Research Design in the Experimental Study of Intellectual Disability

SOME significant problems in the design of research with subjects having an intellectual disability are identified. A lack of agreement on the nature of intellectual disability in relation to brain impairment, and discrepancies in the interpretation of deficits in relation to information processing concepts create problems in the definition of subject groups and interpretation of results. Much of the research uses normal subject groups for comparison, matched with disabled groups for either mental age, chronological age, or both, and this has led to further problems in the criteria and procedures used for matching, particularly for mental age. Further difficulties are encountered in variability both within and between subject groups, affecting reliability of results and making replication of studies difficult. In addition, task demands, particularly in relation to variables other than those under immediate investigation, are not always equivalent for all subject groups. Some suggestions are made for reducing or eliminating the effects of some of these problems in experimental research, and the use of alternative designs which take subject variability into account is recommended.