FIGURE 1–1 Internal validity.

NOTES: Level I represents the strongest methodological controls and IV the least strong (see Box 1–1); N is the number of studies.

SOURCES: For social studies, McConnell (1999); for communication studies, Goldstein (1999); for problem behavior studies, Horner (2000); for intervention studies, Kasari (2000); for sensory-motor studies (Baranek, 1999).

ment), in terms of percentages of studies falling into different levels of rigor with respect to internal validity, external validity, and generalization. This information is discussed in more detail in Chapter 15 and in the chapters describing those content areas. (For details on the coding of individual studies, see the appendices of the papers cited in the figures.)

One of the difficulties in interpreting research, particularly longitudinal studies, is that standards for scientific research within different theoretical perspectives have changed enormously in the last 20 years, and they continue to evolve. Twenty years ago, behavioral researchers were not as concerned with rigorously standardizing measures or diagnoses, maintaining independence between intervention and assessment, or analyzing the effects of development. Similarly, group designs based on a clinical trials model were not expected to monitor treatment fidelity, equate participants for intellectual or language level, address generalization or maintenance of effects, or justify measures by their clinical value. Therefore, particularly when depicting outcomes from longitudinal studies, reviewers of the literature often have to piece together information

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