| Data Smoothing: Loess |
Data Smoothing: Loess
The Loess analysis is intended for scatter plot
smoothing. Given bivariate data 
, the Loess
analysis fits a regression function 
whose value at
a point 
is obtained by evaluating a local regression function at that
point.
This function is constructed based on
data within a neighborhood of . Although the fit in each local
neighborhood is parametric, the construction of the function
depends on many neighborhoods. Consequently,
the resulting function is nonparametric.
You can run a Loess analysis by selecting Analysis 
Data
Smoothing Loess from the main menu. The computation of the
loess regression function, confidence limits, and related statistics
is implemented by calling the LOESS procedure in SAS/STAT.
See the
LOESS procedure documentation in the SAS/STAT User's Guide for additional
details.
Note:
Fitting a loess curve to data sets with more than several thousand
observations might require you to wait a while for the computation
to finish, especially if you are computing confidence limits or
performing an exhaustive search to find the optimal value of the
smoothing parameter. Because of this, the Loess analysis presents a
warning message (shown in Figure 18.1) when your data
contain more than 5000 observations. A similar warning appears if you
are performing an exhaustive search and there are more than 1000 observations.
|
Figure 18.1: A Warning Message
Example
Example: Comparing Smoothers
Specifying the Loess Analysis
Analysis of Selected Variables
References