This example demonstrates how to create a Mixed Population analysis that combines two frequency curves from previously-computed analyses using the probability of union concept. Specifically, information from two previously-computed Bulletin 17 analyses will be used to compute a flow-frequency curve that incorporates the likelihood of more than one type of flood in a year. To open the Mixed Population analysis editor for this example, either double-click on the analysis labeled Mixed Population Test 27 from the study explorer, or from the Analysis menu select open, then select Mixed Population Test 27 from the list of available analyses. When the analysis is opened, the Mixed Population Analysis editor will appear as shown in Figure 1.
Shown in Figure 1 are the general settings for this mixed population analysis. The Number of Curves was set to two. These two curves are representative of different flood causal mechanisms (i.e. populations) that occur within the Kettle Creek watershed in north-central Pennsylvania upstream of Alvin R. Bush Dam. Specifically, the first population was meant to represent floods that occur in winter and spring months (i.e. snowmelt and rain-on-snow floods) while the second population was meant to represent floods that occur in summer and fall months (i.e. thunderstorms and tropical storm remnants). The default Do Not Extrapolate Input Data, Use Log Transform, Flow Data Type, Do Not Compute confidence limits, and Output Frequency Ordinates were not modified. However, "Flow" and "cfs" were entered for the Data Label and Data Units, respectively.

Figure 1. General Tab Shown for Mixed Population Test 27.
Shown in Figure 2 is the Mixed Population analysis editor with the Frequency Curves tab selected. A name of "Summer" was entered for Frequency Curve 1 while a name of "Winter" was entered for Frequency Curve 2. The results of an existing Bulletin 17 analysis was imported for both frequency curves; Summer utilizes the KettleCreek_Summer_B17C analysis while Winter utilizes the KettleCreek_Winter_B17C analysis. 

 Figure 2. Frequency Curves Tab for Mixed Population Test 27.
Figure 3 shows the plotted frequency curves for this analysis. 

Figure 3. Plotted Frequency Curves for Mixed Population Test 27.
Press the Compute button to perform the analysis. Once the computations have been completed, a message window will open stating Compute Complete. Close this window and then select the Results tab. The results tab should resemble Figure 4. The left portion of the Results tab window displays the computed mixed population frequency curve. The right portion of the Results tab displays the input frequency curves in addition to the computed mixed population frequency curve in a plot. To view the input and results in a separate window, either click Plot Mixed Population Frequency Curves button or double click on the plot within the Results tab.
 
Figure 4. Results tab for Mixed Population Test 27.
In addition to the tabular and graphical results, there is a report file that shows input data and results. To review the report file, press the View Report button at the bottom of the analysis window. When this button is selected a text viewer will open the report file and display it on the screen. Shown in Figure 5 is the report file for Mixed Population Test 27.

Figure 5. Report File for Mixed Population Test 27.