You Ask, I Answer: A/B Tests for Non-Random Situations?

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You Ask, I Answer: A/B Tests for Non-Random Situations?

Kanako asks, “What are best practices for conducting A/B tests on non-random situations?”

The short answer is for randomized controlled trials, there isn’t a best practice for doing things non-randomly. You need randomization to reduce the impact of confounding variables, and this is something marketers do wrong many, many times.

Ways A/B tests go wrong:
– Extrapolating a selected population to the whole population
– Allowing self selection
– Group randomization of individuals
– Insufficient sample size

Be clear about what the population is, then randomize from it using true random selection. That’s the only way to avoid things like selection bias.

You Ask, I Answer: A/B Tests for Non-Random Situations?

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Machine-Generated Transcript

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In today’s episode Conoco asks what are the best practices for conducting a B tests on non random situations? All I mean, the short answer is don’t.

An A B test is a more properly named randomized controlled trial.

You have a hypothesis you have a control group of people who don’t receive the test.

You have a group of people who do receive the test, and then you have the measurement to see if there is a statistically significant difference between the control group and the test group.

If you do things non randomly when it comes to the selection of who is in each group, you are going to potentially blow up the test.

Create a an outcome That is statistically not meaningful, or hat worse off has some kind of bias.

And so in order to test properly, you need to have that randomization to reduce the impact of those biases.

So real simple example, let’s say, let’s say you’re running a test on your website.

And you want people to try out the new interface, right? So you say, hey, try out our new interface.

At this point, now you’re doing what’s called self selection where people can opt to be in either the group that stays the same, or the group that is different.

Why would this be bad? Well, think about this, what is the behavioral change that makes somebody choose to be in a different group? Right, they may have a different personality type.

They may have a potentially a different skew towards it.

Gender a skewed towards and ethnicity is skewed towards behavior.

Maybe most smokers might choose that out whether smoking is being a smoker or a nonsmoker is is relevant to your population may or may not be something you want to investigate, but just the fact that someone can opt into a test group automatically sorts those people by something other than randomization.

And that group may behave differently than the group that’s left behind as opposed to a randomized group where everybody has the same chance of behaving in a similar situation where the control group of the test group, so I can’t really think of situations where you would want to have a non random controlled trial.

You can certainly do things like focus groups, maybe To get qualitative information do you want to try and, and get a well mixed sample of your target population rather than the eight people that you know that to come talk about something because again, they may not be representative of the population as a whole.

There are four ways that we see a B Tests, Surveys, all sorts of things go kind of really off the rails.

That self selection is sure one of them the randomizing of individuals in groups, as if as long as it’s not done randomly, like say, you’re going to take everybody with a gmail address.

Well, that’s not really a great way to, to, to assign people to a controlled trial, particularly doing anything regarding email marketing, for example, because Gmail addresses may behave differently than hotmail addresses or AOL addresses for those people who still have them.

So you want to have avoid doing group randomization of individuals.

If you’re testing the individual randomized at the individual.

If you’re testing our group grand, you can randomize at the group level, but then you should not be using individual level data.

Third would be extracting, oh yeah, extracting a extrapolating a population, to the whole population that’s been selected.

So this is one that we see a lot with email marketing, and testing.

People will send out email to a specific email list and get it behavior, whether it’s an A B test running on site or whatever.

And then extrapolate that behavior to the whole.

You see this go wrong a lot with testing a lot with surveys.

You will send out a survey to your email list.

And then you would say well marketers believe that AB testing is overpriced.

Well, no, only the people that you email believe that.

And if your sample is not representative of all marketers, then it’s your conclusion is invalid.

There are a number of companies that have been doing this with surveys over the years.

They email their their database about their product or service or niche.

And then say, marketers believe this will know only the people who are on your email list who have a self selection bias, right? your email list has a self selection bias people do not opt into emails that have companies that they don’t care about.

So only the people who are on your list care enough about you to have given your your them you their email, and they have a bias.


People who are in for example, my analytics for markers slack group, which if you’re interested in joining, go to Trust slash analytics for marketers.

People who joined that slack group, by default have a bias towards being interested in analytics.

If they did not care about analytics, they wouldn’t join.

So if I were to run a survey about analytics to an analytics group, I’m going to get very different results than if I ran the same survey to a random group of markers.

So the extrapolation is, is important.

And the fourth one is insufficient sample sizes.

Again, when it comes to to a b tests, if you run a test and you test, you know, 10 people, and your website gets a million visitors a day.

You’ve got a statistically invalid test that’s that.

So small a sample cannot be used to extrapolate to the behaviors whole boxes is too much too large of a chance that you’ve got people in there who don’t behave like the rest of the population.

So when you’re running, testing For a B tests, you have to make sure that you have a large enough sample.

Now the good news is, for a lot of a b testing software like Google Optimize, it decides when the sample is statistically significant.

So you may want to run a test in a certain timeframe, it won’t let you it will run the test until it has an answer of some kind or can’t come to a valid conclusion.

That one’s not as much of a problem but generally speaking, there are no best practices for non random controlled trials, because they should not exist.

So be clear about your population is randomized at using true random selection, we have a computer Assigned Numbers randomly assigned groups randomly.

That’s how you avoid things like selection bias or confounding variables and stuff without that randomization.

Things just get messy and you’ll end up with bad data from which you will make bad decisions.

So do it well do it with randomization and and you struggle with that.

There’s plenty of software and tutorials on how to do the randomization.

But make sure that you’re doing it.

You’re not doing non random controlled trials.

Good question.

Technically complicated question.

So if Yeah, if the last few minutes were a lot of math words, it’s okay.

But just be aware that of the idea that you always want to have a random sample.

You don’t want to have non random samples.

I can’t really think in the Navy test of a situation where you’d want something like that.

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