Confounding Variables – The Third Variable

28.01.23 Types of variables Time to read: 6min

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Confounding variables are influential factors in research methodology, especially, in relation to experiments, observations, or studies. Rogue confounding variables may seriously mislead researchers, and incorrect or distorted conclusions about the relationship(s) between two exclusive factors can bias results. Qualitative research design examines and mitigates potential confounding factors before they come into play. Guarding your study against being confounded improves internal validity, reliability, and replicability. Learn more about confounding variables in this article.

Confounding Variables – In a Nutshell

The following article covers:

  • What confounding variables are
  • How confounding variables relate
  • How to spot and use a confounding variable
  • Methods to limit confounding variables
  • Frequently asked questions about confounding variables

Definition: Confounding variables

Each confounding variable is a unique, independent factor closely correlated with the independent variable under examination. Confounding variables can arise from coincidences, unrepresentative study samples, and irreplicable or unusual test circumstances.

Confounding variables are detected logically by applying these rules:

  • Every extraneous confounding variable (Z) will always correlate with the independent variable (X) and have a causal relationship with the dependent variable (Y). They can be either qualitative or quantitative.
  • However, only the hidden confounding variable (Z) really influences dependent results (Y) – confounding them. Dual causatives are a possible yet distinct phenomenon.
  • A causal relationship between (X) and (Z) is sometimes (but not always) present.
  • That (X), (Y) and (Z) all connect and confound is often due to sheer coincidence, unexamined multi-stage causal relationships, or another unknown factor.

Finding a confounding variable is no reason to despair! Discovering a third causative or casual factor can easily demonstrate a null hypothesis or open up new areas for later research. Likewise, confounding variables aren’t always bad news. Using confounded examples to illustrate points can improve a study’s detail, scope, and nuance. Identifying and preventing known confounding variables from interfering with (dis)proving your hypothesis is still a proactive move.


A metastudy sets out to disprove a seemingly absurd tabloid newspaper hypothesis that ice cream sales (X) correlate positively with beachfront shark attack rates on humans (Y) within Australia. Logically, the null hypothesis should be easily provable.

However, our researchers examine past datasets and reach a shocking conclusion. Shark attacks (in hundreds) and ice cream sales (in thousands) map closely to each other on a recurring year-on-year curve. A plotted p-value analysis returns a statistically significant result of 0.4, suggesting a reasonably probable real correlation.

What on earth has happened?

Better context solves the mystery. By closely examining local climate data, researchers discovered that ice cream sales and shark attacks almost always peak simultaneously in the warmest months. A broader set correlation test of daily centigrade levels against both factors consistently returns better results of P=0.6~0.8.

Ambient weather is the confounding variable and the actual causative.

QED: More warmth draws more people towards Australian beaches and ice cream. The confounding variables, sunshine, and weather (Z), exhibit statistically demonstrable control over both variables and comfortably fit a better qualitative explanation.


The importance of confounding variables

Correctly identifying as many confounding variables as possible will improve your internal validity. By examining and testing evidence holistically for cause-and-effect relationships, you can construct a better probable model of how factors and variables interact in reality.


A pharmaceutical company wishes to test whether a pill can effectively lower blood pressure to resolve hypertension. Their scientists plan a biomedical trial.

However, human bodies and lives are complex. Many known and unknown extraneous variables can confound general results.

Previous research shows that nominal pressure levels cluster by level across age, stress, physical activity, and type of diet. Market researchers also point out that patients who are actively prescribed hypertension medication, are most probably an unrepresentative, extreme subset.

Considering these factors, the scientists alter their research design to avoid known confounding variables:

  • They acknowledge known confounding variables and create different, themed control groups via a detailed entry questionnaire and stratified randomization.
  • Potential subjects with known, severe confounding risks (e.g. those taking other medication, the chronically ill) are segmented to separate groups.
  • Stratified sampling and matched general population polling recruit a broader selection of representative test subjects.
  • Correlation testing post-trial analyses any remaining unsecured variables for unknown causatives.
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Methods of using confounding variables

If you want to eliminate confounding variables from your study, there are four main methods to do so. How and where to apply them depends on what you’re studying, the type of sample set used, the complexity of your research, and how many potentially confounding variables are present.

Restriction method

Applying strict restriction criteria unifies all test subjects in a group. Dataset homogeneity lowers the risks of unexpected correlations and casual relationships occurring. More than one restriction usually applies. By removing known and potential confounding variables, cause-and-effect becomes easier to establish.


A closely restricted medical trial group might limit subject participation by (banded) age, weight, height, and gender to eliminate extraneous confounding variables.

  • Easy and cheap to set up and run
  • Useful for established areas of research
  • Effort required to source useful participants
  • Restriction can hide confounding variables

Matching method

Matching replicates your initial experiment’s test group and reruns the process to see if the measurable results were meaningful (i.e., replicable) or a fluke. Matching can also help create broader and better representative population models (e.g., focus groups) via sampling. Matched groups are created by examining the original participants (or data points) and then identifying new ones that mimic them as closely as possible.


A medical study might look at twenty patient profiles and then hand-pick a selection of people (i.e., a panel) as close to the originals in age, weight, height, and gender makeup as possible. Some light variation between groups is usually allowed (and all but inevitable in most studies). Matching can be re-applied many times over to improve validity.

  • Useful for examining unpredictable populations with high variability
  • Ideal for large sample studies and repetitive lab tests
  • Creating effective matched sets can prove tricky
  • Replicates inherent confounding variables

Statistical control method

Statistical controls weight post-collection results to illustrate and remove the influence of confounding variables. Averaging demographic measurements to create a standard distribution can also highlight outliers, extraneous factors, and fluke results.

Statistical control relies on the imposition of hypothetical control variables. Control figures are constants that substitute a reliable base value for actual results. Our medical study might explore the effects of a regressive control value of 5’9” for height, set by measuring the mean of all participants.

  • Easy to set up and use with past data
  • Can explore hypothetical, base, and extreme scenarios
  • Regressions can’t remove all inherent confounding variables in collected data
  • Controls may obscure interesting phenomena

Randomization method

Randomization applies sample scrambling to set groups to ensure anomalies have less chance of forming. It’s excellent for studies with small, recurring participant blocs. Our medical trial would use it as a standard research design component.

Why? Confounding variables are often caused by studying unusually homogenous or heterogeneous groups. Randomization seeks to establish a golden average between all. By mixing up data points, confounding trends that might interfere are broken apart and scattered.

Control groups (i.e., homogenized, representative populations), selected groupings, and stratification can also refine randomization. These tangents allow researchers to examine and better define the effects of causatives.

  • Effective – Great for comparative studies
  • Randomization often catches unknown confounding variables
  • Only useful for treatment or variable groups
  • Must be applied thoroughly before starting
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Any factor, qualitative or quantitative, that might skew or falsify results by hiding the true causative in a studied cause-and-effect relationship between independent and dependent variables.

No. Correctly examined, acknowledged, and separated out, known confounding variables can add valuable context, improve validity, and enhance topical knowledge.

Yes. In rare cases, researchers might accidentally disregard a real, sought independent-dependent correlation (X-Y) and falsely credit an unrelated, spurious factor found with causation.