Theil
Mehod is used to determine statistically significant trends of
visibility and major aerosol chemical components in IMPROVE sites with
long-term (e.g. > 10 years) monitoring data.
An excel macro is developed to do
the trend analysis. To run the trend analysis macro, right click the link
and choose "Save Target As..." to save the excel file in your computer;
Enter any time series of the data in sheet1 of the excel file as shown in
the example; Push the "Trend Analysis" button on the bottom left of
sheet1. The analysis will be done and the results, including the slope,
S-Value and P-Value, will be outputted to sheet1.
The slope (units/year) indicates the overall trend of the time series
of the data (positive indicates increase and negative indicates decrease).
The cumulative probabilities (P-Value) are calculated based on the S-Value
and the Kendall's Tau statistics, and used as significance criterion for
trend evaluation. In EPA 1997 National Air Quality Trends Report, a 5%
P-Value has been chosen as the significance criterion (if P-Value < 5%,
the trend is considered statistically significant).
