Basics of Research Process

Internal vs. External Validity: Key Differences to Know

Internal vs External Validity
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When conducting scientific research, it is important to consider validity. It is the compliance of the experimental base which was chosen by a research paper author with generally accepted standards. It also makes it possible to achieve impeccable results in a search for a solution to a problem. 

Compliance with the results of experiments and the goal of a study is determined by the degree of validity (or efficacy, in some sources). At the same time, there is internal and external validity. It begins to sound complicated, isn’t it? But don’t worry! In this article, you will find out what are both internal and external validity. Also you will learn all key differences between internal validity and external validity so that you never mix these concepts up again. Stay with our research paper writing service to nail this topic.

Internal Validity vs. External Validity: Definition 

So, validity is one of the most important characteristics of high-quality scientific research. Every relevant scientific work means that it is reliable. A validity shows its compliance with academic standards, reasonability, and usefulness. 

Let us explain it with a simple example. When you are writing a research paper, you have academic writing rules or standards, plus some requirements of your professor. When you conduct research itself, you also have specific requirements and standards that allow you to get a useful and meaningful result. 

During any study, there is a need to carry out practical work the results of which are subject to evaluation. The result is always correlated with such a concept as a flawless experiment. If the matches are as high as possible, the degree of efficacy is also considered high.

Also, there is internal and external validity. Below you will find out their definitions.

Internal validity is an indicator that reflects the reliability of a conclusion that was obtained after a series of real experimental studies. This is the main requirement that is put forward to each result of an experiment.

External validity is the reliability of a result of a study compared to the results of experiments that are aimed at full compliance with an “ideal” result.

Internal Validity vs External Validity

Characteristics of Internal and External Validity 

What about the key characteristics of both internal validity and external validity? Let’s delve deeper into their traits as this will help you realize their importance in research. 

Internal Validity Features

The characteristics of internal validity play a crucial role in scientific research as they enable researchers to establish the extent to which their study can demonstrate a causal relationship between independent and dependent variables.

Key features include:

  • Control groups: rule out alternative explanations for observed effects.
  • Randomization: ensures that the groups are equivalent in all respects except for an impact of an independent variable.
  • Manipulation checks: help to verify that the independent variable has been successfully manipulated in the intended manner.
  • Blinding: hides certain aspects of a study from participants and/or researchers to reduce the risk of bias.
  • Valid and reliable rates: ensure that observed effects aren’t influenced by measurement error or other factors.
  • Replication: rules out alternative explanations and increases the efficacy of received results.

>> Read more: Difference Between Validity and Reliability

External Validity Features 

Regarding characteristics of external efficacy, these help establish generalizability of research results for other groups, conditions, and situations. External validity refers to the degree to which the results of a study can be generalized outside a specific context of a study.

Some of the key features that help establish external efficacy include:

  • Random sampling: ensures that study participants represent a broader population of interest.
  • Multiple tools: help to assess the generalizability of results to different settings.
  • Interventions: are similar to those used in our real world.

Internal vs. External Validity Compromise 

There is a need to realize that external validity is often a compromise with internal one. Studies that are strictly controlled and have high internal efficacy may be less applicable to real-world settings. While studies that are more environmentally valid may have weaker internal validity. Researchers must balance these factors to achieve both external and internal validity.

Trade-off Between Internal and External Validity

It is also important to know that a trade off of internal and external efficacy is one of those factors which can reduce the credibility of every result of a study. There is a need for assessing internal and external validity.

During some studies, researchers may lose some control over the variables that affect the course of their study. Therefore, the risk of a possible complication reduces the internal one. However such research usually has high external efficacy, since existing conditions are closer to real-life situations and everyday problems, and lower in opposed cases.

So, there is a need to control such threats while designing a research study. You also have to ensure that the results are trustworthy and can be generalized without a negative correlation between validity.

>> Read more: What Is the Control in an Experiment?

Example of External vs. Internal Validity Relationship

Here’s a good example that shows a trade-off relationship between these 2 types of validity. 

An example of the relationship between internal validity and external validity can be a study that aims to test how effective a new drug is in a specific disease. In such a case, internal validity would refer to the extent to which study results are accurate and can be attributed to the intervention being tested. External validity can be ensured by including a diverse sample of participants from different settings and geographical locations.

Thus, a study with a high internal validity but dropping external one may have good accuracy for one group and may not be accurate for other populations and settings.Conversely, a study with high external efficacy but low internal one may have generalizable results but may not accurately attribute effects to any intervention being tested.

Replication: Solution to Trade-Off 

What is replication? In simple words, it is used to increase the reliability in research. The essence of replication is that a study is repeated in different situations and subjects. It allows researchers to identify whether the main results of an original study can be applied in other circumstances. That is, it determines how applicable a study is to other settings.

In other words, this is a chance to test your experiment and confirm its reliability. Replication is important because it allows you to identify not only how effective your research is in different situations but also make sure of its practical application. For example, replication is important in medicine and psychology.

