The confidence interval may be thought of as the range of probable true values for a statistic. In general, as a population or sample size increases, the confidence interval gets smaller. Estimates with smaller confidence intervals are referred to as more "precise." Less precise estimates, such as those calculated from small numbers, will have wider confidence intervals.

Even for complete count datasets, such as birth and death certificate datasets, random fluctuations over time will yield estimates that are not reliable. For instance, the death rate for a short time period from a small population will not reflect the true underlying death risk for that population.

Go to the page on measurement reliability and validity for more information.