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KAIS 2007 1 - Kenya National AIDS & STI Control Programme ...

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DATA IN CONTEXTUNDERSTANDING STATI<strong>STI</strong>CAL SIGNIFICANCEWhenever a population is sampled for a survey, there is some degree of uncertaintyassociated with the results obtained; results from samples of human populations are alwaysestimates. Standard errors represent the degree of uncertainty around an estimate, includingeach estimate in 2003 KDHS and <strong>2007</strong> <strong>KAIS</strong>. The formula for calculating standard errors isdescrubed in Appendix C. Because of the uncertainty of survey estimates, statistical testsusing standard errors can provide a range of potential values of the true estimate; this rangeis referred to as the confidence interval (CI). A 95% CI, for example, means that if a surveywas repeated 100 times in the same population, the CI would be expected to contain the trueestimate 95 times out of 100. The 95% CIs presented in this chapter are shown as lines at thetop and bottom of each bar in the figures.When comparing an estimate between two surveys in the same population, there are formalstatistical methods to test the probability that the differences seen between the two surveysare real and not due to chance. When comparing estimates from 2003 KDHS to estimatesfrom the <strong>2007</strong> <strong>KAIS</strong>, we used the z-test, which computes the probability that the differencewas due to chance alone. We used a conservatively low probability, less than 5%, todetermine whether these differences were likely to be real and not due to chance. If theprobability that chance caused the differences was less than 5%, we said the results werestatistically significant. If the probability was between 5% and 10%, we said the results weremarginally significant. Visually, one way to approximate whether point estimates from the2003 KDHS and the <strong>2007</strong> <strong>KAIS</strong> are different is to visually assess whether the 95% CIs forthe two estimates overlap; that is, the CI of one survey overlaps with the CI of the other. If so,the estimates are likely to not be different, and conversely, if they do not overlap, thedifference is most likely significant.<strong>KAIS</strong> <strong>2007</strong> 53

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