loess.control
Computational Options for Loess Fitting
Description
Specifies computational options used within the
control argument of loess.
Usage
loess.control(surface = c("interpolate", "direct"), statistics =
c("approximate", "exact"), trace.hat = c("exact", "approximate"),
cell = 0.2, iterations = 4, ...)
Arguments
| surface |
determines how the fitted surface is computed.
- "interpolate" (the default) specifies that the fitted surface is
computed using an interpolation method. Use "interpolate"
unless special circumstances warrant.
- "direct" specifies that the fitted surface is computed directly
at all points.
|
| statistics |
determines how the statistical quantities are computed.
- "approximate" (the default) specifies an approximate computation.
- "exact" specifies an exact computation. Use only for
testing the approximation in statistical development. It is not meant
for routine usage because computation time can be horrendous.
|
| trace.hat |
- If statistics is "approximate",
determines the computational method used to compute the trace of the hat
matrix, which is used in the computation of the statistical quantities.
- If statistics is "exact", an exact computation is done.
Normally this computation is quite fast on the fastest machines until n,
the number of observations, is 1000 or more, but for very slow machines,
computation can slow at n = 300.
The default is "exact" for n < 500 and "approximate" otherwise.
If statistics is "exact", an exact computation is always done for the
trace.
|
| cell |
if surface is specified as interpolate, cell
specifies the maximum cell size of the k-d tree.
For example, suppose k <- floor(n*cell*span)
where n is the number of observations.
Then a cell is further divided if the number of observations within it
is greater than or equal to k.
|
| iterations |
if the family argument of loess is "symmetric", the number of iterations of
the robust fitting method.
|
Details
This function can be used to create a list that is given as the
value of the argument
control of the function
loess.
Most users need not specify
control
because its defaults provide satisfactory performance
in most cases.
- Changing the method of evaluating surface
to direct can increase the computational burden
slightly for small data sets and by a substantial
amount for large data sets.
- Changing the method
of computing statistics to exact can cause
a catastrophic computation time, except perhaps
in supercomputer environments.
The arguments of
loess.control can
be specified directly in a call to
loess for a temporary
change in the computational methods. However, you can
achieve a lasting change by rewriting
loess.control with new defaults.
Value
returns a list with the following components:
| surface |
the value of the argument surface.
|
| statistics |
the value of the argument statistics.
|
| trace.hat |
the value of the argument trace.hat.
|
| cell |
the value of the argument cell.
|
| iterations |
the value of the argument iterations.
|
See Also
Examples
loess(ozone ~ radiation, Sdatasets::air, span = 1/2, drop.square = "C",
parametric = "C", control = loess.control(surface = "direct"))
# The following has the same result:
loess(ozone ~ radiation, Sdatasets::air, span = 1/2, drop.square = "C",
parametric = "C", surface = "direct")