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Title screen: Descriptive Statistics and skewness. Copyright ©2017 McGraw-Hill Education. All rights reserved.
Narrator: "This video demonstrates how Excel's Descriptive Statistics output can be used to determined skewness of a data set. When data sets are graphed, they commonly display one of four shapes—symmetric, positively skewed, negatively skewed, and bimodal. In a symmetrically-shaped graph, data are equally dispersed around the mean, forming a bell-shaped curve. A set of values is skewed to the right, or positively skewed, if plotted data show a single peak, but the values extend further to the right of the peak than to the left. Let's consider an example of positive skew. In this data set, a sample of 70 participants between the ages of 40 and 50 are represented by the size of their online investment portfolios in column 'A.' Here is the Descriptive Statistics output for that column. As a reminder, Descriptive Statistics is found in the Data tab in the Data Analysis menu. Skewness is 1.45. A positive value indicates positive or right skew. Let's insert a histogram and take a look at the data. I'll select anywhere in the column, Insert, Charts, All Charts, and Histogram."
The bottom right corner of the Charts section, found within the Insert tab, is selected to open the Insert Chart dialog box. At the top of the box, the All Charts tab is clicked to select Histogram. Upon selection, the histogram chart appears on the sheet, to the right of the data.
Narrator: "And as you can already see from the preview image, this data is skewed to the right. As you can see, the values in the graph trail to the right. The mean is also to the right of or greater than the median, which we can see in our Descriptive Statistics table. The mean of 242.7 is larger than the median of 199.75. Now let's consider a negative-skewed example. In this data set, we have the number of families who used a YMCA daycare service during a 30-day period, and we have a -.67 skew. We will expect to see values extending further to the left than to the right. This example is not as significantly skewed as our last example. The skewness is a smaller value. Let's insert a histogram and look at the results. The mean of 42.7 is the left of the median, or is less than the median, which is the peak of 44 in this third column, and values trail slightly to the left. In this video, we have demonstrated how to find skewness using Descriptive Statistics, and how to determine whether data are positively or negatively skewed using the skewness value in our Descriptive Statistics output."
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