Specifying Weights

The discussion below refers to criteria weights, and also applies to decision maker weightings, indicated by the    icon, in GDM.

To specify the weights for a set of criteria, first create a weighting ruler under the weights icon for the criteria folder you are interested in, and then double click on the weighting ruler icon   . This will open the weighting ruler in a tab in the content pane of the Tetra window. Once this ruler is open, you can add and edit the weights for criteria.

Adding criteria to the weighting ruler

When a weighting ruler is first opened it is empty. You need to add the criteria to the ruler that you want to specify weights for. To add criteria you can:

• Right click on blank space in the weighting ruler, choose Add Criterion, and then select either Add All, or choose the criterion you want to add, or
• Choose Add Criterion from the Ruler menu, and then select either Add All, or choose the criterion you want to add.

Once you have added criteria to the weighting ruler it will look similar to the image below.

Click on the image for a larger version. To return to this page, press the back button.

Weighting Criteria

Once you have the criteria on the weighting ruler, you can specify values for the relative weights of each of the criteria in two ways:

• Numerically — enter values into the table below the ruler.
• Graphically — drag the criteria along the ruler to change the values.

Note that it is only the relative values of weights that are significant. For example, the same result would be obtained if two criteria, C1 and C2 were given weights of 1 for C1 and 2 for C2 or 50 for C1 and 100 for C2.

Since the weights are relative, they are specified by setting one criterion as a "standard" against which the importance of another one is measured. Tetra initially sets one arbitrary criterion as the reference weight (Cost in the figure), with a magnitude indicated by the red bar. The blue bar is associated with the selected criterion's weight (Location in the figure). The length of the blue bar relative to the red bar, quantified by the boxed percentage between the rulers, corresponds to the relative importance of these two criteria (i.e. Location is 80% as important as Cost in the figure).

The weights are also shown using a pie chart representation. Each fraction of the pie is colored according to the index next to the numerical values, showing the contribution of each criterion to the total weight. (The pie chart representation is for information purposes only, and cannot be edited.)

If you want to change the reference criterion, right click on the one you want to use as the reference in the ruler and choose Set Right, or select it and choose Set Right from the Ruler menu. If you want to expand the display so that the reference criterion is shown at the right hand side of the ruler, right click in a blank area of the ruler, and choose Expand. Note that this does not change any of the values, but just changes the area of the ruler that is displayed. If you "hover" the mouse over a criterion for a moment, the numerical value for the criterion will be displayed.