The standard deviation quantifies how much the values vary from each other. A measure of the spread of individual observations around the mean value of the sample. Studies are performed on a sample of the population, not the whole population. Confidence intervals give us some idea of how likely the sample mean represents the population mean. A measure of the precision of the estimate.
Most clinical trials study a sample of the population at risk.
Because a sample is a subset of a population, the mean value obtained for the sample studied may not be same as the mean value if the entire population was studied. Results from a sample population with a wider range of values will have broader confidence intervals than results from a study with a narrower range of values. Derived from the sample mean and the standard error. Variability by the same observer. Can also test the effect of individual factors independently.