Internal Validity – Definition, Threats and Examples

To understand research results, you must understand the basic ideas of research design. Internal validity is one of the most important of these ideas. It can be seen as the most important part of any research project because it sets the stage for meaningful, correct, and reliable results. Before discussing how complicated internal validity is, let’s first talk about what it is.

What is Internal Validity?

Internal Validity Definition

Internal validity is a term used in scientific research, especially in experimental design, to describe how much the results of a study can be attributed to the specific variables that the researcher has chosen to change, as opposed to other factors that could be influencing the results. It is the assurance that the effect seen in a study was caused by the experimental intervention and not by something else.

Let’s say a group of researchers is running an experiment to see how a new way of teaching math affects how well students do in math. A study with high internal validity would show that any changes in the students’ scores are caused by the teaching method and not by other things, like natural academic growth or the teacher paying more attention to the students during the experiment.

The idea of internal validity isn’t just for experimental studies; it can also be used for other research. For example, in survey research, internal validity is how well the survey questions measure what they are supposed to measure.

The Importance of Internal Validity

Internal validity is important in any research study for a number of reasons:

Causal Inference

In many studies, especially experimental designs, determining the relationships between causes and effects is the primary goal. Conclusions about these connections can be drawn with confidence thanks to internal validity, which guarantees that any effect seen is due solely to the independent variable under study and not to any confounding variables.

Reliability of Results

High internal validity in a study means the findings are reliable representations of the phenomenon under study. This strengthens the validity of the results, making it more likely that they will provide useful insights into the relevant research area.

Foundation for Further Research

When a study has a high level of internal validity, it creates a solid base for more research. Future studies can build on these valid results by looking at other parts of the phenomenon, improving theoretical models, or using the results in different situations or with diverse groups of people.

Informing Policies and Practices

Policies and practices are directly affected by research findings in many fields, such as medicine, psychology, and education. High internal validity ensures that these real-world applications are based on strong evidence, making it more likely that the interventions put into place will work.

Resource Utilization

Research takes time, money, and, often, people. High internal validity ensures that these resources aren’t wasted on making wrong or misleading results.

Ethical Considerations

From an ethical point of view, it’s very important to ensure that a study’s results are correct. When research results are used to make decisions that affect the real world, bad things can happen.

Enhancing Scientific Integrity

High internal validity helps keep the scientific process as a whole honest. It makes scientific findings more credible and trustworthy, which allows the public to have more faith in science.

Internal Validity Threats

Researchers try to get a high level of internal validity, but many things can get in the way. Most of the time, these risks come from how the research is set up, the factors of the participants, or other things that can’t be controlled. Here are some of the most common things that can hurt internal validity.

a. History

This refers to events occurring concurrently with the study that are unrelated to the experiment but have the potential to affect the results. For instance, a major global event or natural disaster occurring during the study could influence the responses or mental states of the participants, skewing the results.

b. Maturation

This threat comes from the fact that people change naturally as they age, grow, or develop throughout the study. For example, in a longitudinal study that looked at the effects of a new way of teaching over a year, any improvements in the students’ performance could have been caused by their natural learning process rather than by the new way of teaching.

c. Testing

This threat is also called the “test-retest” effect. It happens when taking a test affects how well you do on a later test. People might do better on a post-test because they are used to how the test is set up or because they remember the answers from the pre-test. This doesn’t necessarily mean that the intervention was a success.

d. Instrumentation

This threat happens when the results change because of changes in the measuring tools or the standards observers judge. During the study, an observer may change how they put certain behaviors into groups, or a measuring tool may lose its calibration, leading to different measurements.

e. Selection Bias

Selection bias happens when significant differences between the groups at the beginning of the study could affect the results. For example, in a study that compares the effects of two different teaching methods on two other classes, if one class has generally better students, the results may reflect this difference in initial performance rather than the effectiveness of the teaching methods.

f. Attrition

This is also called “experimental mortality” and happens when participants drop out of a study before it is over. Quitting because they didn’t like the study (for example, because it was too stressful) can make the results less accurate.

How Can These Threats Be Addressed?

Threats to internal validity must be addressed if researchers are to produce credible findings. Researchers can employ a number of methods to deal with these threats:


The randomization of study participants is a powerful method for maintaining the study’s internal validity. This helps to even out any differences that may already exist between groups, which lessens the risk of selection bias.


Counterbalancing is a method for minimizing the impact of order effects (a form of testing threat) in within-subjects designs where all conditions are presented to each participant. The order in which participants experience the conditions is changed to randomize any effects that may arise from the order of the conditions.

Control Groups

Threats to internal validity can be reduced through the use of control groups. The experimental group is given the treatment, while the control group is given no treatment. This facilitates an improved ability to attribute treatment-related differences when comparing outcomes between the two groups.

Repeated Measures

In longitudinal studies, it is important to account for the effects of maturation by taking multiple measurements at different times. Researchers can account for and adapt to natural trends by tracking changes.


The term “blinding” refers to concealing, or “blinding,” participants and/or researchers from knowledge of the experimental condition. The gold standard of experimental research typically consists of double-blind studies, in which neither the participants nor the researchers know the group assignments.

Pre-testing and Post-testing

Participants can be tested before and after the experiment to see if any changes occurred due to the treatment. Using a control group in conjunction with this is optimal.

Instrument Checks

Validity risks associated with instruments can be mitigated through routine maintenance and calibration. If human observers are used instead, they must also be taught to apply the observational criteria consistently.

Statistical Adjustment

Adjusting for potential confounding variables that could affect internal validity can be accomplished through statistical methods. Regression, matching, and analysis of covariance are just some of the methods that can account for such potential biases.

Addressing Attrition

To address attrition, researchers should closely monitor dropout rates, especially for longitudinal studies. Researchers can use statistical methods to account for attrition or implement strategies to keep participants engaged and reduce dropouts if dropout rates are high or vary across groups.

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Internal Validity Examples

Let’s look at real-world examples to understand the meaning of internal validity better.

i. Pharmaceutical Testing

Consider a study that wants to see how well a new drug treats a particular illness. Researchers would randomly put patients into two groups: a treatment group that gets the new drug and a control group that receives a fake drug. To ensure a lot of internal validity, the researchers would have to control for things like dosage, timing, how well patients took their medicine, and other health problems. If the people in the treatment group get better than those in the control group, and all other factors have been considered, researchers can confidently say that the new drug works. This shows that there is a lot of internal validity.

ii. Educational Intervention

Consider an experiment conducted by a group of educational psychologists who wish to examine the impact of an after-school mentoring program on students’ academic achievement. The students ‘ grades are collected before and after the program’s implementation. Students’ grades improve as a result of the program. The school implemented other changes during the same period, including reduced class size and improved teacher training. These additional modifications may have contributed to the students’ grade increases. This situation threatens internal validity because it is challenging to attribute the improvement in grades solely to the mentoring program.

iii. Marketing Research

An organization wants to know if a new advertising campaign helps sell more products. They put the campaign out in certain places and watch how sales change. If sales increase in the targeted areas, it might be tempting to think the advertising campaign is the only reason. But other things, like a general rise in market demand or the end of a competitor’s business, could have also affected sales. The study’s internal validity could be hurt if these things aren’t considered.

The Bottom Line

In conclusion, a study’s credibility rests on its internal validity. Researchers can all rely on the solid, reliable, and trustworthy results of an investigation with high internal validity. Several factors can undermine the internal validity of a study. Still, if researchers are aware of these dangers, they can design their experiments to reduce or eliminate them, supporting the reliability of their results.