Chance, or uncertainty in estimates, is mostly determined by sample size – the larger the sample, the greater the chance that the sample estimate of the population mean is closer to the true population mean. The impact of chance on the results of a study are usually expressed as P values and confidence intervals. In any epidemiological study, the starting point is to assume that there is no difference between the two groups being tested (i.e. the null hypothesis). If any difference is observed, then it is important to determine whether this observed difference could be due to chance or whether it is a real difference
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