ABSTRACT
A major objective of research design is ensuring that any effects found are caused only by the independent variable and not by other factors in the research situation. These other factors are extraneous variables. Extraneous variables provide alternative explanations for the observed effect; if they are present, the researcher cannot conclude that the independent variable caused the effect. For example, in Chapter 2 we noted that in cross-sectional developmental research it is diffi cult to separate the effects of age, the independent variable in the research, from cohort effects, which provide a possible alternative explanation for any differences found between age groups. To the extent that there are plausible alternative explanations for the effect of an independent variable, we say that a study exhibits low internal validity; conversely, to the extent that we can have confi dence that the results of a study are due only to the effects of the independent variable, we can conclude that a study exhibits high internal validity.
