Basics of Research Process

Internal Validity in Research: Definition, Types, Threats & Examples

Internal validity
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Internal validity is a measure of trustworthiness in a research study. It boils down to the degree to which a study demonstrates a clear cause-and-effect relationship. In other words, the results of your study should truly depend on the variables you're examining, and not on other factors.

Whether you’re a student or an experienced researcher, you need to establish internal validity. This is an essential aspect of any study that can help you determine cause-and-effect relationship between variables. 

In this blog post, we'll explain:

Let’s embark on this research journey together with StudyCrumb!

What is Internal Validity: Definition

Internal validity refers to the degree to which a research study is free from systematic error or bias. It shows whether a study accurately demonstrates a causal relationship between the independent and dependent variable. In simpler terms, internal validity measures reliability of the research outcomes. 

Achieving high internal validity is crucial in scientific research, as it boosts the confidence in the findings. If a study is internally valid, the findings about a causal relationship are right. 

An internal validity in research can be compromised by various factors, ranging from experimenter bias to unmeasured confounding variables. For this reason, you must take steps to address potential risks before proceeding further.

Example of internal validity 

Imagine that a scientist wants to explore the influence of a new medication on cholesterol level. In this case, it is crucial to double-check that the medication is the only aspect that affects cholesterol levels. A researcher needs to exclude such factors as age, gender, diet, or exercise which may lead to inaccurate outcomes.

Why Is Internal Validity Important?

Strong internal validity is important for a number of reasons:

  • Ensures that study results are valid and reliable
  • Demonstrates how well research was conducted
  • Shows if no errors or flaws may have impacted your findings
  • Provides a basis for replication
  • Determines whether modifications in the independent variable have a direct impact on the changes in your dependent variable.

Overall, a study with high internal validity warrants that you have done your best to eliminate any internal factors which could alter the findings.

Importance of Internal Validity: Example

As an example, let's consider a study looking at how caffeine increases alertness. The independent variable in this case is the amount of caffeine consumed and the dependent variable is alertness. To maintain higher internal validity, researchers must control for any internal factors that may influence results such as sleep deprivation or other stimulants.

If a researcher only provides participants with coffee but not tea or soda which also contain some levels of caffeine, this would be seen as a limitation since a scientist does not take all sources of caffeine consumption into account.

How to Test for Internal Validity in Research?

Checking internal validity in research ensures that your outcomes are tried and tested. Here are some conditions that must be satisfied: 

1. Independent and dependent variables must be correlated and change simultaneously.

Suppose you are examining the effectiveness of a new medication for depression. All changes in the level of depression should occur concurrently with medication intake.

2. The response should follow the treatment, not the other way around. 

If the researcher measures cognitive function after administering the medication, it is difficult to know whether any observed changes are influenced by the supplement or other factors.

3. No other internal or external factors should affect the outcomes of your study.

To preserve good internal validity, researchers must take steps to control for extraneous variables that may have an impact on the study's outcome. These are factors like the participants' age, sex, and other health conditions. Researchers also should manipulate their independent variable (the medication) by giving it to some participants but not to others.

By fulfilling these 3 criteria, researchers can confidently attribute a cause-and-effect relationship to their findings.

How to Increase Internal Validity of a Study?

Here are some methods researchers can employ to enhance internal validity. 





This method allows to reduce bias in a study by omitting certain identifying information from a researcher and participants.

For example, if conducting a drug trial, researchers can ‘blind’ their participants so that they do not know which group is receiving the treatment or control.

Experimental manipulation

This tactic involves deliberately changing one factor of an experiment to observe its impact on results while keeping other factors constant.

A study exploring the effectiveness of an educational program for children would manipulate an educational program.

Random assignment

This method increases internal validity by randomly assigning participants to either the treatment group or control group. In turn, this will help eliminate any pre-existing variations between groups that could distort your conclusion.

If conducting a study on the impact of exercise on health, participants should be randomly assigned to either an exercise or sedentary group.

Random selection

This approach implies randomly selecting individuals from a population for inclusion in a study.

If participants in a study examining the link between smoking and lung cancer risk are randomly drawn from a pool of smokers, such study can be considered internally valid. The risk of lung cancer is measured after a period of time during which the participants smoked.

Rigorous research protocol

This process involves following a detailed plan such as conducting multiple trials and controlling for extraneous variables.

If you are testing a hypothesis whether cognitive behavioral therapy (CBT) has an impact on depression symptoms, you should strictly follow a procedure. Such protocol involves controlling for the participants' age and gender, administering multiple trials of CBT, and carefully assessing outcomes.

Internal replication

Make a repetition of an experiment within its own internal environment, usually with different participants. 

If a study on the effects of physical activity on academic performance was conducted in one school, the same experiment can be conducted to see if results are consistent.

By following these suggestions, scholars can ensure their revelations can be trusted.

>> Learn more: Difference Between Internal and External Validity

Internal Validity Threats and How to Reduce Them

There are numerous examples of threats to internal validity that can emerge and affect your results. 

Common risks to internal validity include:

  • Selection bias
  • Maturation
  • Testing effects
  • Instrumentation
  • Statistical regression.

Keep in mind that potential risks depend on the type of study being conducted – single-group or multi-group. Let's explore each potential risk and efficient ways to combat them.

Threats to Internal Validity in Single-Group Studies

A single-group study is a research design that integrates only one group of participants.

In single-group studies, the following types of internal validity threats can occur.





