21-Robust regression.wxb

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```WEIGHTED LEAST SQUARES - used to give more emphasis to selected points in the analysis
What are weighted least squares?
Recall, in OLS we minimize
Q = ! %32 = ! (Y3 - &quot;! - &quot;&quot; X3 )2
or
n
n
i=1
i=1
_ - X&quot;
_ - X&quot;
Q = (Y
_ )w (Y
_)
In weighted least squares, we minimize
–
2
Q = DDe23 œ DDw3 cY3  Y  b&quot; X3 )d
i j
i j
The normal equations become
b! Dw3 + b&quot; Dw3 X3 = Dw3 Y3
b! Dw3 X3 + b&quot; Dw3 X2i = Dw3 X3 Y3
For the intermediate calculations we get (where w3 is the weight)
t n3
D D w3 X3 , DD w3 Yij , DD w3 X3 Yij , DD w3 X23 , DD w3 Y2ij , DD w3
i j
i j
i j
i j
i j
i j
Calculation of the corrected sum of squares is
–
DD y2ij = D D w3 (Yij  Y..)2 = DDw3 Y2ij 
i j
i j
i j
the slope is
^ = b =
&quot;
&quot;
&quot;
DD x3 yij
i j
DD x23
i j
œ
DDw3 Yij X3 
i j
DDw3 X32 
i j
(DDw3 Yij)2
i j
DDw3
i j
(DDw3 YijX3 )
i j
Dw3
(DDw3 X3 )2
i j
DDw3
i j
–
–
the intercept is calculated with X and Y as usual, but these are calculated as
–
Y.. =
DDw3 Yij
i j
DDw3
i j
and
–
X.. =
DDw3 X3
i j
DDw3
i j
It the weights are 1, then all results are the same as OLS
The variance is 532 = 5%23 = 5Y2 3 =
52
w3
WEIGHTED LEAST SQUARES handout
Q =!%32 = !(Y3 - &quot;! - &quot;&quot; X3 )2 or Q = (Y - X&quot;)w (Y - X&quot;)
In OLS we minimize
n
n
i=1
i=1
In weighted least squares, we minimize
Q = De23 œ Dw3 cY3  b! - b&quot; X3 d2
i
i
If the weights are 1, then all results are the same as OLS
The normal equations become
b! Dw3 + b&quot; Dw3 X3 = Dw3 Y3
b! Dw3 X3 + b&quot; Dw3 X2i = Dw3 X3 Y3
For the intermediate calculations we get (where w3 is the weight)
t n3
D D w3 X3 , DD w3 Yij , DD w3 X3 Yij , DD w3 X23 , DD w3 Y2ij , DD w3
i j
i j
i j
i j
i j
i j
Calculation of the corrected sum of squares is
_
DD y2ij = D D w3 (Yij  Y..)2 = DDw3 Y2ij 
i j
i j
i j
the slope is
^ = b =
&quot;
&quot;
&quot;
DDw3 Yij X3 
i j
DDw3 X23 
i j _
(DDw3 Yij)2
i j
DDw3
i j
(DDw3 YijX3 )
i j
Dw3
(DDw3 X3 )2
i j
DDw3
i j
_
the intercept is calculated with X and Y as usual, but the means are calculated as
_
Y.. =
DDw3 Yij
i j
DDw3
i j
2
53 = 5%23 =
and
_
X.. =
DDw3 X3
i j
DDw3
i j
2
The variance is
5Y2 3 = 5w3
For multiple regression
Q = DDe32 = DDw3 cY3  b!  b&quot; X&quot;i  b# X#i  ...  b:-&quot; X:-&quot;,i )d2
i j
i j
the normal equations
the regression coefficients
the variance-covariance matrix;
(Xw WX)B = (Xw WY)
B = (Xw WX)-&quot; (Xw WY)
5,2 = 52 (Xw WX)-&quot;
52 is estimated by MSEA , based on weighted deviations; MSEA =
^ )2
Dw3 (Y3 Y
3
np
In a multiple regression, we minimize
–
2
Q = DDe23 œ DDw3 cY3  Y  b&quot; X&quot;i  b# X#i  ...  b:-&quot; X:-&quot;,i )d
i j
i j
The matrix containing weights is W8x8
&Ocirc; w&quot;
&Ouml; 0
= &Ouml;
0
&Otilde; 0
0
w#
0
0
0 0 &times;
0 0 &Ugrave;
&Ugrave;
... 0
0 w8 &Oslash;
The matrix equations for the regression solutions are,
(Xw WX)B = (Xw WY)
the normal equations
B = (Xw WX)-&quot; (Xw WY)
the regression coefficients
the variance-covariance matrix; 5,2 = 5 2 (Xw WX)-&quot;
where 5 2 is estimated by MSEA , based on weighted deviations
MSEA =
^ )2
Dw3 (Y3 Y
3
np
=
Dw3 /32
n p
Estimating weights
For ANOVA this may be relatively easy if enough observations are available in
each group. Variances can be estimated directly.
