External validity
Encyclopedia
External validity is the validity of generalized (causal) inferences in scientific studies, usually based on experiments as experimental validity
Validity (statistics)
In science and statistics, validity has no single agreed definition but generally refers to the extent to which a concept, conclusion or measurement is well-founded and corresponds accurately to the real world. The word "valid" is derived from the Latin validus, meaning strong...

.

Inferences about cause-effect relationships based on a specific scientific study are said to possess external validity if they may be generalized from the unique and idiosyncratic settings, procedures and participants to other populations and conditions. Causal inferences said to possess high degrees of external validity can reasonably be expected to apply (a) to the target population of the study (i.e. from which the sample was drawn) (also referred to as population validity), and (b) to the universe of other populations (e.g. across time and space).

The most common loss of external validity comes from the fact that experiments using human participants often employ small samples obtained from a single geographic location or with idiosyncratic features (e.g. volunteers). Because of this, one cannot be sure that the conclusions drawn about cause-effect-relationships do actually apply to people in other geographic locations or without these features.

Threats to external validity

"A threat to external validity is an explanation of how you might be wrong in making a generalisation." Generally, generalizability is limited when the cause (i.e. the independent variable) depends on other factors; therefore, all threats to external validity interact with the independent variable.
  • Aptitude-Treatment-Interaction: The sample may have certain features that may interact with the independent variable, limiting generalisability. For example, inferences based on comparative psychotherapy studies often employ specific samples (e.g. volunteers, highly depressed, no comorbidity). If psychotherapy is found effective for these sample patients, will it also be effective for non-volunteers or the mildly depressed or patients with concurrent other disorders?

  • Situation: All situational specifics (e.g. treatment conditions, time, location, lighting, noise, treatment administration, investigator, timing, scope and extent of measurement, etc. etc.) of a study potentially limit generalizability.

  • Pre-Test Effects: If cause-effect relationships can only be found when pre-tests are carried out, then this also limits the generality of the findings.

  • Post-Test Effects: If cause-effect relationships can only be found when post-tests are carried out, then this also limits the generality of the findings.

  • Reactivity (Placebo, Novelty, and Hawthorne Effects
    Hawthorne effect
    The Hawthorne effect is a form of reactivity whereby subjects improve or modify an aspect of their behavior being experimentally measured simply in response to the fact that they know they are being studied, not in response to any particular experimental manipulation.The term was coined in 1950 by...

    ): If cause-effect relationships are found they might not be generalizable to other settings or situations if the effects found only occurred as an effect of studying the situation.

  • Rosenthal Effects: Inferences about cause-consequence relationships may not be generalizable to other investigators or researchers.

External, internal, and ecological validity

In many studies and research designs, there may be a "trade-off" between internal validity
Internal validity
Internal validity is the validity of inferences in scientific studies, usually based on experiments as experimental validity.- Details :...

 and external validity: When measures are taken or procedures implemented aiming at increasing the chance for higher degrees of internal validity, these measures may also limit the generalizability of the findings.
This situation has led many researchers call for "ecologically valid" experiments. By that they mean that experimental procedures should resemble "real-world" conditions. They criticize the lack of ecological validity
Ecological validity
Ecological validity is a form of validity in a research study. For a research study to possess ecological validity, the methods, materials and setting of the study must approximate the real-life situation that is under investigation. Unlike internal and external validity, ecological validity is not...

 in many laboratory-based studies with a focus on artificially controlled and constricted environments.
External validity and ecological validity
Ecological validity
Ecological validity is a form of validity in a research study. For a research study to possess ecological validity, the methods, materials and setting of the study must approximate the real-life situation that is under investigation. Unlike internal and external validity, ecological validity is not...

 are closely related in the sense that causal inferences based on ecologically valid research designs often allow for higher degrees of generalizability than those obtained in an artificially produced lab environment. However, this is not always the case: Some findings produced in ecologically valid research settings may hardly be generalizable, and some findings produced in highly controlled settings may claim near-universal external validity.
Thus, External and Ecological Validity are independent – a study may possess external validity but not ecological validity, and vice-versa.

Qualitative research

Within the qualitative research
Qualitative research
Qualitative research is a method of inquiry employed in many different academic disciplines, traditionally in the social sciences, but also in market research and further contexts. Qualitative researchers aim to gather an in-depth understanding of human behavior and the reasons that govern such...

 paradigm, external validity is replaced by the concept of transferability. Transferability is the ability of research results to transfer to situations with similar parameters, populations and characteristics.

See also

  • Statistical conclusion validity
    Statistical conclusion validity
    Statistical conclusion validity refers to the appropriate use of statistics to infer whether the presumed independent and dependent variables covary...

  • Internal validity
    Internal validity
    Internal validity is the validity of inferences in scientific studies, usually based on experiments as experimental validity.- Details :...

  • Ecological validity
    Ecological validity
    Ecological validity is a form of validity in a research study. For a research study to possess ecological validity, the methods, materials and setting of the study must approximate the real-life situation that is under investigation. Unlike internal and external validity, ecological validity is not...

  • Construct validity
    Construct validity
    In science , construct validity refers to whether a scale measures or correlates with the theorized psychological scientific construct that it purports to measure. In other words, it is the extent to which what was to be measured was actually measured...

  • Content validity
    Content validity
    In psychometrics, content validity refers to the extent to which a measure represents all facets of a given social construct. For example, a depression scale may lack content validity if it only assesses the affective dimension of depression but fails to take into account the behavioral dimension...

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