History threat to internal validity involves events that occur during the study and aren’t related to an independent variable but can affect a dependent variable.

You want to measure how a new teaching method impacts student performance. During your study, a major event occurs, such as a school shooting or a natural disaster, which affects all students' mental states, causing a change in their performance.


Participants naturally change over time, leading to internal validity concerns.

A study testing the effect of a new teaching curriculum on student performance is conducted over multiple weeks. Students are gradually exposed to more challenging materials as the study progresses, thus improving their performance at the end of an experiment due to maturation rather than the independent variable.


The testing threat to internal validity occurs when participants become more proficient at a task or test by taking it multiple times. This can lead to inflated scores or distorted results.

After multiple trials, participants become more familiar with completing tests used to measure their anxiety levels, thus improving their performance.


Changes in the measurement tools or procedures used during the study may affect a dependent variable.

If you are using a questionnaire to assess participants' stress levels, modified questions may affect the outcomes of your psychology research.

How to Minimize Threats to Internal Validity in Single-Group Research

If you want to make sure that your single-group research is on point, here are a few strategies to counter threats to internal validity:

  1. Using a control group to compare the effects of the intervention against no treatment or a placebo. This strategy allows researchers to determine whether the intervention itself caused the observed changes.
  2. Minimizing practice effects by randomly ordering tasks or tests. This strategy reduces the impact of learning or fatigue effects on the results and increases the accuracy of the measurements.
  3. Maintaining bigger sample sizes to reduce spurious results due to random chance.
  4. Monitoring participants throughout the experiment to make sure they remain engaged and won't drop out.

By using these strategies, researchers can dot their i's and cross their t's to prevent low internal validity in single-group research.

Threats to Internal Validity in Multi-Group Studies

Multi-group studies can be tricky to navigate, as there are several threats to internal validity that researchers need to watch out for. Here are some common ones, along with examples.




Selection bias

The selection of participants for the different groups in the study may not be completely random.

If a study on the effectiveness of a new drug only includes people who are already healthy, the results may not establish a causal link.

Regression to the mean

Regression to the mean is a statistical phenomenon where extreme results tend to balance out over time, like what goes up must come down.

If a basketball player scores an unusually high number of points in one game, their next game is likely to be less spectacular.

Social desirability

People may give answers that they think are socially desirable rather than the truth.

If you are studying attitudes towards climate change, participants may be more likely to report favorable views of green initiatives even if their true beliefs differ.

Attrition bias

Some participants are more likely to drop out of a study if they are in the control group, resulting in biased results.

In a study comparing two different teaching methods, participants in the control group may feel like they're not getting the same benefit as those in the experimental group and thus drop out more often.

How to Improve Internal Validity in Multi-Group Research

To maximize internal validity in multi-group research, you should:

  1. Use appropriate randomization techniques, such as simple randomization or block randomization, to ensure that participants are assigned to groups in an unbiased manner.
  2. Take pre-test and post-test measurements to control for individual differences.
  3. Apply a single-blind or double-blind design to eliminate experimenter bias. This boils down to eliminating any influence by your expectations or perceptions.
  4. Keep participants on board and engaged throughout the experiment.

With these methods, researchers can maintain internal validity and be sure that their results are as right as rain.

Internal Validity Examples

Internal validity can be seen in many aspects of qualitative research. Here are some examples:

Example 1

When studying the effectiveness of a new medication on migraine sufferers, make sure the outcomes aren't skewed by any external factors.

Example 2

In a study that investigates the relationship between smoking and lung cancer, researchers need to control for environmental exposure to pollutants. This will guarantee that any observed relationship isn't biased.

Example 3

While carrying out a survey on public opinion about climate change initiatives, formulate questions in an unbiased way. Determine whether a sample is representative of the population being studied.

Example 4

While assessing the impact of different parenting styles on child development, researchers must make sure that their results are not influenced by genetic factors.

Bottom Line on Internal Validity

Maintaining internal validity is critical in any type of research, whether it involves a single-group or multi-group study. 

By controlling external factors that could undermine the results, researchers can increase the accuracy and validity of their findings. Additionally, internal validity allows researchers to make causal inferences based on their results. 

By following the strategies outlined above and taking steps to mitigate threats, researchers can ensure that their results are meaningful, trustworthy, and stand the test of time.

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FAQ About Internal Validity of a Study

1. What is internal validity in research?

Internal validity in research shows whether a study precisely measures the causal relationship between variables. If it’s high, this means that there is a strong connection between independent and dependent variables. As a rule of thumb, internal validity is achieved by diminishing the influence of extraneous variables.

2. What does internal validity measure?

Internal validity allows to measure whether the relationship between independent and dependent variables is error-free. Simply put, it determines whether the observed effect is created by intervention or another factor.

3. What biases affect internal validity?

There are 3 types of biases that can weaken internal validity and lead to misleading outcomes:

  1. Experimenter bias: occurs when the researcher’s expectations disturb results.
  2. Selection bias: emerges when samples aren’t chosen randomly.
  3. Attrition bias: arises when participants drop out of a study.

For this reason, you need to keep these biases in mind when conducting research. 

4. What is the biggest threat to internal validity?

There is no single biggest threat to internal validity in research, as it depends on the specific study design and context. However, history, maturation, and selection bias are generally considered to be the most significant risks undermining results in many studies.

Article posted on:Apr 13, 2023
Article updated on:May 12, 2023


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