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Free download from www.hsrcpress.ac.zapay attention to the criterion of external validity. As we indicated in Chapter 2,(see diagram on p. 49), it is customary to distinguish between internal andexternal validity in research. As far as internal validity is concerned, we havepaid fairly detailed attention to its most important dimensions which comprisetheoretical validity, measurement validity, reliability, and inferential validity.In our discussion of inferential validity we have, however, only paid attentionto the requirement of valid inference with regard to the data collected from thesample that was studied. In those cases, however, in which the researcherwishes to make inferences about the population from which the sample wasdrawn, (in other words studies of more general interest) the question that arisesis whether the conclusions are externally valid.The term external validity is used as a synonym for generalizability. It is forthis reason that we referred to external validity within the context of researchof a more general interest. Typically, this research would proceed along thefollowing lines: a target population of people or situations or periods would bedefined as clearly as possible. For example, all first year psychology studentsat universities who use English as the language of instruction. Following this,samples representative of this population would be drawn and studied. At theconclusion of the study the aim would be to apply the findings based upon thesample to the defined population or, in other words, to generalize the findingsto the defined population. From this brief exposition of the issue of externalvalidity it is sufficiently clear that the greatest threat to external validity is to befound in the extent to which the sample is representative of the population. Thesubjects included in the sample could, for example, be more highly motivated,older, more conservative, more intelligent, or more urbanized than thepopulation, and these factors would all have a negative influence on theexternal validity of the findings. Cook and Campbell (1979: 73-74) distinguishthree threats to external validity.SELECTION EFFECTSThe way in which the sample is selected can have a significant effect upon thegeneralizability of the findings. This effect is particularly evident inexperimental studies in which ethical considerations frequently compel theresearcher to use volunteers as participants. In a classical study on thevolunteer subject Rosenthal and Rosnow’s (1969) findings indicated thatvolunteer subjects were frequently more highly educated, more intelligent, of ahigher professional status, and displayed lower levels of authoritarianism thandid non-volunteer experimental subjects. A number of studies have beenpublished since the appearance of the Rosenthal and Rosnow study in whichthe selection effects on psychological research have been indicated where alarge proportion (some authors claim that it is as high as 90%) of the studieshave been conducted exclusively with psychology students as participants.119