Internal validity

Definition
Internal validity is the validity of (causal) inferences in scientific studies, usually based on experiments as experimental validity.

Details
Inferences are said to possess internal validity if a causal relation between two variables is properly demonstrated. A causal inference may be based on a relation when three criteria are satisfied:
 * 1) the "cause" precedes the "effect" in time (temporal precedence),
 * 2) the "cause" and the "effect" are related (covariation), and
 * 3) there are no plausible alternative explanations for the observed covariation (nonspuriousness).

In scientific experimental settings, researchers often manipulate a variable (the independent variable) to see what effect it has on a second variable (the dependent variable) For example, a researcher might, for different experimental groups, manipulate the dosage of a particular drug between groups to see what effect it has on health. In this example, the researcher wants to make a causal inference, namely, that different doses of the drug may be held responsible for observed changes or differences. When the researcher may confidently attribute the observed changes or differences in the dependent variable to the independent variable, and when he can rule out other explanations (or rival hypotheses), then his causal inference is said to be internally valid.

In many cases, however, the magnitude of effects found in the dependent variable may not just depend on
 * variations in the independent variable,
 * the power of the instruments and statistical procedures used to measure and detect the effects, and
 * the choice of statistical methods (see: Statistical conclusion validity).

Rather, a number of variables or circumstances uncontrolled for (or uncontrollable) may lead to additional or alternative explanations (a) for the effects found and/or (b) for the magnitude of the effects found. Internal validity, therefore, is more a matter of degree than of either-or, and that is exactly why research designs other than true experiments may also yield results with a high degree of Internal Validity.

In order to allow for inferences with a high degree of internal validity, precautions may be taken during the design of the scientific study. As a rule of thumb, conclusions based on correlations or associations may only allow for lesser degrees of internal validity than conclusions drawn on the basis of direct manipulation of the independent variable. And, when viewed only from the perspective of Internal Validity, highly controlled true experimental designs (i.e. with random selection, random assignment to either the control or experimental groups, reliable instruments, reliable manipulation processes, and safeguards against confounding factors) may be the "gold standard" of scientific research. By contrast, however, the very strategies employed to control these factors may also limit the generalizability or External Validity of the findings.

Threats to internal validity
The following example will be used in defining and understanding the threats against internal validity. A research study was done to determine what fruit children liked best. The researcher surveyed all 290 children in one K-3 grade school and all 582 of the children in a K-5 grade school (N = 872). At the K-3, school, 81% preferred apples, 14% preferred bananas, and 5% preferred oranges. At the K-5 grade school, 80% preferred apples, 15% preferred bananas, and 5% preferred oranges. The completed result was 80% (a = 698) of the children liked apples best. 15% (b = 131) liked bananas, and the remaining 5% (o = 43) preferred oranges. The researcher asks to what extent the causal inference that school change effects favoring apple as a snack (over oranges or bananas).

Confounding
A major threat to the validity of causal inferences is Confounding: Changes in the dependent variable may rather be attributed to the existence or variations in the degree of a third variable which is related to the manipulated variable. Where Spurious relationships cannot be ruled out, rival hypothesis to the original causal inference hypothesis of the researcher may be developed.

Selection (bias)
Selection bias refers to the problem that, at pre-test, differences between groups exist that may interact with the independent variable and thus be 'responsible' the observed outcome. Researchers and participants bring to the experiment a myriad of characteristics, some learned and others inherent. For example, sex, weight, hair eye, and skin color, personality, mental capabilities, and physical abilities, but also attitudes like motivation or willingness to participate. During the selection step of the research study, if an unequal number of test subjects have similar subject-related variables there is a threat to the internal validity. For example, a researcher created two test groups, the experimental and the control groups. The subjects in both groups are not alike with regard to the independent variable but similar in one or more of the subject-related variables. It would be difficult for the researcher to determine if the discrepancy in the groups is due to the independent variable or to the subject-related variables. Selection bias may be reduced when selection/inclusion processes are controlled for and group assignment is randomized. However, in most cases, it may never be ruled out completely as relevant between-group differences may go unnoticed.

