The Meaning of "Internal Validity"

• When a study lacks internal validity, one or more alternative hypotheses exist to explain the outcomes of the study. These alternative hypotheses are referred to by researchers as "threats to internal validity."
• When a study has internal validity, it means that any relationship observed between two or more variables is unambiguous as to what it means, rather than being due to something else.

Threats to Internal Validity

• Some of the more common threats to internal validity are differences in subject characteristics, mortality, location, instrumentation, testing, history, maturation, attitude of subjects, regression, and implementation.
• The selection of people for a study may result in the individuals or groups differing (i.e., the characteristics of the subjects may differ) from one another in unintended ways that are related to the variables to be studied.
• No matter how carefully the subjects of a study (the sample) are selected, it is common to lose some of them as the study progresses. This is known as "mortality." Such a loss of subjects may affect the outcomes of a study.
• The particular locations in which data are collected, or in which an intervention is carried out, may create alternative explanations for any results that are obtained.
• The way in which instruments are used may also constitute a threat to the internal validity of a study. Possible instrumentation threats include changes in the instrument, characteristics of the data collector(s), and/or bias on the part of the data collectors.
• The use of pretest in intervention studies sometimes may create a "practice effect" that can affect the results of a study. A pretest can also sometimes affect the way subjects respond to all intervention.
• On occasion, one or more unanticipated, and unplanned for, events may occur during the course of a study that can affect the responses of subjects. This is known as a history threat.
• Sometimes change during an intervention study may be due more to factors associated with the passing of time than to the intervention itself. This is known as a maturation threat.
• The attitude of subjects toward a study (and their participation in it) can create a threat to internal validity.
• When subjects are given increased attention and recognition because they are participating in a study, their responses may be affected. This is known as the Hawthorne effect.
• Whenever a group is selected because of unusually high or low performance on a pretest, it will, on the average, score closer to the mean on subsequent testing, regardless of what transpires in the meantime. This is called a regression threat.
• Whenever an experimental group is treated in ways that are unintended and not a necessary part of the method being studied, an implementation threat can occur.

Controlling Threats to Internal Validity

• There are a number of techniques or procedures that researchers can use to control or minimize threats to internal validity. Essentially they boil down to four alternatives: (1) standardizing the conditions under which the study occurs; (2) obtaining and using more information on the subjects of the study; (3) obtaining and using more information on the details of the study; and (4) choosing an appropriate design.