Observer Bias: Definition, Examples & Prevention

Observer bias is a type of experimenter bias that occurs when a researcher’s expectations, perspectives, opinions, or prejudices impact the results of an experiment. This type of research bias is also called detection bias or ascertainment bias.

This typically occurs when a researcher is aware of the purpose and hypotheses of a study and holds expectations about what will happen.

If a researcher is trying to find a particular result to support the hypothesis of the study or has a predetermined idea of what the results should be, they will have the incentive to twist the data to make them more in line with their predictions.

This bias occurs most often in observational studies or any type of research where measurements are taken and recorded manually. In observational studies, a researcher records behaviors or takes measurements from participants without trying to influence the outcome of the experiment.

Observational studies are used in a number of different research fields, most specifically medicine, psychology, behavioral science, and ethnography.

For Example

You are performing an observational study to investigate the effects of a new medication to treat nausea. Group A receives the actual treatment with the new medication, while group B receives a placebo.

The participants do not know which group they are a part of, but you – the researcher – do.

Unconsciously, you treat the two groups differently, framing questions more negatively towards Group B and commenting that those in Group A seem more energized and upbeat.

Impact of Observer Bias

Observer bias can result in misleading and unreliable results. A researcher’s biases and prejudices can affect data collection and observer interpretation, leading to results that fail to represent accurately what exists in reality.

It might also result in inaccurate data sets, misleading information, or biased treatment from researchers. 

Observer bias can damage scientific research and policy decisions and lead to negative outcomes for people involved in the research studies.

Why Observer Bias Can Happen 

Subjective Methods

Subjective research methods are those that involve some type of interpretation before you record the observations. Subjectivity refers to the way research is influenced by the perspectives, values, emotions, social experiences, and viewpoints of the researcher.

This could lead a researcher to record some observations as relevant while ignoring other equally important observations.

Even if a researcher is subconsciously primed to see only what they expect to observe, subjective research methods could lead to skewed conclusions. 

Objective Methods

Although objective research tends to be impartial and fact-based, observer bias might still influence studies that use more objective methods.

This is because researchers tend to interpret or record readings differently, skewing the results to be more in line with their predictions.

For example, when measuring blood pressure using a blood pressure monitor, a researcher might round up the blood pressure to the nearest whole number.

Or, due to familiarity with the procedures of measuring blood pressure, a researcher might be less careful when taking the measurements and thus record inaccurate results.

How to Minimize Observer Bias 


Blinding, or masking, ensures that the participants and researchers are all unsure of the goals of the study.

This will help eliminate some of the research expectations that come from knowing the study’s purpose so observers are less likely to be biased.

Additionally, in double-blind studies, neither the researchers nor the subjects know which treatments are being used or which group they belong to.

Random Assignment

Randomly assigning subjects to groups instead of choosing the subjects themselves will help minimize observer bias. 

Multiple Observers

Having multiple researchers involved in the research study will ensure that your data is consistent and make it less likely that one researcher’s biases will significantly affect the project’s outcome.

It can also be beneficial to use multiple data collection methods for the same observations to corroborate your findings and check that they line up with each other.

Train Observers

Before beginning a study, it is beneficial to train all observers in the procedures to make sure everyone collects and records data in exactly the same way.

This will eliminate any variation in how different observers report the same observation, keeping interrater reliability high and minimizing observer bias.

Standardized Procedures

It is important to create standardized procedures or protocols that are easy for all observers to follow.

You can record these procedures so that the researchers can refer back to them at any point in the research process.

Related Biases

Observer bias is closely related to several other types of research bias.

Observer-Expectancy Effect

The observer-expectancy effect occurs when a researcher’s cognitive bias causes them to subconsciously influence the results of their own study through interactions with participants.

Researchers might unconsciously or deliberately treat certain subjects differently, leading to unequal results between groups.

For example, a researcher might ask different questions or give different directions to one group and not another, or influence certain participants’ behavior by changing their body language, posture, tone of voice, or appearance in certain ways.

Actor-Observer Bias

Actor-observer bias is an attributional bias where a researcher attributes their own actions to external factors while attributing other people’s behaviors to internal causes.

This bias can help explain why we are inclined to blame others for things that happen, even when we would not blame ourselves for acting in the same way.

For example, if you perform poorly on a test, you might blame the result on external factors such as teacher bias or the questions being harder than usual.

However, if a classmate fails the same test, you might attribute their failure to a lack of intelligence or preparation. 

Hawthorne Effect

The Hawthorne effect refers to some participants’ tendency to work harder and perform better when they know they are being observed.

This effect also suggests that individuals may change their behavior due to the attention they are receiving from researchers rather than because of any manipulation of independent variables.

Experimenter Bias

Experimenter bias is any type of cognitive bias that occurs when experimenters allow their expectations to affect their interpretation of observations.

Experimenter bias typically refers to all types of biases from researchers that might influence a study, including observer bias, the observer-expectancy effect, actor-observer bias, and the Hawthorne effect.

When a researcher has a predetermined idea of the results of their study, they might conduct the study or record results in a way that confirms their theory.


What is the difference between observer bias and confirmation bias?

Observer bias is a type of experimenter bias where a researcher’s predetermined expectations, perspectives, opinions, or prejudices can impact the results of an experiment.

Confirmation bias is a type of cognitive bias that occurs when a researcher favors information or interprets findings to favor their existing beliefs.

Unlike observer bias which can be intentional in some instances, confirmation bias happens due to the natural way our brains work, so it cannot be eliminated. 

What is the difference between observer bias and the observer effect?

The observer effect in psychology is also known as the Hawthorne effect. It refers to how people change their behavior when they know they are being observed in a study.

Observer bias is a related term in the social sciences that refers to the error that results from an observer’s cognitive biases, particularly when observers overemphasize behavior they expect to find and fail to notice behavior they do not expect.

The observer effect is not to be confused with the observer-expectancy effect or the actor-observer bias, discussed above.

Further Reading

Burghardt, G. M., Bartmess‐LeVasseur, J. N., Browning, S. A., Morrison, K. E., Stec, C. L., Zachau, C. E., & Freeberg, T. M. (2012). Perspectives–minimizing observer bias in behavioral studies: a review and recommendationsEthology118(6), 511-517.

Hróbjartsson, A., Thomsen, A. S. S., Emanuelsson, F., Tendal, B., Hilden, J., Boutron, I., … & Brorson, S. (2013). Observer bias in randomized clinical trials with measurement scale outcomes: a systematic review of trials with both blinded and nonblinded assessorsCmaj185(4), E201-E211.

Salvia, J. A., & Meisel, C. J. (1980). Observer bias: A methodological consideration in special education research. The Journal of Special Education14(2), 261-270.

Saul Mcleod, PhD

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Educator, Researcher

Saul Mcleod, Ph.D., is a qualified psychology teacher with over 18 years experience of working in further and higher education.

Julia Simkus

Research Assistant at Princeton University

Undergraduate at Princeton University

Julia Simkus is a Psychology student at Princeton University. She will graduate in May of 2023 and go on to pursue her doctorate in Clinical Psychology.