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In single-subject research, external validity is frequently questioned due to the reliance on very small sample sizes. Critics point out the difficulty in knowing whether treatment effects observed in just a few participants will successfully generalize to others in the broader population. For example, if a single-subject study demonstrates that a treatment reduces self-injury in two specific children with intellectual disabilities, advocates of group research might question how researchers can be confident that the identical treatment will be effective for other children with intellectual delays.

External Validity Concerns in Single-Subject Research

Single-subject researchers address external validity concerns by relying on the strong, consistent effects they observe, which are likely to generalize even from small samples. Additionally, they place a strong emphasis on replication; by attempting to reproduce effects with different participants or under slightly varying conditions, they systematically build confidence in the generalizability of their results. This methodological approach is supported by the historical success of principles like classical and operant conditioning, which were primarily discovered using single-subject research and have been successfully generalized across an incredibly wide range of species and situations.

Addressing External Validity in Single-Subject Research

An example of external validity concerns in single-subject research involves a study showing that a specific treatment reduces self-injury in two children with intellectual disabilities. While the treatment effect may be strong for these two individuals, researchers face the challenge of determining whether this success can be generalized to other children with intellectual delays who were not part of the original study.

Example of External Validity Concerns in Single-Subject Research

A common question regarding external validity in single-subject research is whether a treatment that effectively reduces self-injury in a couple of children with intellectual disabilities will also work for other children with similar delays in the broader population.

Generalization of Single-Subject Self-Injury Treatment

To build confidence in the external validity of their findings, single-subject researchers rely heavily on systematic replication. They attempt to reproduce observed effects using different small samples, often with slightly different types of participants or under varied environmental conditions.

Replication for External Validity in Single-Subject Research

Single-subject researchers defend their methods by noting that the strong and consistent effects they typically focus on are likely to generalize to others in the broader population, even when these effects are initially observed in very small samples.

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