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Internal Validity Concerns An experiment is internally valid is sufficient control of extraneous variables allows differences observed in an experiment to be attributed solely to the experimental treatment (Tuckman, 1978). Campbell (1971) lists eight variables that threaten the internal validity of a nonequivalent control group design and thus may confound the effect of the experimental stimulus: history, maturation, testing, instrumentation, statistical regression, selection, experimental mortality, and selection-maturation interaction. History refers to the occurrence of any event which is not part of the experimental treatment but which may affect performance on the dependent variable. Maturation refers to physical and mental changes that may occur within the subjects over a period of time. While the researcher has no control over the occurrence of history and maturation, they are somewhat controlled, as events, other than treatment, that might produce difference in the experimental group would also produce a difference in the control group. In relation to research that shows that girls often have a more positive attitude toward reading, it has already been noted that the attitude of boys toward academic reading is not significantly different that the girls’ attitude toward academic reading. A pretest may sensetize subjects to treatment, causing the effects of the treatment to be partially attributable to this sensitization, rather than from the treatment (Tuckman, 1978). Using the 2002 ITBS scores and comparing them to the 2003 ITBS scores, and by having a 28-week space between the two administrations of the ERAS controlled pretest sensitization. Instrumentation refers to deterioration or changes in the accuracy of instruments, devices, or observers used to measure the dependent (outcome) variable. Several safeguards were put in place. To guard against observers recording observations inaccurately, the researcher utilized the school’s assistant principal and reading specialist. Both had been extensively trained in the Clinical Supervision model, which has an observation component of verbatim notes of interactions within classrooms. At the start of the study all material was photocopied and put in a filing cabinet, ensuring that the ERAS student survey was photocopied in exactly the same manner, and that the observation sheets were exactly the same. The ITBS was delivered to the school at the designated test time and was kept under lock and key until the test administration. The Chicago Board of Education calculated the ITBS test results, and the school’s assistant principal calculated all other measures. Grouping participants on the basis of extreme scores may result in inaccurate categorizations, because extreme scorers tend to regress toward the group mean on repeated testing. Pretreatment analysis of between group differences showed groups to have no significant difference, so any regression occurring should be similar in control and experimental groups.
Approximate Word count = 1753
Approximate Pages = 7
(250 words per page double spaced)
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