History
Events outside of the study/experiment or between repeated measures of the dependent variable may also provide for alternative explanations than the researcher's cause-effect hypothesis. Suppose that, in the example above, the researcher observed students at K-3 and again two years later, at K-5, which snack offered they would take. Furthermore, suppose that, in the meantime, a shortage of apples had led to dramatic increases in food prices, in turn leading to parents offering their children apples less often. Would you still believe that school change effects favoring apple as a snack?

Maturation
Subjects change during the course of the experiment or even between measurements. For example, young children might maturate and their ability to concentrate may change as the grow up. Both permanent changes, such as physical growth and temporary ones like fatigue, provide "natural" alternative explanations; thus, they may change the way a subject would react to the dependent variable. So upon completion of the study, the researcher may not be able to determine if the cause of the discrepancy is due to time or the independent variable.

Repeated testing
Repeatedly measuring the participants may lead to bias. Participants may remember the correct answers or may be conditioned to know that they are being tested. Repeately taking (the same or similar) intelligence tests usually leads to score gains, but instead of concluding that the underlying skills have changed for good, this threat to Internal Validity provides good rival hypotheses.

Instrument change
The instrument used during the testing process can change the experiment. This also refers to observers being more concentrated or primed. If any instrumentation changes occur, the internal validity of the main conclusion is effected, as alternative explanations are readily available.

Regression to the mean
This type of error occurs when subjects are selected on the basis of extremes score (one far away from the mean) during the first test but score closer to the mean with the second test. For example, when in 3rd grade children with the worst reading skills are selected to participate in a nationwide reading skills program, may it be validly concluded that positive changes at the end of the program are due to the educational efforts? A good alternative conclusion is regression to the mean and occurs due to a regression artifact. If the children had been tested again before the start of the program, they would likely have obtained less extreme scores, even if repeated testing effects could have been ruled out.

Mortality/differential attrition
This error occurs if inferences are made on the basis of only those participants that have participated from the start to the end. However, participants may have dropped out of the study before completion, and maybe even due to the study or programme or experiment itself. For example, the percentage of group members having quit smoking at post-test was found much higher in a group having received a quit-smoking training program than in the control group. However, in the experimental group only 60% have completed the program. If this attrition is systematically related to any feature of the study, the administration of the independent variable, the instrumentation, or if dropping out leads to relevant bias between groups, a whole class of alternative explanations is possible that account for the observed differences.

Selection-maturation interaction
This occurs when the subject-related variables, color of hair, skin color, etc., and the time-related variables, age, physical size, etc., interact. In the fruit experiment, the ages of the children in one school is 4-12 and the other 4-9 years old. If a discrepancy between the two groups occurs between the testing, the discrepancy may be due to the age differences in the age categories.

Diffusion
If treatment effects spread from treatment groups to control groups, a lack of differences between experimental and control groups may be observed. This does not mean, however, that the independent variable has no effect or that there is no relationship between dependent and independent variable.

Compensatory rivalry/resentful demoralization
Behaviour in the control groups may alter as a result of the study. For example, control group members may work extra hard to see that expected superiority of the experimental group is not demonstrated. Again, this does not mean, that the independent variable produced no effect or that there is no relationship between dependent and independent variable. Vice-versa, changes in the dependent variable may only be effected due to a demoralized control group, working less hard or motivated, not due to the independent variable.

Experimenter bias
This occurs when the researcher influences the outcome. Once again, refer back to the fruit study. If the researcher as the children came up to choose their piece of fruit told the children, “You don’t want to pick a banana, it’s turning brown.” The researcher is influencing the outcome of the study, thus causing an internal validity error. It is a variant of the effect commonly known as Rosenthal-effect.