Introduction to ChemStat
Introduction to ChemStat
ChemStat is an application for the statistical analysis of ground water monitoring data at RCRA facilities. ChemStat is not a data management program. Data must be imported from other data management applications such as USEPAGRITS/STAT, or Starpoint SoftwareChemPoint.
ChemStat operates on data for only one facility at a time. Once one data file is opened, the file must be closed before another can be opened. Statistical methods are selected from either the Distribution menu, or the Analysis menu. Graphs are selected from the Graphs menu.
The Distribution menu includes methods to test the data for normal distribution, homogeneity of variance, skewness, trend analysis, or outlier detection. The Analysis menu includes statistical methods to determine if a statistically significant increase has occurred in down-gradient wells. The Graphs menu includes graphs for the statistical analysis of data, and time-concentration plots to view trends in data over time. The Options menu includes selection for program options such as fonts, colors, and customization. The Select menu includes functions specific to specific analyses. Context-sensitive right-click menus are available throughout the program. To access a right-click menu, click the right mouse button over the window of the current analysis. All menu selections on the Select menu are available from the appropriate right-click menu. The Workbook menu contains commands specific to the Workbook.
Selecting a Parameter
Each statistical method or graph is performed only for a single parameter (except the multi-parameter graph). The parameter is selected from the drop-down list box on the main window button bar.
Selecting a Well
Most statistical comparisons are performed for all compliance (down-gradient) wells. However some methods, such as the Wilcoxon Rank-Sum method or intra-well comparisons, are performed only for a selected compliance well. For analyses that require a specific well, the well is specified from a drop-down list on the tool bar button.
Selecting an Analysis Method
Select the desired statistical report or graph from the appropriate menu. A new window will open displaying the analysis or graph for the selected parameter or well. Changing the parameter or well selection will update all displayed statistical reports and graphs with the new parameter/well combination.
Selecting From the Method Tree
You can also select statistical analysis methods from the Method Tree. The tree lists all statistical methods in a hierarchical structure, so you can find methods based on guidance documents, or data set requirements. Double click on a statistical method to perform the test.
Some statistical reports or graphs are displayed with default values that you may want to change. For most methods, options to change default values are selected from the right-click menu. For example, graphs have a choice of connection lines; prediction intervals have a choice of recent sampling dates for the comparison; and normality tests have a choice of the well groups to test for normality.
ChemStat allows for the statistical analysis of the original data, or one of many transformations. Non-detects can be represented by the detection limit, ½ of the detection limit, 1, or 0, or adjusted with Aitchison's or Cohen's methods. Select Options | Transform Data to change the detection limit. All displayed methods will be updated with the new transformation and non-detect representation.
Although data can only be imported into ChemStat via an ASCII text file, there are advantages to saving your data in ChemStat's binary format. Binary data files load as much as 99% faster than ASCII text files. The binary format can not be access or edited by any other program, so the data are not subject to accidental alteration. Additional information is saved with the binary file including a description of the file. You can enter this description, along with other information from File | Properties. Binary files all contain all of your Workbook information. Binary ChemStat files have a "csd" file name extension.
Displaying Statistical Analyses
Results for any statistical method are quickly displayed in a new window. Text results are referred to as reports. Other results appear as graphs. Reports and graphs can both be printed. From File | Header/Footer, page header and footer options can be selected. Options for headers and footers include the representation of non-detects, and the data transformation.
Reports can also be exported to tab-delimited ASCII text files, or rich-text formatted files. Tab-delimited ASCII text files can be imported into spreadsheet applications. Rich-text format can be imported into most word processors, while preserving the font and color scheme.
Report text that show a statistically significant increase in contamination can be displayed in with different colors than text not showing a statistically significant increase in contamination. From Options | Colors, set the transient color for the color that does not show a statistically significant increase in contamination, and set the alert color for the color the does show a statistically significant increase in contamination. By default, transient is green and alert is red. Because results may be printed on a black-and-white printer, the background color for alert text can be specified. By setting the alert background to gray, alert text will appear shaded when printing in black-and-white.