## Scatter diagram & regression line
## DescriptionIn a scatter diagram, the relation between two numerical variables is presented graphically. One variable (the independent variable X) defines the horizontal axis and the other (dependent variable Y) defines the vertical axis. The values of the two variables on the same row in the data spreadsheet, give the points in the diagram. ## Required inputThe dialog box for the scatter diagram is similar to the one for Regression: ## Variables**Variable Y**and**Variable X**: select the dependent and independent variables Y and X.**Weights**: select a variable containing relative weights that should be given to each observation (for weighted least-squares regression). Select the dummy variable "*** AutoWeight 1/SD^2 ***" for an automatic weighted regression procedure to correct for heteroscedasticity (Neter et al., 1996). This dummy variable appears as the first item in the drop-down list for Weights.**Filter**: you may also enter a data filter in order to include only a selected subgroup of cases in the statistical analysis.
## Regression equationBy default the option MedCalc offers a choice of 5 different regression equations (X represents the independent variable and Y the dependent variable):
When you select an equation that contains a Logarithmic transformation for one of the variables, the program will use a logarithmic scale for the corresponding variable. ## Options**95% Confidence**: two curves will be drawn parallel to the regression line. These curves represent a 95% confidence interval for the regression line. This interval includes the true regression line with 95% probability.**95% Prediction**: two curves will be drawn parallel to the regression lines. These curves represent the 95% prediction interval for the regression curve. The 95% prediction interval is much wider than the 95% confidence interval. For any given value of the independent variable, this interval represents the 95% probability for the values of the dependent variable.**Line of equality**: option to draw a line of equality (y=x) line in the graph.
## ResidualsIn regression analysis, To obtain a residuals plot, select this option in the dialog box. This graph will be displayed in a second window. ## SubgroupsClick the ## Examples
When you click a point on the regression line, the program will give the x-value and the f(x) value calculated using the regression equation. You can press If you want to repeat the scatter diagram, possibly to select a different regression equation, then you only have to press function key ## ExtrapolationMedCalc does only show the regression line in the range of observed values. As a rule, it is not recommended to extrapolate the regression line beyond the observed range. For particular applications however, such as evaluation of stability data, extrapolation may be useful, see for example the ICH guideline Evaluation of Stability Data (PDF). To allow extrapolation, right-click in the graph and select ## Residuals plotWhen you select the option ## Literature- Altman DG (1991) Practical statistics for medical research. London: Chapman and Hall.
- Eisenhauer JG (2003) Regression through the origin. Teaching Statistics 25:76-80.
- Neter J, Kutner MH, Nachtsheim CJ, Wasserman W (1996) Applied linear statistical models. 4
^{th}ed. Boston: McGraw-Hill.
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