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17. Calculate Correction Factor

To calculate correction factors, select Calculate Correction Factors from the Break Point menu or the calculator symbol on the tool bar. The Calculate Correction Factor screen will appear.

This screen provides two graphs, one being the double mass curve of the estimate of raw data and the other showing indicators on the double mass curve plotted against a linear time scale. The break point whose correction factor is to be calculated is displayed in black on both graphs. The station id, name and station history are also displayed along with the season (if appropriate).

After determining and saving a correction factor for a particular breakpoint, the user can move to either other stations or other breakpoints for the current station by pressing the Next Station, Previous Station, Next Break Point or Previous Break Point buttons.

Note: The ability to calculate correction factors by season is available. The user may select which season to calculate correction factors for if the data was producted by the preprocessor for seasons. If the user does select a season, then the next and previous break point buttons will only scroll through those break points for the specified season. To get to the break points for the other season, the user must select the other season. If the user selects the All option then the next and previous break point buttons will scroll through the break points in order of time of the break points.

In addition to defining the value of a correction factor, the user must also define the range over which the break point is applied.

A correction factor is applied to a specific segment of the double mass plot called the "break point range". The break point range runs from the breakpoint under consideration to an end point defined by the next break point along the curve or the end of the curve, whichever comes first. The user can move the current break point by entering a new date. This will take effect as soon as the user moves the cursor out of the field.

There are two ways that correction factor can be derived. The first is to enter a value directly and the second relies on identifying a slope on the double mass plot that the break point range should match. The slope can be chosen arbitrarily but is usually the slope of the double mass curve plot in the segment prior to the breakpoint.

Once a correction factor is specified, it is saved by pressing the "Save Correction" button. Correction factors must be saved in this way so that they can be used elsewhere in the IDMA tool. This action does not save correction factors to the preprocessor input file. Saving the correction factors to the preprocessor input file is discussed elsewhere.

At any point, the user can arbitrarily change the value of the correction factor that is displayed, however it must always be saved by pressing the Save Correction button for it to be used elsewhere in the tool.

In the case of direct entry of the correction faction, the user merely enters a value for the correction.

In the case of identifying a slope segment that the break point range should match, the user identifies the slope segment (as discussed below) and then presses the "Compute Breakpoint" button to instruct the IDMA tool to compute the appropriate correction factor.

The slope segment is identified either by entering the beginning and ending dates of the slope segment directly or by dragging the cursor from the beginning point of the slope segment on the double mass plot to the ending point of the slope segment. When the slope segment is identified by dragging the cursor, a line is drawn on the double mass plot showing the slope visually.

Once all correction factors have been saved, press the close button to return to the Double Mass Analysis screen.