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Free download from www.hsrcpress.ac.zaGreen (1970), and Webb et al. (1966), have discussed this matter. Kolson andGreen, for example, draw attention to the existence of the tendency to gambleamong children who are not certain of the meaning of items. Similar responsepatterns that have been noted, particularly when the meaning of items isobscure, include the tendency to endorse only the extremes on scaled items(extreme checking style), or to check the mid-points of the scale (centraltendency). For the purpose of our discussion, we shall pay more attention totwo well-known types of response pattern: social desirability and acquiescenceresponse set.The Hawthorne effect is clearly an example of a social desirability tendency.As Selltiz et al succinctly state the matter: Most persons will try to giveanswers that make themselves appear well-adjusted, unprejudiced, rational,open-minded and democratic (quoted in Smith, 1975: 136). Rosenberg wasalso able to confirm that those individuals who attained high scores onMarlow-Crone’s Social Desirability Scale were more inclined to supplyextremely positive responses than those with low scores on the scale.The tendency to answer either yes or no to virtually all the items in aquestionnaire or scale is referred to as acquiescent response set. As early as1937 Sletto found that respondents were more likely to agree with a statementthan to disagree with the inverse of that statement. In a more recent anddetailed study of this issue, Schuman and Presser (1981, chapter 8) were ableto confirm earlier findings on this topic. Apart from the fact that they were ableto confirm the existence of this type of response pattern (it can producedifferences that range between 10 and 15 %), they also found indications thatthis phenomenon is more likely to occur amongst respondents with a low levelof education than amongst, for example, university graduates. Schuman andPresser, however, maintain that we have not yet built up a sufficiently largebody of research into the phenomenon of acquiescent response set to be able toprovide an adequate interpretation of the reasons underlying this type ofresponse pattern.MEASURING INSTRUMENT EFFECTSIn the literature, observer effects that are directly related to the nature andstructure of the measuring instruments are dealt with almost exclusively withinthe context of questionnaire or scale effects. In view of the fact that theseeffects are dealt with in detail in two recent publications (Schuman & Presser,1981; Sudman & Bradburn, 1982),- we shall merely list the most importanteffects.(i) Item or question sequence effects(ii) Open and closed question effects(iii) Don’t know effects(iv) Midposition effects89