My 7 Rules for A/B Testing That Triple Conversion Rates

I really don’t care how any given A/B test turns out.

That’s right. Not one bit.

But wait, how do I double or triple conversion rates without caring how a test performs?

I actually care about the whole SYSTEM of testing. All the pieces need to fit together just right. If not, you’ll waste a ton of time A/B testing without getting anywhere. This is what happens to most teams.

But if you do it right. If you play by the right rules. And you get all the pieces to fit just right, it’s simply a matter of time before you triple conversions at any step of your funnel.

I set up my system so that the more I play, the more I win. I stack enough wins on top of each other that conversion rates triple. And any given test can fail along the way. I don’t care.

What does my A/B testing strategy look like? It’s pretty simple.

  • Cycle through as many tests as possible to find a couple of 10-40% wins.
  • Stack those wins on top of each other in order to double and triple conversion rates.
  • Avoid launching any false winners that drag conversions back down.

For all this to work, you’ll need to follow 7 very specific rules. Each of them is critical. Skip one and the whole system breaks down. Follow them and you’ll drive your funnel relentlessly up and to the right.

Rule 1: Above all else, the control stands

I look at A/B tests very differently from most people.

Usually, when someone runs a test, they’ll consider each of their variants as equals. The control and the variant are both viable and their goal is to see which one is better.

I can’t stand that approach.

We’re not here for a definitive answer. We’re here to cycle through tests to find a couple of big winners that we can stack on top of each other.

If there’s a 2% difference between the variant and the control, I really don’t care which one is the TRUE winner. Yes, yes, yes, I’d care about a 2% win if I had enough data to hit statistical significance on those tests (more on this in a minute). But unless you’re Facebook or Amazon, you probably don’t have that kind of volume. I’ve worked on multiple sites with more than 1 million visitors/month and it’s exceedingly rare to have enough data hitting a single asset in order to detect those kinds of changes.

In order for this to system to work, you have to approach the variant and control differently. Unless a variant PROVES itself as a clear winner, the control stands. In other words, the control is ALWAYS assumed to be the winner. The burden of proof is on the variant. No changes unless the variant wins.

This ensures that we’re only making positive changes to assets going forward.

Rule 2: Get 2000+ people through the test within 30 days

So you don’t have any traffic? Then don’t A/B test. It’s that simple. Do complete revamps on your assets and then eyeball it.

Remember, we need the A/B testing SYSTEM working together. And we’re playing the long-term. Which means we need a decent volume of data so we can cycle through a bunch of different test ideas. If it takes you 6 months to run a single test, you’ll never be able to run enough tests to find the few winners.

In general, I look for 2000 or more people hitting the asset that I’m testing within 30 days. So if you want to A/B test your homepage, it better get 2000 unique visitors every month. I even prefer 10K-20K people but I’ll get started with as little as 2000/month. Anything less than that and it’s just not worth it.

Rule 3: Always wait at least a week

Inside of a week, data is just too volatile. I’ve had tests with 240% improvements at 99% certainty within 24 hours of launching the test. This is NOT a winner. It always comes crashing down. Best-case scenario, it’s really just a 30-40% win. Worse case, it flip-flops and is actually a 20% decline.

It also lets you get a full weekly cycle worth of data. Visitors don’t always behave the same on weekends as they do during the week. So a solid week’s worth of data gives you a much more consistent sample set.

Here’s an interesting result that I had on one of my tests. Right out of the gate, it looked like I a had 10% lift. After a week of running the test, the test does a COMPLETE flip-flop on me and becomes a 10% loser (at 99% certainty too):

Flip Flip A/B Test

One of my sneaking suspicions is that most of the 250% lift case studies floating around the interwebs are just tests that had extreme results in the first few days. And if they had ran a bit longer, they would have come down to a modest gain. Some of them would even flip-flop into losers. But because people declare winners too soon, they run around on Twitter declaring victory.

Rule 4: Only launch variants at 99% statistical significance

Wait, 99%? What happened to 95%?

If you’ve done an A/B test, you’ve probably run across the recommendation that you should wait until you hit 95% significance. That way, you’ll only pick a false winner 1 out of every 20 tests. And none of us want to pick losers so we typically follow this advice.

You’ve run a bunch of A/B tests. You find a bunch of wins. You’re proud of those wins. You feel a giant, happy A/B testing bubble of pride.

Well, I’m going to pop your A/B testing bubble of pride.

Your results didn’t mean anything. You picked a lot more losers than just 1 in 20. Sorry.

Let’s back up a minute. Where does the 95% statistical significance rule come from?

