Histogram Plots

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The histogram, or relative frequency, plot is the most commonly used in risk analysis. A histogram plot of simulation data can be produced in ModelRisk by selecting the variable in the Simulation Results window and clicking:

The plot is produced by grouping the data generated for a model’s output into a number of bars or classes. The number of values in any class is its frequency. The frequency divided by the total number of values gives an approximate probability that the output variable will lie in that class’ range. We can easily recognise common distributions like a triangular, normal, uniform, etc, and we can see whether a variable is skewed. The figure below shows a typical plot:

Graphing controls

allows you to copy the graph as a bitmap image

to print the graph

to zoom in on part of the graph

to change the horizontal axis range

to change between line and bar plots

to switch sliders on/off

to define the position of sliders

to plot together the same variable for multiple simulation runs

to turn the legend on/off

opens a comprehensive   dialog to edit the graph

The most common mistake in interpreting a histogram is to read off the y-scale value as the probability of the x-value occurring. In fact, the probability of any x-value, given the output is continuous (and most are), is infinitely small. If the model’s output is discrete, the histogram will show the probability of each allowable x-value, providing the class width is less than or equal to the distance between each allowable x-value.