help practitioners in running trustworthy controlled experiments.

What others are saying

  • Web Analytics forum 4/13/2008 (was
    here's an excellent article on A/B and multivariate testing...
    it is indeed a great resource
  • Web Analytics forum 10/15/2008 (was

For those of us that are wondering about this in A/B tests, there is a nice
short paragraph about assigning equal users to treatments.
It is located in section 6.2.4, of the paper located here:
This paper is a great read!

(Moderator, Analyst@Intuit)



author = {Kohavi, Ron and Longbotham, Roger and Sommerfield, Dan and Henne, Randal M.},
title = {Controlled Experiments on the Web: Survey and Practical Guide},
journal = {Data Mining and Knowledge Discovery},
issue_date = {February  2009},
volume = {18},
number = {1},
month = feb,
year = {2009},
issn = {1384-5810},
pages = {140--181},
url = {},
doi = {10.1007/s10618-008-0114-1},
acmid = {1485091},
publisher = {Kluwer Academic Publishers},
address = {Hingham, MA, USA},
keywords = {A/B testing, Controlled experiments, MVT, MultiVariable Testing, Website optimization, e-commerce},


ACMRef: Ron Kohavi, Roger Longbotham, Dan Sommerfield, and Randal M. Henne. 2009. Controlled Experiments on the Web: Survey and Practical Guide. Data Mining and Knowledge Discovery 18, 1 (February 2009), 140-181. DOI=10.1007/s10618-008-0114-1

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