Normal Probability Plot for Excel 2007

by Stephanie Ellen

A normal probability plot for Excel 2007 shows the normal distribution, sometimes called a "Bell Curve." The normal distribution is a type of statistical graph that shows a spread of data. Microsoft Excel has a built-in function, the Normdist function, that can plot the normal distribution in a spreadsheet.

Significance

The normal distribution is a type of statistical distribution that allows a statistician or researcher to see how many members of a particular population will fall into a certain category. For example, class grades, IQ scores and employee performance can all be plotted using a standard normal distribution. With a normal distribution, 68 percent of group members will fall into the center of the graph (you can think of this as average scores, or C grades).

The Normdist Function

The Normdist function has the syntax "=Normdist(x,mu,sigma,cumulative)." "X" is the input for the command, mu is the average and sigma is the standard deviation. The last term, "cumulative" is nearly always set to "1" for true because normal distributions are generally considered cumulative functions (they build upon each other). The standard deviation is a measure of how spread out the data is around a mean (an average figure).

Example

An example of a Normdist function in Excel might be =Normdist(A1,0,1,0). This tells Excel that the input for the function is in cell A1, and that the function should calculate the normal distribution with an average of 0 and a standard deviation of 1. The last digit in this case is a '0" because we are entering a single cell's data (cell A1) into the function so this isn't a cumulative function.

Graphs

Once you have used the Normdist function to define all of your data points in Excel, you can graph the bell curve by clicking on "Insert" and then "Scatter." Choose a line graph and Excel will plot a normal distribution curve based on your data.

Layout

Excel doesn't automatically lay out the normal probability plot on a standard X Y axis: it's up to you to lay out the graph according to your needs. In Excel 2007, click on the "Layout" tab, select "Axes" and then set the axes to your own specifications. How you set the axes will determine the overall look of your bell curve (for example, it might make the curve look thin or fat).

About the Author

Stephanie Ellen teaches mathematics and statistics at the university and college level. She coauthored a statistics textbook published by Houghton-Mifflin. She has been writing professionally since 2008. Ellen holds a Bachelor of Science in health science from State University New York, a master's degree in math education from Jacksonville University and a Master of Arts in creative writing from National University.