© 2016 Microsoft Corporation. All Rights Reserved. Page 10 2/24/2017 F41xx14 FIGURE 41-14 A negatively skewed histogram of data plotting days from conception to birth. F41xx14: This figure shows a negatively skewed histogram of days from conception to birth. • Multiple peaks When a histogram exhibits multiple peaks , it usually means that data from two or more populations are being graphed together. For example, suppose the diameter of elevator rails produced by two machines yields the histogram shown in Figure 41-15. (See cell Q11 in the file Twinpeaks.xlsx.) F41xx15 FIGURE 41-15 A multiple-peak histogram. F41xx15: This figure shows a histogram with two peaks. In this histogram, the data clusters into two separate groups. In all likelihood, each group of data corresponds to the elevator rails produced by one of the machines. If you assume that the diameter you want for an elevator rail is 0.55 inches, you can conclude that one machine is producing elevator rails that are too narrow, whereas the other machine is producing elevator rails that are too wide. You should follow up with your interpretation of this histogram by constructing a histogram that charts the elevator rails produced by each machine. This example shows why histograms are a powerful tool in quality control. What can I learn by comparing histograms from different data sets? Analysts are often asked to compare different data sets. For example, you might be asked how the monthly returns on GM and Cisco stock differ. To answer a question such as this, you can construct a histogram for GM by using the same bin ranges as for Cisco and then place one histogram above the other, as shown in Figure 41-16. See the Histograms worksheet of file Stock.xlsx.