Avoiding Bias in Conversion Rate Optimization

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Conversion rate optimization (CRO) refers to the systematic process of increasing the percentage of website visitors who take a desired action – things like making a purchase, filling out a form, or subscribing to a newsletter. For online businesses, CRO plays a critical role because it can directly affect the bottom line by improving the number of conversions and revenue. 

However, there are a number of factors that contribute to the effectiveness of CRO initiatives. Biases that can occur throughout various stages of the process – data collection, analysis, decision-making, etc. – sometimes result in suboptimal outcomes or even harm the credibility and reputation of a business organization. 

In this article, we will discuss some of the practical strategies and techniques to identify and mitigate the occurrence of biases in conversion rate optimization efforts. We will offer insights into common biases that affect CRO, their impact on the decision-making process, and ultimately how to prevent them.

Understanding Validity Threats and Bias in CRO

Essentially, validity threats and bias are issues that can compromise the accuracy and reliability of certain CRO initiatives. Validity threats may include things like selection bias, measurement bias, and maturation bias, which can undermine the integrity of the results.

Bias can occur during any stage of CRO and can lead to incorrect conclusions, false assumptions, and decreased trust in the validity of results. Identifying and avoiding bias in CRO is necessary to ensure that the results can be trusted and utilized to make data-driven decisions that genuinely optimize website conversion rates. 

History Effect

The history effect is a kind of validity threat that happens when external factors influence the results of a CRO experiment. These external factors may include things like changes to the website or business, fluctuating market trends, and even global, socio political events that may impact consumer behavior. The history effect can lead to inaccurate conclusions about the effectiveness of an optimization technique, as the observed changes in conversion rates may be attributed to external variables beyond the control of the experiment. 

In order to identify and avoid the history effect in CRO experiments, organizations must closely monitor external factors that could impact the experiment. Ideally, external factors like changes to the website, marketing campaigns, and other internal initiatives could be temporarily paused to provide greater control during the experiment. 

Real-world examples of the history effect in CRO include a case in which a retailer implemented a new pricing strategy to increase conversion rates, but not long after the experiment began, a major competitor launched a big sale that drove the retailer’s conversion rates down. As a result, the observed changes in conversion rates could reasonably be attributed to the external event – the competitor’s sale – rather than the retailer’s new pricing strategy. 

Instrumentation Effect

The instrumentation effect is another type of validity threat that can occur when there are changes to the measurement tools used to collect data during a CRO experiment. These changes may affect how the data is being collected, the timing of data collection efforts, or alterations to the metrics used to measure the effectiveness of the intervention. The instrumentation effect can invalidate experimentation results, as changes in measurement tools may artificially inflate or deflate the observed changes in conversion rates. 

To circumvent the instrumentation effect in CRO experiments, organizations must utilize consistent measurement tools throughout the entire duration of the experiment, or carefully document any changes made to the measurement tools. It may also be helpful to utilize multiple metrics to measure the effectiveness of an intervention, as changes in one metric may not accurately reflect changes to overall performance. 

In the real world, the instrumentation effect can be difficult to identify, especially when there are multiple parties involved that require frequent communication. A business may change the way they track user behavior on their website during an experiment, which could lead to differences in the observed conversion rates. Another example could be a change in the timing of data collection efforts – like collecting data at different times of day or on different days of the week. This, while subtle, can also impact the observed conversion rates.

Selection Effect

The selection effect can occur when the sample of participants in a CRO experiment does not accurately reflect the overall population. This might happen if certain groups of users are more likely to participate in the experiment or if the sample is biased in another way. The selection effect can invalidate experiment results, as the observed changes in conversion rates may not be generalizable to the broader population of a business’s website visitors. 

To identify and thwart the selection effect in CRO experiments, a business must carefully consider all aspects of the sample selection process to ensure that the sample is representative of the overall population. This may involve utilizing random sampling techniques, setting inclusion and exclusion criteria, and collecting demographic data about participants. 

