This example demonstrates how to create a Mixed Population analysis that combines three frequency curves using the probability of union concept. Specifically, two parameterized analytical distributions along with a single manually-defined empirical distribution 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 28 from the study explorer, or from the Analysis menu select open, then select Mixed Population Test 28 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 three. These three curves are representative of different flood causal mechanisms (i.e. populations). The default Allow Extrapolation of Input Data option was selected. Also, "Flow" and "cfs" were entered for the Data Label and Data Units, respectively. The Use Log Transform and Flow Data Type were selected. The Compute Using Order Stats confidence limits option was selected and an Equivalent Years of Record of 45 was entered. The default 5 and 95% confidence limits were selected. Finally, the 0.1-percent ordinate was added to the list of Output Frequency Ordinates.

Figure 1. General Tab Shown for Mixed Population Test 28.
Shown in Figure 2 is the Mixed Population analysis editor with the Frequency Curves tab selected. A name of "LP3" was entered for Frequency Curve 1, a name of "GEV" was entered for Frequency Curve 2, and a name of "Manual" was entered for Frequency Curve 3. The LP3 frequency curve was set to utilize a Log Pearson Type III distribution and the required parameters were entered. Similarly, the GEV frequency curve was set to utilize a Generalized Extreme Values distribution and the required parameters were entered. Finally, the Manual frequency curve was set to utilize a manually-entered empirical distribution. 

Figure 2. Frequency Curves Tab for Mixed Population Test 28.
Figure 3 shows the plotted frequency curves for this analysis. 

Figure 3. Plotted Frequency Curves for Mixed Population Test 28.
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 and confidence limits. The right portion of the Results tab displays the input frequency curves in addition to the computed mixed population frequency curve and confidence limits within 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 28.
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 28.

Figure 5. Report File for Mixed Population Test 28.