Brand Search Incrementality Testing: a Guide for Performance Marketers

Here’s how to design and execute a brand search ad incrementality test to find out if you could save some of your budget.

Brand search ads tend to be cheap at a CPC level but the high volume of clicks for most large brands can result in a large spend. Incrementality testing is a great way of estimating the true value of your brand search campaigns, allowing you to make an informed decision about the amount of money you spend on this kind of marketing. For example, one brand we worked with had an annual spend of €500k for their brand ad campaign - through an incrementality test we found that only 11% of conversions were incremental and up to €445k could be saved through budget/bid decreases and exclusions of non-incremental audiences.

Hopefully you’re considering running a test on your brand search ads and are looking for some information to get you started. There are many approaches you could take, depending on the circumstances of your campaign. Information about each approach is spread out across multiple resources and these often use language that is heavily technical and not so accessible to those working in the marketing field. We’ve created this guide for marketers as a comprehensive overview of each of our recommended method. This will point you in the right direction on designing, executing and analysing a brand incrementality test. There are a number of approaches included here that work for a variety of businesses and requiring varying levels of technical ability. So whether you work purely in marketing, or have some experience with coding and statistics, at the end of this article you will have a good idea of the next steps to take for you.

Before starting

The goal of an incrementality test

Firstly, let’s clarify the goal of incrementality testing, and the output we aim to get.

The goal of these tests is to estimate the impact on conversions as a result of brand campaigns being active. This is usually represented by an average number of conversions gained (or lost) over a period (e.g. an hour or day) as well as the % of last click conversions that are incremental.

For example if a campaign reported 200 conversions per day, and the incrementality test found that the incremental value was 21.5 conversions on average, we could report this as:

  • Average incremental conversions per day: 21.5
  • Incrementality %: 10.75%

From this we could also calculate the cost per incremental conversion (CPiA). If the campaign spends £500 day the CPiA would be £23.25


The goal of an incrementality test is not to save money - although that may be the reason for carrying out the test, and an outcome of finding a low incrementality. Carrying out the test in an unbiased way is crucial. The goal is also not to speculate on why on why a campaign might be incremental or not, although it may provide some hints as to why that is the case

Which conversions should you look at?

Estimate the incrementality in purchases is usually the objective - as that the ultimate goal for most businesses. However you could do this for any conversion or micro-conversion. Conversions that happen in a short window are best to use with this testing as the impact on these is clearer. In some cases this means using bookings, trial optins or signups if a purchase is expected to happen more than 7 days after a click.

The volume of data is a key factor - if you have 100 purchases per day, a 10% uplift would be quite clear - however if you’re averaging 1 purchase a day it would be difficult to see changes in this metric. Using basket adds, signups or even traffic would add a lot of additional data if purchase data is sparse.

Which traffic should you look at?

One of the hardest, and most important parts of incremental testing is deciding on which traffic you want to look at. When testing the incrementality of brand search campaigns, you want to only be looking at users searching for the brand to get clear results - an uplift in Facebook Ad users when the campaign is switched off is probably unrelated and will obscure the results. There’s no magic way to look at these users and creating a best estimate is the only option.

You should start by looking at your brand search SERP to get an idea of all the routes a user can get to your website. Then you will need to filter the data you are looking at to just these users - for example in Google Analytics this could involve creating segments for each source.

Some common examples of traffic coming from brand searches could be:

  • Paid brand search ad campaigns (duh!)
  • Source = “google” + medium = “organic” + landing page =”/” (Organic brand listings)
  • Google My Business links

Links to social media pages often rank high for brand search terms. Organic social traffic is sometimes included in a brand incrementality test but I’d only recommend this if organic social activity is low - if you are regularly posting to a high reach then the impact of brand search users on traffic from this channel is likely to be low.

Methods of testing

When trying to determine any causality, a controlled experiment is seen as the ultimate strategy. This would involve being able to control all of the impacting variables so that you can be more sure that any change in your dependent variable is caused by your independent variable. The closest we can get to this in marketing is an AB test, in which an audience is randomly split and given a different treatment - and while there are factors you can’t control (time, demographic) the random allocation of users to a group gets us a reasonably effective test. For brand search incrementality we don’t have the privilege of running a controlled experiment of any sort, so the methods we need to use fall into a category called quasi-experiments. These are test designs that best deal with non-randomised non-controlled environments, in a way that minimises any bias as much as possible. It should be noted that while these can produce reliable results, they may be influenced by factors outside of the test more than a standard AB test.

