A Better Approach to A/B Test Analysis | Hacker News Reader

A Better Approach to A/B Test Analysis

blog.sendwithus.com

51 pointsbvanvugt21 comments

Hey! I'm the author of Confidence.js. Emily Malcolm and I have been working hard on this new approach for the past few weeks and we're super excited to share it!

We're both here to answer any questions :)

This is really cool. I can tell you guys actually know statistics.

w/r/t testing multiple hypotheses, I've long wondered why false discovery rate never gets mentioned in the A/B-testing context. Any thoughts?

Thanks for your question! I'll pass it on to Emily, our stats expert.

>First, we use Chi Squared Tests to determine if differences in the A/B test data are meaningful or not.

Could you explain why you've chosen to take a binary approach to determining whether differences are meaningful or not?

To me it seems like a continuous approach would be both more useful and more realistic. Creating an artificial threshold for significance seems a bit silly (and it also makes the model harder for users to use, because different applications might need different significance levels to justify an action).

From my perspective, every data point contains information and if you wait for significance you're essentially ignoring early information.

Edit: Also, when switching costs are small, significance levels become mostly pointless and you just want to switch to the best A/B option immediately. As evidence swings the other way, you just switch back.

Hi! This is Emily, the stats brains behind this. My research indicated that 80% significance was commonly used for A/B Testing (and of course any significance level >80% would be even more conservative). I definitely see your point of the advantage of a continuous test and I think for the expert being able to see the exact significance level for the test would be very useful. However, our thoughts were that the average user might not necessarily have the expertise to know what a "good enough" level of significance would be for any test. Rather than having to educate every user on what significance means and how to interpret it we decided that a "yes, significant" or "no, not significant" would be more easily interpreted by all, regardless of their statistical background. If there is demand for a more continuous approach, it certainly could easily be implemented.

Thanks for the reply. I guess it's a good reminder for everyone building statistical products (or even non-statistical products) that the goal is not an optimal formula, but a result that makes users comfortable.

Worth pointing out that our (sendwithus) js library for calculating all of this is open source: github.com/sendwithus/confidence

The statistical methods we use now were created in the context of long-running experiments that had to be set up in advance and run in parallel. In that situation, you have to decide up-front how many subjects to test on, and the methods reflect this.

I'd like to see someone takle creating a method aimed at our situations where results steadily trickle in. There ought to be a way to come up with adaptive thresholds such that at any given time we can ask, "Do we have statistically significant results yet, or do we keep the test running?"

Yes! This is very important to us in dealing with transactional email.

Our Z-Test Method in Confidence.js pinpoints when results become statistically significant by calculating the required sample size at any given time using the standard error, zScore, and margin of error. If we have more samples than required, we can stop our test with confidence that we have statistically significant results.

We're still working out a way to do this with the new method - we have a hunch it might have to do with identifying when the confidence level becomes stable (as it does with more samples over time).

Hi! This is Emily, the stats brains behind this. I think this is a really interesting and important question. I know this question comes up in other contexts (clinical trials) and there has been a lot of work done on how to find "stopping times" from a statistical standpoint. I'd love to see some of these statistical methods for sequential analysis (developed for clinical trials) integrated into how we do A/B testing.

I totally agree. And I think Bayesian methods (which compute the likelihood of the model given the data) tend to work much better for these rolling data applications than frequentist methods (which compute the likelihood of the data given the model). The problem for frequentist methods here is that when data is constantly rolling in it's hard to specify the space of all possible data collected (because traditional bounds like the number of data points no longer work).

Here's a link to a paper on a Bayesian approach to the multi-armed bandit problem: http://onlinelibrary.wiley.com/doi/10.1002/asmb.874/abstract...

Hi! This is Emily, the stats brains behind this. I will definitely look into this and other Bayesian methods. Thanks!

I would also suggest switching from chi-square to the g-test.

If you look at the history, I believe that you'll find that Pearson originally came up with the g-test as an approximation to an exact test, and then found the chi-square as an easier to compute alternative. This mattered back in the days of pencil and paper, but there is no excuse today to use the worse technique.

I'm going to avoid long discussions about the advisability of taking multiple looks at results with a classical statistical test. But see my incomplete series at http://elem.com/~btilly/ab-testing-multiple-looks/index.html for some of the considerations.

I never got into Bayesian statistics in there. In general they depend on the existence of a prior distribution. Careful treatments will talk about this. Sloppy ones assume one, don't talk about the one that they assume, and then quote results without letting you know about this important assumption. As long as you accept that assumption, they work well. But sometimes can be confusing to explain. (Until people "get" it. Then it can become irritating getting them to STOP explaining it!)

If you want to discuss these issues more, my email is my name at gmail.com.

"Upgrading" to the g-test is certainly something we could implement in the future - Pearson's Chi Square was simply a starting place. I will definitely have a look over the link you provided. Thanks!

> There ought to be a way to come up with adaptive thresholds such that at any given time we can ask, "Do we have statistically significant results yet, or do we keep the test running?"

Relevant keywords here include 'trial sequential analysis', 'adaptive trials', & 'multi-armed bandits'.

This is exactly the test you are looking for:

http://www.bayesianwitch.com/blog/2014/bayesian_ab_test.html

It can be drastically improved in the case of emails by building a Bayesian model tuned to email itself. Most people just blindly apply a testing procedure to their situation, but if you tune the test to your situation you can make do with a LOT fewer samples.

Here's another paper that's relevant to this topic: http://www.qubitproducts.com/sites/default/files/pdf/most_wi.... And discussion here: https://news.ycombinator.com/item?id=7287665

There are a number of testing tools relying on this methodology. This is useful: Evan's Awesome A/B Tools (http://www.evanmiller.org/ab-testing/). It includes a Chi-Squared test, sample size calculator, two sample T-test, and Poisson Means test.

At my company we've implemented a Bayesian A/B test in order to minimize the amount of time a test has to run.

http://visualrevenue.com/blog/2013/02/tech-bayesian-instant-...

I will definitely have a look through this. Thanks!

Nice to see someone questioning old statistical methods and then bringing new methods to the masses.

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