However, in regression we usually have a &quot;smooth&quot; function of changing residuals
with changing X3 values&THORN;
To estimate these we note that 53 can be estimated by the residuals |e3 |.
We can then estimate the residuals with OLS, and regress the residuals
(squared or unsquared) on a suitable variable (usually one of the X3
^ ).
variables or Y
3
The procedure can be iterated. That is, if the new regressions coefficients
differ substancially from the old, the residuals can be estimated again
from the weighted regression and the weights can be calcalated again and
the regression fit again with the new weights.
Weighting may be used for various cases
1) One common use is to adjust for non-homogeneous variance.
One common approach is assume that the variance is non-homogeneous
because it si a function of X3 .
Then we must determine what that function is, for simple linear regression,
the function is commonly
532 = 5 2 X3 ,
532 = 5 2 &Egrave;X3
532 = 5 2 X32 ,
to adjust for this, we would weight by the inverse of the function
2) Another common case is where the function is not known, but the data can be
subset into smaller groups. These may be separate samples, or they may
be cells from an analysis of variance (not regression, but weighting also
works for (ANOVA).
Then we determine a value of 532 for each cell i, and the weighting function
is the reciprocal (inverse) of the variance for each subgroup ’ 512 “
3
3) The values analyzed are means, then mean will differ between “observations&quot; if
the sample size are not equal. Since we may still assume that the variance
of the original observations is homogeneous, The the variance of each
2
point is a simple function of n3 (where the variance of a mean is 5n , or
5 2 1n )
we would weight by the inverse of the coefficient, of simply n
If we could not assume that the variances were equal then for each mean we would
2
have the variance of 5n33 , and the weight would be 5n32
3
NOTE: the actual value of a weight is not important, only the proportional value
between weights, so all weights could be multiplied or divided by some
constant value. Some people recommend weighting by 5n32 without
532
3
concern for the homogeneity of variance. Since all
are equal to 5 2
when the variance is homogeneous, this is the same as taking all n3
weights, and multiplying by a constant 512
8. ROBUST REGRESSION  There are many regressions developed that call
themselves &quot;robust&quot;. Some are based on the median deviation, others on
trimmed analyses. The one we will discuss uses weighted least squares,
and is called ITERATIVELY REWEIGHTED LEAST SQUARES.
a) ordinary least squares is considered &quot;robust&quot;,
but is sensitive to points which deviate greatly from the line
this occurs 
(1) with data which is not normal or symmetrical
 data with a large tail
(2) when there are outliers (contamination)
b) ROBUST REGRESSION is basically weighted least squares where the where
the weights are an inverse function of the residuals
(1) points within a range &quot;close&quot; to the regression line get a weight of 1 if
all weights are 1 then the result is ordinary least squares regression
(2) points outside the range get a weight less than 1
(3) ordinary least squares minimize
^ 2 = (residual)2
D(Yij  X3 &quot; )2 = D(Yij  Y..)
robust regression minimizes
Dw3 &Ograve;(Yij  X3 &quot; )2
c5
where w3 is some function of the standardized residual and w3 should be
chosen to minimize the influence of large residuals. The w3 chosen is a
weight, and
if the proper function is chosen, the calculations may be done as a weighted
least squares, minimizing
Dwij (Yij  X3 &quot; )2
(4) the function chosen by Huber was chosen such that if the residual is small ( Ÿ
^ ) there is no weight
1.3455
First we define a robust estimate of variance as called the median absolute
7/.3+8&Ouml;l/3 7/.3+8&ETH;/3 &Ntilde;l&times;
deviation (MAD) such that MAD =
, where the
!&THORN;'(%&amp;
constant 0.6745 will give an unbiased estimate of 5 for independent
observations from a normal distribution.
MAD is an alternative estimate of &Egrave;MSE
Then we define a scaled residual as .&szlig; where . =
/3
Q EH
^
then weight wij = 1 if leij l Ÿ 1.\$%&amp;5
wij =
&quot;&THORN;\$%&amp;
l.l
^
for keij k &gt; 1.3455
where 1.345 is called the tuning constant, and is chosen to make the
technique robust and 95% efficient. Efficiency refers to the ratio of
variances from this model relative to normal regression models.
(5) The solution is iterative
(a) do regression, obtain eij values
(b) compute the weights
(c) run the weighted least squares analysis using the weights
(d) iteratively recalculate the weights and rerun the weighted least
squares analysis  UNTIL
(1) there is no change in the regression coefficients
 to some predetermined level of precision
(2) note that the solution may not be the one with the minimum sum
of squares deviations
(6) PROBLEMS
(a) this is not an ordinary least squares, but the hypotheses are likely to be
tested as if they were
the distributional properties of the estimator are not well documented
(b) Schreuder, Boos and Hafley (Forest Resources Inventories Workshop
Proc., Colorado State U., 1979)
suggest running both regressions
if &quot;similar&quot;, use ordinary least squares
if &quot;different&quot; find out why; if because of an outlier then robust regression is
&quot;probably&quot; better
they further suggest using robust regression as &quot;a tool for analysis, not a cure all
for bad data&quot;
The technique is useful for detecting outlierS.
```