Dig up any academic or scientific journal that that has quantitative research and you’ll find 95% statistical significance everywhere. It’s the golden standard.

When marketers started running tests, it was a smart move to use this same standard to see if our data actually told us anything. But we forgot a key piece along the way.

See, you can’t just run a measure of statistical confidence on your test after it’s running. You need to determine your sample size first. We do this by deciding the minimal improvement that we want to detect. Something like 5% or 10%. Then we can figure out the statistical power needed and from there, determine our sample size. Confused yet? Yeah, you kind of need to know some statistics to do this stuff. I need to look it up in a textbook each time it comes up.

So what happens if we skip all the fancy shmancy stats stuff and just run tests to 95% confidence without worrying about it? You come up with false positives WAY more frequently than just 1 out of 20 tests.

Here’s an example test I ran. In the first two days, we got a 58.7% increase in conversions at 97.7% confidence:

Chasing Statistical Significance with A/B Tests - 2 Day Results

That’s more than good enough for most marketers. Most people I know would have called it a winner, launched it, and moved on.

Now let’s fast-forward 1 week. That giant 58.7% win? Gone. We’re at a 17.4% with only 92% confidence:

Chasing Statistical Significance with A/B Tests - 1 Week Results

And the results after 4 weeks? Down to a 11.7% win at 95.7% certainty. We’ve gone from a major win to a marginal win in a couple of weeks. It might stabilize here. It might not.

Chasing Statistical Significance with A/B Tests - 4 Week Results

We have tests popping in and out of significance as they collect data. This is why determining your required sample size is so important. You want to make sure that a test doesn’t trick you early on.

But Lars! It still looks like a winner even if it’s a small winner! Shouldn’t we still launch it? There are two problems with launching early:

  1. There’s no guarantee that it would have turned out a winner in the long run. If we had kept running the test, it might have dropped even further. And every once in awhile, it’ll flip-flop on you to become a loser. Then we’ve lost hard-earned wins from previous winners.
  2. We would have vastly over-inflated the expected impact on the business. A 60% win moves mountains. They crush your metrics and eat board decks for breakfast. 11% wins, on the other hand, have a much gentler impact on your growth. They give your metrics a soothing spa package and nudge them a bit in the right direction. Calling that early win at 60% gets the whole team way too excited. Those same hopes and dreams get crushed in the coming weeks when growth is far more modest. Do that too many times and people stop trusting A/B test results. They’ll also take the wrong lessons from it and start focusing on elements that don’t have a real impact on the business.

So what do we do if 95% statistical significance is unreliable?

There’s an easier way to do all this.

While I was at Kissmetrics, I worked with our Growth Engineer, Will Kurt, at the time. He’s a wicked smart guy that runs his own statistics blog now.

We modeled out a bunch of A/B testing strategies over the long term. There’s a blog post that goes over all our data and I also did a webinar on it. How does a super disciplined academic research strategy compare to the fast and lose 95% online marketing strategy? What if we bump it to 99% statistical significance instead?

We discovered that you’d get very similar results over the long term if you just used a 99% statistical significance rule. It’s just as reliable as the academic research strategy without needed to do the heavy stats work for each test. And using 95% statistical significance without a required sample size isn’t as reliable as most people think it is.

The 99% rule is the cornerstone of my A/B testing strategy. I only make changes at 99% statistical significance. Any less than that and I don’t change the control. This reduces the odds of launching false winners to a more manageable level and allows us to stack wins on top of each other without accidentally negating our wins with a bad variant.

Rule 5: If a test drops below a 10% lift, kill it.

Great, we’re now waiting for 99% certainty on all our tests.

Doesn’t that dramatically increase the time it takes to run all our tests? Indeed it does.

Which is why this is my first kill rule.

Again, we care about the whole system here. We’re cycling to find the winners. So we can’t just let a 2-5% test run for 6 months.

What would you rather have?

  • A confirmed 5% winner that took 6 months to reach
  • A 20% winner after cycling through 6-12 tests in that same 6 month period

To hell with that 5% win, give me the 20%!

So the longer we let a test run, the higher that our opportunity costs start to stack up. If we wait too long, we’re forgoing serious wins that we could of found by launching other tests.

If a test drops below a 10% lift, it’s now too small to matter. Kill it. Shut it down and move on to your next test.

What if we have a 8% projected win at 96% certainty? It’s SO close! Or what if we have enough data to find 5% wins quickly?

Then we ask ourselves one very simple question: will this test hit certainty within 30 days? If you’re 2 weeks into the test and close to 99% certainty, let it run a bit longer. I do this myself.

What happens at day 30? That leads us to our next kill rule.

