Difference Series Plot
Description
A way to assess potential discontinuities within one station's data is to compare its data with another station that has no known discontinuities during the time period in question. This tool computes a difference between Station 1 and Station 2, plots the difference series, and tests whether the series defined as Segment 1 has a statistically different mean than Segment 2.
Discussion of the Output Data
This tool assumes a discontinuity occurred between Segment 1 and Segment 2 (e.g., a station move). Visual and statistical inspection is required to assess the potential inhomogeneity introduced by the suspected discontinuity. Output consists of subtracting Station 2 from Station 1 to yield a difference series. This difference series is then plotted in two different colors (Segment 1 colored blue and Segment 2 colored red; Fig. 1). A t-test is then run to determine if the mean of Segment 1 is statistically different from the mean of Segment 2. The p-value of the test and the computed difference between the two segments is reported within the "T-Test Results" area (Fig. 1). Any statistically significant difference, trend in one segment and not in another, or clear changes in variance between the two segments would suggest inhomogeneity in the supsect station.
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Figure 1. In this example, a potential discontinuity occurred at Leavenworth on 1 January 1873. Difference series between daily mean temperatures at Manhattan and Leavenworth before (Segment 1, blue) and after (Segment 2, red) the discontinuity suggest a statistically significant change in the mean (reported under "T-Test Results"). Note also the trend in the difference series during the spring of 1873. |
Input
Input File
Select the input file by clicking on "Browse" and navigating to the proper file.
Start Row
This input is the row number where the main data to be processed begin in the dataset. An example of how row numbers are entered is shown in Figure 2 located in the Running HOB Tools topic.
Station 1 Column
This input is the column number where the station 1 data are located in the input file selected above. An example of how column numbers are entered is shown in Figure 2 located in the Running HOB Tools topic.
Station 2 Column
This input is the column number where the station 2 data are located in the input file selected above. An example of how column numbers are entered is shown in Figure 2 located in the Running HOB Tools topic.
Y-Axis Label
Enter the y-axis label for the plot.
Input Data Format
If Year / Month / Day / Data is selected:
If Year / Month / Data is selected:
If Year / Data is selected:
Each "/" is a tab (i.e., data should be tab-delimited). Station 1 through Station N are the names of the individual stations. All columns must be filled with observations and in the same units. If a station does not have observations during a particular year or day, then a “-99” should be entered indicating a missing value.
Segment 1
Start Plot
Enter the year and, if necessary, the month and the day when the first segment should start. The day and month fields will be disabled if they are not needed based on the "Input Data Format" selected above.
End Plot
Enter the year and, if necessary, the month and the day when the first segment should end. The day and month fields will be disabled if they are not needed based on the "Input Data Format" selected above.
Segment 2
Start Plot
Enter the year and, if necessary, the month and the day when the second segment should start. The day and month fields will be disabled if they are not needed based on the "Input Data Format" selected above.
End Plot
Enter the year and, if necessary, the month and the day when the second segment should end. The day and month fields will be disabled if they are not needed based on the "Input Data Format" selected above.