Use a smoother line to help you explore the potential relationships between two variables without fitting a specific model, such as a regression model. Smoother lines are most useful when the curvature of the relationship does not change sharply. Smoother lines are calculated using the lowess smoothing method.
To add smoother lines when you create a graph, click Data View. To add smoother lines to an existing graph, click the graph and choose Editor > Add > Smoother. To edit smoother lines, select and double-click the smoother lines that you want to edit. For more information on selecting smoother lines, go to Select groups and single items on a graph. You can change the following items on the Options and Attributes tabs.
Lowess smoothing parameters
Degree of smoothing
A lowess smoother generally works best when the fraction (f) of points is large enough to give a smooth fit without distorting the underlying relationship between the variables. Cleveland1 suggests that you make f as large as possible, but maintain unrelatedness in a separate lowess plot of the y-value residuals versus the x-values.
Number of steps
To limit the influence of outliers on the smoothed y-values, you can set the number of iterations of smoothing. Each step reduces the weights given to outliers in the next iteration of weighted linear regression, based on the size of residuals in the previous lowess step. For more details, refer to step 4 of the lowess method. When you set the number of steps to 0, step 4 of the lowess method is eliminated entirely. Cleveland suggests that two robust steps adequately smooth outlier effects for most data.
Double-click a smoother line to change its color, size, or type.
1 W.S. Cleveland (1979). "Robust locally weighted regression and smoothing scatterplots," Journal of the American Statistical Association, 74, 829-836.