The 68-95-99.7% Rule and z-scores. Normal distribution. The shape of histograms sometimes approximate what is known as a “bell-shape.”. Normal distribution. By joining the columns of the histogram together, a bell-shaped curve is formed.
68% of data have values within 1 standard deviation of the mean.
95% of the data have values within 2 standard deviations of the mean.
99.7% of the data have values within 3 standard deviations of the mean.
where x is the score, is the mean, and s is the standard deviation.