A two-sided confidence interval quantifies our knowledge about the true population mean by bounding the range of likely values on both sides of the sample mean. For example, after taking 25 random soil samples from a survey unit and measuring the lead concentrations, we might calculate a two-sided 90% confidence interval around the mean as 11.9 ppm \( \pm \) 0.6 ppm. The two-sided 90% confidence interval for this example is 11.3 ppm to 12.5 ppm. In general, use a two-sided confidence interval instead of a one-sided confidence interval when bounds are desired above and below the sample mean.