If we think about a normal curve / bell curve and the 68-95-99.7 rule, we know that the majority of data will lie within 1, 2, or 3 standard deviations from the mean. The mean is in the center of the curve, and to each side of the mean, 34% of the data lies 1 standard deviation on either side of the mean. Therefore, we need to add and subtract one standard deviation from the mean.
