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Eight Threats To Internal Validity

1. HISTORY: Unplanned events that occur between the first and second measurements. Examples include changes in local drug use patterns, drug supply interruptions, changes in treatment approach, and seasonal drug use patterns. In general, the more time that elapses between measurements, the greater the threat from historical effects.



2. MATURATION: Developmental changes that naturally occur in clients. Growing older, more experienced, or more independent may be especially important maturation effects with adolescent client populations. Other maturation effects raise particular concerns in samples from special population in transition, such as pregnant women, HIV positive clients, and ex-offenders re-entering society from prison.



3. TESTING: Effects of taking a measurement on the results of subsequent measurement. Repeated urine tests for drug use tend to discourage later drug use, for example, while repeated ability tests tend to raise scores and subjects practice their test-taking skills, even if no real increase in ability occurs.



4. INSTRUMENTATION: Effects of changes in a measurement instrument, or in criteria for recording behavior, during the course of an evaluation. Common examples are a change in the cutoff point for a Adrug positive@ determination by a urinalysis laboratory or a change in police criteria for making arrests (for example, during a neighborhood crackdown).



5. STATISTICAL REGRESSION: Effects of selecting samples on the basis of extreme behavior -- over time, their behavior tends to Aregress@ toward the overall group average. In drug treatment program evaluation, prodrug attitudes in an extremely prodrug sample, heroin consumption in a sample selected during periods of heavy use, and self-esteem in a sample selected on the basis of low self-esteem, will all tend to be less extreme on a second measurement.



6. SELECTION: Effects of unmeasured difficulties between a group receiving treatment and a nonequivalent group not receiving treatment. Common examples of unmeasured difficulties include clients' motivation to seek treatment, family and social support structures, and expectations about future drug use. The problem tends to arise when treatment is given to volunteers and withheld from nonvolunteers, instead of assigning volunteers randomly to treatment.



7. ATTRITION: Effects of unequal dropout rates among different subgroups in the sample. In drug treatment program evaluation, common examples include differences between those who receive treatment and those who do not, between heavier drug users and lighter users, or between more and less satisfied clients.



8. HAWTHORNE EFFECTS: Effect of changes which are due to the fact of being included in an evaluation.