Rule 6: If no winner after 1 month, kill it.

Chasing A/B test wins can be addictive. JUST. ONE. MORE. DAY. OF. DATA.

We’re emotionally invested in our idea. We love the new page that we just launched. And IT’S SO CLOSE TO WINNING. Just let it run a bit longer? PLEEEEEASE?

I get it, each of these tests becomes a personal pet project. And it’s heartbreaking to give up on it.

If you have a test that’s trending towards a win, let it keep going for the moment. But we have to cut ourselves off at some point. The problem is that a many of these “small-win” tests are mirages. First they look like 15% wins. Then 10%. Then 5%. Then 2%. The more data you collect, the more that the variant converges with your control.

CUT YOURSELF OFF. We need a rule that keeps our emotions in check. You gotta do it. Kill that flop of a test and move on to your next idea.

That’s why I have a 30-day kill rule. If the variant doesn’t hit 99% certainty by day 30, we kill it. Even if it’s at 98%, we shut it down on the spot and move on.

Rule 7: Build your next test while waiting for your data

Cycling through tests as fast as we can is the name of the game. We need to keep our testing pipeline STACKED.

There should be absolutely NO downtime between tests. How long does it take you to build a new variant? Starting with the initial idea, how long until it goes live? 2 weeks? 3 weeks? Maybe even an entire month?

If you wait to start on the next test until the current test is finished, you’ve wasted enough data for 1-2 other tests. That’s 1-2 other chances that you could of found that 20% win to stack on top of your other wins.

Do not waste data. Keep those tests running at full speed.

As soon as one test comes down, the next test goes up. Every time.

Yes, you’ll need to get a team in place to dedicate to A/B tests. This is not a trivial amount of work. You’ll be launching A/B tests full time. And your team will need to be moving at full-speed without any barriers.

If it were easy, every one would be doing it.

Follow All 7 A/B Testing Rules to Consistently Drive Conversion Up and to the Right

Follow the system with disciple and it’s a matter of time before you double or triple your conversion rates. The longer that you play, the more likely you’ll win.

Here are all the rules in one spot:

  1. Above all else, the control stands
  2. Get 2000+ people through the test within 30 days
  3. Always wait at least a week
  4. Only launch variants at 99% certainty
  5. If a test drops below a 10% lift, kill it.
  6. If no winner after 1 month, kill it.
  7. Build your next test while waiting for your data

I’ll show you how to build an unstoppable growth machine. Don’t miss any of my new essays.

I don\’t share emails. Ever. And your trust means a great deal to me.

{ 16 comments… add one }
  • Alex Chaidaroglou October 1, 2015, 1:03 pm

    Another great article Lars, thanks for sharing these rules.

    On point 7, do you mean to set up copy, landing page or whatever is needed for the next test while the current is running?

    You could A/B test for example the homepage and a whitepaper optin landing page at the same time, right?

    Thanks in advance!

    • Lars Lofgren October 1, 2015, 6:22 pm

      While your current test is running on an asset (like your homepage), get the next test ready for that same asset (the homepage). 🙂

      And yes, test different assets at the same time (like a homepage and landing page).

  • Alex Chaidaroglou October 2, 2015, 9:29 am

    Thanks a lot for the reply Lars, much appreciated!

  • Arend Zwaneveld October 24, 2015, 9:13 am

    Hi Lars,

    I like your article and agree on most of it.
    However, I find it somewhat disturbing that you refer to significance levels as if they are probabilities that the variation is the winner. For example, you say that in some cases you “decide to keep a test running just a bit longer”. This is a big statististics nono, introduced by AB-testing tools that suggest there exists something like a “running significance level”.

    Indeed, this distinction requires reading up on a lot of statistics… something I have done. 🙂 The result is a series of significance calculators at

    Check out the bayesian probability calculator and tell me what you think!

    • Lars Lofgren October 27, 2015, 6:04 pm

      Thanks Arend!

      I completely agree, there’s a lot of nuance that get’s lost in our tools and how we talk about testing.

      But in some cases, that’s actually a good thing. Here’s an example: saying a variation had a 12% lift in conversions that hit certainty. This is another completely incorrect statement. Even if you’re using a bayesian probability calculator, there’s a range of outcomes for the conversion rate. Your conversions might land anywhere between -2% to 26% after you launch.

      Should we stop saying “12% lift in conversions” and always state the probable range? I don’t believe so.

      I’m actually far MORE concerned about how testing is perceived and acted on across the company than I am being 100% factual. If I make my testing too complicated, people tune it out, don’t understand it, and stop trusting it. Then I have a horrible time getting support to launch my variations and approvals for the resources I need to run a full testing program. You and I both know all the nuances and uncertainties in this data. But no one else on the team does, nor do they care.

