What does the span argument control in geom_smooth?
I am using geom_smooth from the ggplot2 package to create a smoothed line on a time series scatter plot (one point for each day of the year, so I have 365 points). One of the arguments is called span, and going into the help file (?geom_smooth) the following description is given:
spancontrols the amount of smoothing for the default loess smoother. Smaller numbers produce wigglier lines, larger numbers produce smoother lines.
However, this doesn't actually tell me what the span argument is controlling. Setting it to 1 is useless, and setting it to 0.1 provides something that looks good.
span = 0.5
span = 0.1
However, when describing the plot, since I'm not totally sure what span actually changes, I'm not sure how to describe the smoothed line. Any pointers?
Answer
LOESS smoothing is a non-parametric form of regression that uses a weighted, sliding-window, average to calculate a line of best fit. Within each "window", a weighted average is calculated, and the sliding window passes along the x-axis.
One can control the size of this window with the span argument. The span element controls the alpha, which is the degree of smoothing. The smaller the span, the smaller the 'window', hence the noisier/ more jagged the line.
Look for documentation under LOESS rather than span.