Internal Validity1

Internal Validity

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What is Internal Validity?

Internal validity is the extent to which a research study establishes a reliable cause-and-effect relationship between the variables being studied: the independent variable (treatment) and the dependent variable (outcome). In other words, it determines whether or not it is reasonable to assume a causal relationship from the observed covariation between the variables.

Why is Internal Validity Important

When researchers conduct experiments, they often are trying to establish clear relationships between the variables being studied. Internal validity allows them to make the findings of the causal relationship in a study credible and trustworthy. When an experiment has low external validity, it cannot reliably establish a causal link between the variables in question.

Threats to Internal Validity

Let’s take a look at some common threats to internal validity:

Historical Events:

Historical events may have an effect on the outcome of studies that take place over a particular period of time. This is because certain events may alter how participants feel or respond in regard to a specific subject.


This refers to the impact of time on the internal validity of a study. If an experiment takes place over a period of time during which participants could potentially change in any way (example: grew older/became smarter), then it may become difficult to clearly state whether or not the effects identified were simply caused due to the effect of time. Maturation is most threatening to experiments that are conducted over a long period of time.

Experimenter Bias:

If an experimenter exhibits different behaviour to different study groups, this could have an impact on the results and lower the internal validity of the study.


This occurs when participants within the experiment interact and observe one another, impacting the reliability of the findings of the study.

Repeat Testing:

Experiments sometimes involve repeatedly testing the same participants in an attempt to gather more reliable data. However, repeated testing could significantly influence outcomes as participants are likely to perform better as they learn the test or get more familiar with the testing process.

How to Increase the Internal Validity of a Research Study

The internal validity of a study can be increased using the following measures:


Experimental manipulation involves manipulating an independent variable to identify the cause-and-effect relationship between the independent and dependent variables.


This technique refers to eliminating the impact of any extraneous variables in the experiment by identifying them and keeping them constant.

Study Protocols:

Another way in which high internal validity can be achieved is through strict procedures for the administration of the treatment or manipulation of variables.


Internal validity can be improved by assigning participants to treatments and control groups on a completely random basis.

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The Relationship between Internal Validity and External Validity

External Validity can be defined as the extent to which the cause-and-effect relationship found in a study can be generalized. It indicates whether or not the findings of the study can be generalized beyond an experimental setting, to real-life situations. 

The following models can be used to describe the relationship between external and internal validity:

The Trade-off Model:

According to the trade-off model, the relationship between internal and external validity involves a trade-off, and therefore to achieve one, you must compromise the other, and vice versa. This can be owed to the fact that internal validity is achieved when the experiment has increased control while external validity is achieved when the experiment is more natural and representative: these are contradictory ideas that are difficult, if not impossible, to achieve simultaneously.

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The Purcuser Model:

According to this model, internal validity is a purcuser of external validity. It states that if there is no control in the experiment and internal validity is not achieved, then the results of the study cannot be generalized at all (external validity is not achieved). 

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