      I’d much rather be 90% factual instead of the full 100% if it means I can gain the trust of the entire team. Like many fields (finance, fitness, etc), the psychology of your users is more important than being 100% accurate.

      • Arend Zwaneveld November 2, 2015, 4:16 pm

        Hi Lars,

        I agree that there are situations where a white lie is acceptable.
        Personally I believe that stating wether a test result is significant is not one of them. Especially not when the information that is omitted is not fully understood in the first place.

        I agree that communicating test results with a significance interval is often “unacceptable” in business situations, simply because in practice the intervals are so “huge”.

        The only thing the business wants to know is whether the variation beats the control, and with how much certainty. A standard ‘frequentist’ analysis does not answer this question. If you look at the calculator at you’ll find that it does not communicate the results as an interval, but in “probabilities that the variation is x% better than the control.

        Let me know wether you agree this method is both scientifically correct and easy to communicate with the business.

        • Lars Lofgren November 22, 2015, 10:02 pm

          I personally prefer bayesian statistics when calculating certainty. That’s how we were doing it at Kissmetrics when we built our A/B testing program.

          But when it comes to building a growth team and embedding A/B testing across an entire company. It’s pretty low on my list of priorities. I’d much rather focus on:

          • Getting the leadership team and and key folks across the company to support regular testing
          • Improving the quality of hypotheses through high quality brainstorming and deep customer research
          • Building a culture of disciple that’s willing to walk away from a failed test instead of finding excuses to launch a pet project regardless of data
          • Focusing on elements and levers that actually have the ability to impact the trajectory of the company
          • Having a testing system that designers, marketers, and engineers can easily run without me personally involved in every test

          If I fail on any of these, it doesn’t matter if I have a frequentist or bayesian calculator. And my 99% rule gives my team the testing discipline we need in order for us to make decisions without making things too complicated. In other words, frequentist vs bayesian is just too low on my priority list to worry about. It’s a nice-to-have, not a need-to-have.

          To change my mind, I’d have to see analysis that using bayesian vs frequentist statistics materially impacts the quality of decisions and conversions over the long term. The impact would have to be large enough that it’d be worth the effort to instill that habit into a given team. Everything has an opportunity cost.

  • Dave Nevogt January 27, 2017, 3:26 pm

    Hi Lars, thanks for writing this. It’s a very good article on how to get in the right mindset. We’ve designed an AB platform at our business and have made the mistake of spending time testing small things. So the big winners is something that we’ve adopted – it was good to hear you back that up.

    I have a question regarding goals… Should we be setting up tests to measure activation like inviting users, viewing certain pages that have high relevance to our main conversion point (becoming a paid customer) – OR should the primary goal of all tests be the end result (the paid customer). We can test more at a time if we are testing for the activation points. Thank you!

    • Lars Lofgren January 29, 2017, 7:57 pm

      Thanks Dave!

      The more A/B tests I run, the more I prefer to test all the way down the funnel. I’ve run into too many situations where we get amazing wins at the top of the funnel that don’t have any impact on revenue. For example, we used this framework to optimize FB ads and landing pages. We cut our cost per lead from $10 to just over $2. And the impact on revenue or profitability for our paid leads? Zero, it didn’t move an inch. I will run tests concurrently on different assets for each step of the funnel but all of them are measured against the end goal of that funnel.

      • Dave Nevogt February 1, 2017, 9:51 pm

        Good deal, thank you for the response, this is the same answer I’ve gotten from everyone I’ve asked now. Again, I appreciate you writing this, it was a good one!

  • Georgi Georgiev June 27, 2017, 10:56 am

    Hi Lars,

    You seem to share anecdotes in which you do repeated measures of statistical significance, but it is not clear what kind of sequential testing procedure you are using. I hope you were not the victim of “peeking” or “waiting for significance”, which is quite a common occurrence, frankly.

    Then, I have to ask why such arbitrary rules such as #5 and #6? There are proper methods to design a test so it has a given statistical power, or to have a proper futility stopping rule in a sequential test. Also, rule #5 will certainly suffer from the peeking problem, equivalent to the issue for statistical significance calculations, but with regards to statistical power. How do you address that? Aren’t you missing too many true winners by applying this rule, thus wasting resources in developing, deploying and analyzing tests?


    • Lars Lofgren June 29, 2017, 11:23 pm

      Great questions Georgi.