Method 1: Switching off the campaign and seeing what happens

Switching off the campaign and seeing what happens Chart with performance of a campaign that has been turned off

This is by far the simplest way to estimate incrementality - and while it may not be the most reliable method on the list, for most brands this will still give you a hint at the value of the campaigns.

This method basically involves switching off the campaign for a period of time and observing the conversions before and during the test. You can choose any period of time, aiming to cover at least 50 conversions, but including at least a week is recommended to avoid weekday seasonality impacting the results. The test (both the campaign on and off periods) should be over a period in which there are as few other factors that may also impact conversions - pausing the campaign over Black Friday week could make the incrementality of the brand campaign seem negative.

There are two ways to analyse the results, requiring varying degrees of technical work:

  • On the simple side is simply comparing the conversion volumes in each period, with the incrementality of the campaigns being the 1 minus the % decrease in conversions while the campaign is off.

For example, if your brand search traffic has an average of 50 conversions per week, you could choose to switch the campaign off on Monday of one week, switching it back on the following Monday. The “campaign on” period would be the 7 days prior to the “campaign off” period. If you received 45 conversions during the “campaign on” period and 32 during the “campaign off” period you could estimate the incrementality as 13 conversions per week, or 28.89% of reported conversions.

Simple comparison A simple comparison of performance

  • On the more technical side is regression discontinuity design (in this case looking particularly at the RDD in time method). This involves creating a plot in which the y-axis is your target metric (e.g. conversions per day) and the x-axis is a period over time, and then calculating a line of best fit for the periods before and during the test period. The incrementality can be estimated as the difference between the lines just before and after the test is started. This method reduces the impact of confounding factors during the testing period compared to simply comparing the results.

RDD Regression Discontinuity Design

On the pros side, these methods are good because:

  • They’re easy to setup internally, and therefore cheap
  • They don’t require any sophisticated analysis
  • They work well for accounts with a low amount of data

These methods do have significant drawbacks though:

  • Having the test running over two completely different periods allows for a lot of other factors to impact the results
  • Avoiding any impacting factors changing on the website over that time period is very difficult - unless you’re confident in your ability to stop your eCommerce managers changing all the discount codes for at least two weeks
  • It’s not feasible to determine the significance accurately, so subtle impacts can’t be measured
  • It requires your brand campaign to be off for at least one week, which may cause changes in competitor behaviour

If you have a low volume of sales and no budget to run anything more reliable this should still be good place to start to help you make decisions about your brand search ads. If you don’t have these limitations I wouldn’t recommend using these types of test - although these could be used as a way of convincing management to run one of the more sophisticated tests.

Method 2: Geo test

Geo test Chart of campaign performance in a geo test

This method involves finding two distinct geolocations that have users with similar traits and exposing them to different conditions. This test is the one that best avoids the impact of different factors occurring at different times, as each side of the test is running consecutively. As far as brand search incrementality tests go, this method is probably the most desirable - but it does have some very strict limitations.

For this type of test to work in practice, all of these factors need to be true:

  • The campaign needs to be operating over a large enough area to have a significant number of conversions across multiple areas
  • There are no factors that could impact one geo over another (e.g. delivery speed, store density, booking capacity)

If these are met then you first need to find one control and one test geo. This isn’t as simple as saying “Manchester and Liverpool are good because close together” - the geos need to have correlated behaviours. There are a number of ways you can find these areas, from the less-technical such as finding areas with similar CTRs, CPC and conversion rates, to more technical solutions such as finding the multicollinearity between areas with variance inflation factors. The areas don’t need to be identical in terms of volume, but should have similar performance metrics. Creating a better match will reduce noise and improve the clarity of your results.

Once you have these it’s time to run the test - choose a period to test, and switch off the brand ads in your test geo. The test length depends on the volume and variance of data. Similar to the previous test type - I’d recommend testing in increments of 7 days to iron out any weekday seasonality impact. 14 days is often the sweet spot for most brands, but if your conversions are low (<50/week) then extending this to 28 days should give you enough data.

