In research design, history serves as an alternative explanation for changes in a dependent variable, representing a significant threat to internal validity. This threat occurs when extraneous external events happen between the pretest and posttest measurements, unintentionally causing a shift in the participants' scores. For instance, if an anti-drug education program is being evaluated, an unexpected and highly publicized celebrity drug overdose occurring during the study period could independently influence participants' attitudes, confounding the results and making it impossible to determine if the education program was the true cause of the change.

History as a Threat to Internal Validity

Maturation is a threat to internal validity that occurs when participants undergo natural developmental or physiological changes—such as growing older, learning, or becoming fatigued—between the pretest and posttest measurements of a study. These natural, ongoing changes, rather than the experimental treatment itself, may be the actual cause of any observed differences in the dependent variable over time. For example, over the course of a year-long intervention, children naturally become better reasoners, which could be misconstrued as an effect of the intervention.

Maturation as a Threat to Internal Validity

The testing threat to internal validity occurs when the mere act of taking a pretest inadvertently influences a participant's responses on the subsequent posttest. Completing the initial measurement might inspire further thought, conversation, or practice regarding the topic being studied. This increased awareness or familiarity can independently cause a change in the posttest scores, regardless of whether the actual experimental treatment was effective.

Testing as a Threat to Internal Validity

Instrumentation threatens a study's internal validity when the fundamental characteristics of the measuring instrument or process change between the pretest and posttest. This issue frequently arises in observational studies where human observers are used to measure behavior, as they may gradually gain skill, become fatigued, or unconsciously alter their evaluation standards over time. Additionally, instrumentation effects can manifest when the participants themselves change their approach to the measurement tool, such as taking a survey very seriously during the novel pretest but becoming bored and less careful with their responses during the posttest.

Instrumentation as a Threat to Internal Validity

Spontaneous remission is the natural tendency for many medical conditions and psychological problems to gradually improve over time without any formal treatment or intervention. This phenomenon acts as a significant confounding variable in studies that lack a control group; researchers evaluating a treatment using a simple pretest-posttest design may falsely attribute a patient's recovery to their specific intervention, when in reality, the condition would have naturally improved on its own.

Spontaneous Remission

A confounding variable is an extraneous variable that differs on average across levels of the independent variable, meaning it varies systematically alongside it. Because this variable differs systematically across conditions just like the independent variable, it provides a plausible alternative explanation for any observed difference in the dependent variable, making it difficult to establish a clear cause-and-effect relationship. For instance, if an extraneous variable like intelligence is substantially lower in one group than another, it becomes a confounder that confuses the results.

Confounding Variable

What is the primary focus of internal validity in an experiment?

If an experiment is set up to severely test whether influence X really does affect outcome Y then it needs to be clear any systematic changes in Y are wholly due to the change in X. This is the issue of internal validity.

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Validity is often divided into several distinct types, which represent different kinds of evidence researchers use to judge a measure's accuracy. Beyond reliability, researchers evaluate evidence through various forms of validity, such as face validity, content validity, and criterion validity, to ensure the measurement method truly captures the intended construct.

Types of Validity

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Internal validity

Refers to the extent to which research findings would have relevance and application in the real world.

Ecological Validity

For all experiments, there is always compromise across all aspects of validity, at the very least ecological because in order to achieve experimental control, some aspects of the real world need to be constrained. Depending on the complexity of the idea being tested in the experiment, different compromises need to be made. 

Validity as a whole

Face validity is a subjective assessment of how well a measurement tool appears to measure the concept it is intended to, based on a surface-level inspection. It considers whether the measure seems appropriate and relevant for its stated purpose at first glance.

Face Validity

Discriminant validity is the extent to which scores on a measure are not strongly correlated with measures of conceptually distinct variables. Providing evidence of low correlations between differing constructs—such as self-esteem and mood—is crucial to demonstrate that the new measure is uniquely capturing the intended trait rather than inadvertently measuring a different psychological state.

Discriminant Validity

Content validity refers to the extent to which a measure adequately covers all aspects of the theoretical construct it is designed to assess. In psychological measurement, researchers often rely on multiple-item measures to achieve content validity because a single item rarely captures the full complexity of a construct comprehensively.

Content Validity

Criterion validity involves evaluating how effectively a measure aligns with expectations by correlating it with other relevant variables. Researchers assess this by analyzing the correlations among multiple measures of the same psychological construct, or by verifying expected relationships with measures of conceptually distinct constructs. Additionally, a successful experimental manipulation inherently provides evidence for criterion validity. For example, if an experiment manipulates participants' moods by instructing them to think positive versus negative thoughts, and the subsequent mood measure reveals a distinct difference between the two groups, this simultaneously demonstrates that the manipulation worked and that the measurement tool was valid.

Criterion Validity

External validity refers to the ability to generalize the results of a study beyond the specific people and situations that were actually investigated.

External Validity

Construct validity assesses the quality of a study's experimental manipulations and how accurately those procedures operationalize the core conceptual research question. A research design demonstrates high construct validity when the variables are manipulated in a way that clearly isolates the specific phenomenon of interest. This validity depends heavily on the nuances of the experimental design, such as the number of conditions. For example, in Darley and Latané’s study on bystander intervention, testing only two conditions (one versus two other students) might merely demonstrate general social inhibition rather than the specific diffusion of responsibility, resulting in lower construct validity; expanding to multiple conditions helps clarify the phenomenon, though adding too many conditions may eventually plateau without adding further insight.

Construct Validity

In psychological research, what is the primary purpose of evaluating distinct forms of measurement validity, such as face, content, and criterion validity?

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