Preface to First Edition - lib

CHAPTER 4Conditional Inference: GuessingLengths, Suicides, GastrointestinalDamage, and Newborn Infants4.1 IntroductionThere are many experimental designs or studies where the subjects are nota random sample from some well-defined population. For example, subjectsrecruited for a clinical trial are hardly ever a random sample from the setof all people suffering from a certain disease but are a selection of patientsshowing up for examination in a hospital participating in the trial. Usually,the subjects are randomly assigned to certain groups, for example a controland a treatment group, and the analysis needs to take this randomisation intoaccount. In this chapter, we discuss such test procedures usually known as(re)-randomisation or permutation tests.In the room width estimation experiment reported in Chapter 3, 40 of theestimated widths (in feet) of 69 students and 26 of the estimated widths (inmetres) of 44 students are tied. In fact, this violates one assumption of theunconditional test procedures applied in Chapter 3, namely that the measurementsare drawn from a continuous distribution. In this chapter, the data willbe reanalysed using conditional test procedures, i.e., statistical tests wherethe distribution of the test statistics under the null hypothesis is determinedconditionally on the data at hand. A number of other data sets will also beconsidered in this chapter and these will now be described.Mann (1981) reports a study carried out to investigate the causes of jeeringor baiting behaviour by a crowd when a person is threatening to commitsuicide by jumping from a high building. A hypothesis is that baiting is morelikely to occur in warm weather. Mann (1981) classified 21 accounts of threatenedsuicide by two factors, the time of year and whether or not baiting occurred.The data are given in Table 4.1 and the question is whether they giveany evidence to support the hypothesis? The data come from the northernhemisphere, so June–September are the warm months.65© 2010 by Taylor and Francis Group, LLC