After the test has run you’ll have a dataset of conversions over time for your test and control geo. There are a couple of ways to extract incrementality from this - stretching from non-technical to technical. Using a more sophisticated approach can increases the reliability of your findings.

  • Difference in differences is the simplest of the three methods of analysis we’d recommend. This method assumes that while the volume of conversions could be different, the rate of change from before the test period to during the test period should be the same if there are no deliberate differences between each geo. With this method we look at the difference in conversions before and after the test has started for each geo, and then look at the difference between those two (hence the difference in differences).

DiD Difference in differences

  • Causal impact is particularly reliable method in which a counterfactual is created for the test geo. In essence, this method involves using the control geos trend to create a hypothetical version of the test geo, in which nothing changed. The difference between the factual and counterfactual test group results is the estimate of the impact of the brand search ads.

Causal impact Causal impact

  • Synthetic control is a method similar to causal impact, in that you’re comparing the test groups factual vs counterfactual date. The difference here is that with synthetic control only the data before the test period is used to calculate the counterfactual, whereas with causal impact data during the period is used too. This means you can be sure that any impact the test has on the control group is removed, but it does require a bit more data, often requiring more control groups.

Synthetic control Synthetic control

Method 3: On-off

On-off A campaign with an on-off test

The last method we’ll look into can be called an on-off test. In this design we flick a campaign on and off over a period of time to gather many data points with the campaign on and off as possible - often this will be mean having the campaign on for one hour, off for the next hour and so on, often for a distinct period through the day. Here we are only aiming to estimate the short term impact - anything outside of the one hour periods where the campaign is off isn’t looked at.

This type of test is effective in that it can operate in a situation where the above two methods aren’t feasible:

  • If there isn’t a significant amount of data across multiple geos, or if there are factors that mean geos perform differently from each other
  • If the campaign can’t be switched off fully, or if a significant loss in conversions has to be avoided

Tests with this design spend less time in a test period than the other designs so may need to be run over a longer period - however any loss in conversions is minimised (in the case where the campaigns have a high incrementality). The impact on impression share is also lower, making it less likely that a competitor will change their behaviour as a result of the test.

Once the test has completed, you will have a dataset of conversions for periods in which a campaign is on and off. There are a couple of ways to analyse this:

  • Basic comparison between the periods where a campaign is on and off. If you have covered enough periods throughout the day to avoid the impact of hour of day seasonality, simply comparing the conversions when a campaign is on and off can give you a good idea of the incrementality. While this may be biased by factors outside the campaign’s status, this could give you a quick idea of the performance if you need to make the decision within a test to continue or not.

Basic comparison Basic comparison of results

  • Interpolation can be used to create a counterfactual of the periods in which the campaign is switched off. This involves using linear algebra to **“fill in the gaps” **in the datapoints for which the campaign is active, and using these datapoints as counterfactuals to the datapoints where the campaign is paused. This allows relatively effective counterfactuals to be created without the need for a separated control group.

Interpolation Interpolation

  • Regression discontinuity design in time can be used here as well - although here we would be looking at the difference across multiple points in which the test condition is applied. Using this analysis works best when the test periods are longer than 1 hour so that there are sufficient datapoints to create a line of best fit.

RDD Regression discontinuity design

Next steps

By this point in the article you should have a good idea of which kind of test will work for your circumstances. Our aim with this article was to give you an overview into each method of testing - we’ve purposefully avoided adding practical guides to the more technical analyses on here as this would lead to an unreasonably long post. There are many ways to execute tests with each of the designs, both for technical and non-technical marketers. Practical guides and code examples for the above exist and are updated regularly so I encourage you to Google for these terms to find the latest approaches.

If you want to get a head start on any of these methods by having a test set up for you, we offer all of the test design, execution and analyses as a service. We are also building an app to enable fast and effective geo and on-off brand incrementality test setup and execution - join our waitlist for more details.

Related Articles

Join our app waitlist

Our app is currently in a closed beta. To ensure the quality of each test, we’re opening our test to just a handful of users at a time. Signup using the link below to join the waitlist and get more info

Join the waitlist