Similarities Between Internal Validity and External Validity  

Both these concepts are important in research methodology. Here are some key similarities between internal validity and external validity:

  1. Both concepts show the efficacy of your study results. Internal and external ones are related to ensuring the reliability of sny result of a study.
  2. They both depend on study design. Study design can affect both internal and external efficacy. For example, a poorly designed study may have low internal efficacy because it cannot control for extraneous variables or low external efficacy because the sample is not representative.
  3. Bias has an impact. Bias can threaten both internal and external credibility. For example, selection bias can threaten external efficacy if a given sample is not representative of the population. Experimenter bias can threaten internal validity if your expectations influence the results.
  4. They are important for generalizability. Internal validity ensures that the results are true in the context of the study, while external validity ensures that the results can be generalized in other contexts.

Differences Between Internal and External Validity 

Now, let’s find out what are key differences between internal and external validity so that you never feel confused when it comes to these terms. 

  • Internal validity focuses on the causal relationship between an independent variable and a dependent variable. Unlike internal one, external validity focuses on the generalization of results for other conditions.
  • Internal validity examines whether changes in one variable affect changes in another variable. External validity examines whether differences between samples or populations can be attributed to differences between those samples or populations; that is, whether it is suitable for another group.
  • Data collection difference. Internal efficacy depends heavily on experiments in which researchers manipulate variables and measure outcomes. External relies heavily on naturalistic observation, where researchers observe behavior without changing anything about it (for example, by observing the behavior of people at work).
Similarities and Differences Between Internal and External Validity

External Validity vs. Internal Validity Comparison Chart 

Now, check out an internal versus external validity comparison chart to get a clear and visual idea of how they differ, what goals they have and what threats they are prone to.





Reflects the reliability of the conclusions that were obtained after a series of real experimental studies.

Shows reliability of the results of a study compared to the real situations (other settings, populations, and times).


Control of extraneous variables within a study.

Generalization of results beyond a study.


Necessary for establishing cause-and-effect relationships within a study.

Necessary for generalizing the study's results.


History, maturation, testing, instrumentation, selection, and regression to the mean.

Sampling and selection bias, time and ecological factors.


A lab test how caffeine effect on driver’s reaction.

A field study examining the relationship between exercise and cognitive function in older adults in a certain community.

Internal and External Validity Threats

If we talk about indicators of internal efficacy, then it is subject to many threats. For example, conducting experiments may be hindered by many circumstances that occur around. Also, there are natural transformations in the environment. External validity is also threatened in some ways. For example, imagine a study that examines the effects of a particular medication on reducing depression symptoms. If this study only includes participants who are recruited from a single clinic, the sample may not be representative of people with depression in the wider population.

So, let’s check out some threats to internal and external validity in more detail.

Threats to Internal Validity 

The main threat factors are:

  • History There are external events that occur during a study and influence the outcome. For example, if a study on the effectiveness of a new drug is conducted during flu season, the results may be skewed by the high prevalence of influenza.
  • Maturation There are changes that occur naturally over time (physical or psychological) that may affect the outcome of a study.
  • Testing The impact of pre-testing on the outcome of the study. Participants may become more familiar with the test, which may affect their performance on subsequent tests.
  • Instrumentation Changes in the measurement tools used in the study may affect the result.
  • Regression to the mean This refers to the tendency of extremes to become less extreme over time. If extreme values are measured in a study, this phenomenon may affect the results.
  • Selection bias Systematic differences between study groups may also affect the outcome of a study.

Threats to External Validity

The main factors that affect the indicators of external efficacy include:

  • Sampling bias The sample is not representative of the population of interest (so, it becomes difficult to generalize the results).
  • Selection bias This occurs when there are systematic differences between study groups that may affect the generalizability of the results.
  • Reactivity The study participants change their behavior or response because they know they are being studied (difficult to generalize the results to other situations).
  • Time factors Time frames can affect the generalizability of the results. For example, changes in technology, social norms, or economic conditions can make research results irrelevant or outdated.
  • Ecological validity This refers to the degree to which the context and conditions of a study resemble those of the real world. If the research conditions are too artificial or contrived, the results may not be applicable to real-life environments.

Bottom Line on Internal vs. External Validity Difference 

External and internal validity are significantly different. Thus, scientists should keep a balance between internal and external validity in research. They need to ensure that their research is well-controlled and accurate, as well as applicable to real situations and extendable to other populations, settings, and time periods. 

Ultimately, the goal of the research is to produce results that are valid and applicable to the real world. So, understanding the difference between external and internal validity and keeping the validity features in mind is paramount for the success of a study.

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FAQ About External and Internal Validity

1. What is the difference between internal validity and external validity?

Internal validity establishes a causal relationship between an independent and a dependent variable. External validity allows researchers to determine how the results of a study can be extended to other populations, settings, and situations. That is, the internal one is about the accuracy of the conclusions and the external one is about the generalization of the results for the real situation.

2. Which type of validity is more important – internal or external one?

It is important to understand that internal or external validity in experimental research are important when it comes to reliability. But if we talk about which type is more important, then internal validity comes first. The higher this indicator, the more profound a study is.

Article posted on:Apr 19, 2023
Article updated on:Apr 14, 2024


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