Selection effect occurs frequently in real-world applications, so it’s vital for organizations to remain vigilant and scrupulous. An e-commerce website, for instance, may test a new checkout process with users who had previously made a purchase on the site. This test would likely lead to biased results, as users who had not previously made a purchase on the website may have different behavior patterns during the checkout process. To avoid this, experiment runners should have included people who had never made a purchase on the website before in the experiment. 

Sample Distortion Effect (Statistical Regression)

Another type of validity threat is the sample distortion effect – also known as statistical regression. Sample distortion effect occurs when the sample of participants in a CRO experiment is not evenly distributed across different groups or categories. For example, if the sample is heavily skewed towards one particular demographic group, the observed changes in conversion rates will likely not be generalizable to other demographic groups. 

Sample distortion effect can sometimes occur when experimenters are on a deadline or only have access to certain test subjects. Business organizations must carefully consider sample compositions in order to ensure that it is evenly distributed across different groups. 

In the real world, sample distortion effect in CRO could involve a case where an e-commerce website tested a new product page layout with a sample that was heavily skewed toward a hyper localized geographic region, which would lead to biased results. Or perhaps a social media platform tested a new algorithm for content recommendations with a sample that represented only one particular age group. In both cases, the sample distortion effect would impact the validity of results and could lead to false conclusions about the overall effectiveness of the optimization strategy. 

Confirmation Bias

One of the most common cognitive biases among the general population is confirmation bias, which is when people tend to seek out information that confirms their existing beliefs (while simultaneously ignoring evidence that contradicts those beliefs). In CRO, confirmation bias can lead practitioners to interpret data in a way that supports their initial hypothesis or preferred optimization strategies, while ignoring data that might suggest alternative approaches.

Confirmation bias can lead to biased decision-making, and to avoid skewing experiment results, practitioners must approach data with an unbiased mindset and actively seek out contradictory evidence and be willing to revise strategies or approaches based on reliable data. 

Anchoring Bias

Anchoring bias occurs when people rely too much on the first piece of information they encounter when making a decision – in other words, they “anchor” themselves to this information. In the realm of CRO, anchoring bias can affect results, especially when practitioners fail to take a more comprehensive and balanced approach to the data analysis process. For example, if a CRO practitioner witnesses a particularly high or low conversion rate early on in the experiment, they might be “anchored” to that initial data point and overlook additional data that might suggest a different, more comprehensive conclusion. 

Framing Bias (Cognitive Bias in Design) 

Framing bias refers to a cognitive bias in which people’s decisions are influenced by the method in which the information is presented or framed to them. In a CRO context, framing bias would affect the way practitioners or users perceive and respond to certain data. For instance, if a product is framed as a “luxury” item, users might be more likely to purchase the item at a higher price point, even if the product is not worth that much. To avoid framing bias, CRO practitioners must be very mindful of how website elements are presented, and they must test different framing strategies to determine how user behavior is affected. 

Recency Bias

Finally, recency bias is when people give more credence to recent events or information when making decisions. In CRO, recency bias may lead practitioners to overemphasize recent data points when making optimization strategy decisions. For example, if a CRO practitioner observes a sudden increase or decrease in conversion rates, they may overemphasize that recent data while overlooking longer-term trends or the “bigger picture.” To avoid this cognitive bias, CRO practitioners must thoughtfully analyze data from multiple perspectives before making decisions about optimization strategies. 


To summarize, avoiding bias in CRO experiments is vital for ensuring the validity and accuracy of optimization strategies. CRO practitioners must be aware of common cognitive biases and take measures to ensure that biases do not impact experimentation. Practitioners can avoid bias in CRO experiments by thoughtfully designing experiments, selecting appropriate samples, randomizing certain treatments, testing multiple variations, and approaching the process with a data-driven mindset. 

By avoiding bias in CRO experimentation, practitioners can make better-informed decisions about optimization strategies and drive significant improvements in conversion rates. Business leaders must continually monitor for biases in CRO experiments and adjust strategies when necessary to ensure the best possible results. 

About The Author

Matthew Post

Matthew Post

Matthew Post has dedicated over two decades to building and optimizing websites. He has worked in-house for nationwide e-commerce companies and large local firms to increase customer engagement through conversion rate optimization and search engine optimization. His expertise covers both the development and growth of digital properties.