© John Wiley & Sons, Inc.

FIGURE 19-11: The fitted LOWESS curve follows the shape of the data, whatever it may be.

In Figure 19-11, the smoothed curve seems to fit the data quite well across all ages except the lower

ones. The individual data points don’t show any noticeable upward trend until age 12 or so, but the

smoothed curve starts climbing right from age 3. The curve completes its rise by age 20, and then

remains flat until almost age 50, when it starts declining. The rate of decline seems to be greatest

between ages 50 to 65, after which it declines less rapidly. These subtleties would be very difficult to

spot just by looking at the individual data points without any smoothed curve.

Adjusting the amount of smoothing

R’s LOWESS program allows you adjust the stiffness of the fitted curve by specifying a smoothing

fraction, called f, which is a number between 0 and 1. Figure 19-12 shows what the smoothed curve

looks like using three different smoothing fractions.