      First, I don’t worry about peeking. I know this is heresy but when you’re working with a team of marketers, designers, and engineers, everyone peeks. Even if you tell them not to. Even if they all agree it’s a waste of time and can lead to poor decisions. Every single person peeks. Even I peek when it’s a test that I’m personally invested in. The curiosity is too hard to resist.

      Instead of trying to build an A/B testing playbook that’s theoretically perfect but no one can follow, I’d rather build a playbook that’s 90% of what I need that most teams can follow.

      It’s the same problem with using statistical power on most teams. I have never met a marketer than could calculate it, let along one disciplined enough to use it on every test. I even run into regular problems with people applying statistical significance in horribly incorrect ways. Why fight human nature when you can use a different set of rules that work almost as well?

      That’s what my rules of A/B testing shoot for.

      Rules 5 and 6 are about keeping the speed of testing up. This isn’t an academic research scenario where we’re pursuing conclusive answers. For elements that have a minor impact, we don’t care about knowing the real truth and determining a winner. We’re here to grow the business. And to do that, I’d rather cycle through 20-30 tests as quickly as I can, filter out a couple of real winners that improve conversions by 20-30%, stack them on top of each other, and permanently lift that step of the funnel by a large amount. Impact is more important that finding the true winner with every experiment.

  • Georgi Georgiev June 30, 2017, 1:23 pm

    Thank you for the response, Lars.

    It’s actually a great one and confirms what I’ve been noticing happen in marketing, CRO, landing page optimization etc. over the past few years. I’d argue it’s mostly due to marketers and a/b testing “experts” recommending and trying to use statistical methods, developed in the 1930s for testing crops, in online A/B testing. The two are completely different scenarios calling for completely different approaches. The problem with peeking on both significance and power is that it is much worse than “90% of what I need that most teams can follow”. The actual uncertainty is multiple times worse than the nominal reported one, leading to illusory findings that later fail to materialize in the business bottom-line.

    That’s why I’ve spent the past 3 years looking for a solution that is both theoretically perfect and that anyone can follow, with very little training. I looked for an area where the business case is the same as in A/B testing, but where they have good statistical methods. Thus I took methods from medical trials and packaged them in a way which is easy to use, but which doesn’t sacrifice the quality of error-control. Amazingly, it also leads to much more efficient tests, up to 80% more-efficient, when compared to fixed-sample size tests with the same parameters.

    As a product it is available here , while as a methodology that anyone can adopt I’ve described in a white paper: “Efficient A/B Testing in Conversion Rate Optimization: The AGILE Statistical Method”, available for free.

    To address this point of yours: “This isn’t an academic research scenario where we’re pursuing conclusive answers…. I’d rather cycle through 20-30 tests as quickly as I can…”. I see no conflict in doing many tests quickly and doing them properly. Statistical analysis is just so you can measure the uncertainty, not to slow you down.

    There is no need to differentiate between “academic” and “business”, since if someone is asking a question about a phenomena that can be measured objectively, there is just one method to measure it that also comes with control of the uncertainty associated to that measure. Some academic disciplines might require a lower uncertainty, attached to their application of the method, but not necessarily (I can think of many businesses where uncertainty is much less tolerated than most of academia!).

    “Conclusive answers” is a relative term, as every test can require a different level of confidence in the results before one is happy to accept them and move on as if they are “conclusive” enough. 50% or 60% actual statistical significance might be OK in some scenarios, so can 60% actual power (the fact that I can’t think of many doesn’t change that).

    “Impact is more important than finding the true winner with every experiment.” – I’ve heard it from other people before, but I can’t see how these two are contradictory. “True winner” is a relative term, so is “Impact”. You can just design the test in a way that “a true winner” is equal to a “true impactful winner”, and you’re all good. In fact, that is what is usually recommended – that the minimum detectable effect is to be set as the minimum effect you’d be happy to detect with the desired levels of certainty.

    Let me know if you have a chance to examine the AGILE A/B testing method: I’d be happy to discuss how it can be made even more accessible.

  • nick January 26, 2018, 8:08 pm

    Hey Lars, any rules you have for killing tests early, other than the 10% rule? For example, if a new test is showing a 40% decrease at 97% confidence after only 3 days… would you kill it, let it keep bleeding out, or at the very least allocate less traffic to the test?

    • Lars Lofgren January 29, 2018, 4:57 pm

      If something really tanks your funnel and you can already feel it when looking at your weekly/monthly metrics, it’s safe to kill it before the end of the first week or before you hit 99% statistical significance. We do that occasionally when we come across a test like that.

  • Naman Modi December 13, 2018, 8:03 am

    Thanks for the useful steps. I usually pay good attention to how actively plugin is maintained & how often plugin supports questions are answered in the forum.


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