Interpreting the Results of Risk Analysis
Open Plan provides a number of views that you can use to display the results of risk analysis.
You can use the risk spreadsheet, activity risk barchart, or risk analysis histogram to view the mean and standard deviation of the following dates for each activity:
- Early start dates
- Late start dates
- Early finish dates
- Late finish dates
In addition, you can use either the Risk Spreadsheet or the Activity Risk Barchart to display the following additional statistical information about each activity:
- Mean and standard deviation of:
- Total float
- Free float
- Criticality index
This information displays in a risk spreadsheet view.
The mean value of a distribution is one measure of central tendency and may be roughly thought of as the weighted average of the possible values in a distribution. It is important to remember that the value for standard deviation is not the maximum amount of variation possible in either direction from the mean value. Instead, you can use the standard deviation value as a measure of confidence as follows:
- There is an approximately 68% probability of an outcome occurring that falls within one standard deviation of the mean.
- There is an approximately 95% probability of an outcome occurring that falls within two standard deviations of the mean.
- There is an approximately 99% probability of an outcome occurring that falls within three standard deviations of the mean.
Thus, you could expect with 95% confidence that an activity with a mean early start of May 1 and a standard deviation of six days would start within 12 days of that date.
This technique provides only an approximation of confidence levels. For one thing, the actual distribution is often skewed in one direction or the other, rendering confidence estimates based on mean and standard deviation values less effective. To get a complete picture, an activity should be designated as a key activity as shown in a risk histogram.
It is also possible to show clearly the relationship between criticality index of an activity and the mean and standard deviation of the total float. Activities with total floats that have large standard deviations relative to their means are more likely to be critical than activities with relatively small standard deviations.
Open Plan calculates values for free float and total float based on the project calendar. Depending on the option you select on the Options tab of the Risk Analysis dialog box, standard deviations for dates can be calculated with or without regard to the activity calendar.