 Dataplot Vol 2 Vol 1

# BIWEIGHT LOCATION

Name:
BIWEIGHT LOCATION (LET)
Type:
Let Subcommand
Purpose:
Compute a biweight based location estimate for a variable.
Description:
Mosteller and Tukey (see Reference section below) define two types of robustness:

1. resistance means that changing a small part, even by a large amount, of the data does not cause a large change in the estimate

2. robustness of efficiency means that the statistic has high efficiency in a variety of situations rather than in any one situation. Efficiency means that the estimate is close to optimal estimate given that we distribution that the data comes from. A useful measure of efficiency is:

Efficiency = (lowest variance feasible)/(actual variance)

Many statistics have one of these properties. However, it can be difficult to find statistics that are both resistant and have robustness of efficiency.

For location estimaors, the mean is the optimal estimator for Gaussian data. However, it is not resistant and it does not have robustness of efficiency. The median is a resistant estimate, but it has only moderate robustness of efficiency.

The biweight location estimator is both resistant and robust of efficiency. Mosteller and Tukey recommend using the median for exploratory work where moderate efficiency in a variety of situations is adequate and the biweight in situations when high performance is needed.

The biweight location estimate is defined as:

$$y* = \frac{\sum_{i=1}^{n}{w_{i}y_{i}}} {\sum_{i=1}^{n}{w_{i}}}$$

where

$$w_{i} = (1 - (\frac{y_{i} - y*}{cS})^{2})^{2} \hspace{0.5in} \mbox{for } (\frac{y_{i} - y*}{cS})^{2} < 1$$

$$w_{i} = 0 \hspace{0.5in} \mbox{otherwise}$$

and

$$S = \mbox{median}\{|y_{i} - y*|\}$$

c = 6 (using 6 means that residuals up to approximately $$4 \sigma$$ are included)

Note that this is an iterative estimate since y* depends on wi and wi depends on y*.

Dataplot will compute up to 10 iterations (computation is terminated if the biweight location estimate does not change in value by more than 0.000001).

Syntax:
LET <par> = BIWEIGHT LOCATION <y>           <SUBSET/EXCEPT/FOR qualification>
where <y> is the response variable;
<par> is a parameter where the computed biweight location is stored;
and where the <SUBSET/EXCEPT/FOR qualification> is optional.
Examples:
LET A = BIWEIGHT LOCATION Y1
LET A = BIWEIGHT LOCATION Y1 SUBSET TAG > 2
Note:
Dataplot statistics can be used in a number of commands. For details, enter

Default:
None
Synonyms:
None
Related Commands:
 BIWEIGHT SCALE = Compute a biweight scale estimate of a variable. BIWEIGHT CONFIDENCE LIMITS = Compute a biweight based confidence interval. MEAN = Compute the mean of a variable. MEDIAN = Compute the median of a variable. MIDMEAN = Compute the midmean of a variable. TRIMMED MEAN = Compute the trimmed mean of a variable. WINSORIZED MEAN = Compute the Winsorized mean of a variable. AVERAGE ABSOLUTE DEVIATION = Compute the average absolute deviation of a variable. MEDIAN ABSOLUTE DEVIATION = Compute the median absolute deviation of a variable. STANDARD DEVIATION = Compute the standard deviation of a variable.
Reference:
Mosteller and Tukey (1977), "Data Analysis and Regression: A Second Course in Statistics," Addison-Wesley, pp. 203-209.
Applications:
Robust Data Analysis
Implementation Date:
2001/11
Program 1:
    LET Y1 = NORMAL RANDOM NUMBERS FOR I = 1 1 10000
LET Y2 = LOGISTIC RANDOM NUMBERS FOR I = 1 1 10000
LET Y3 = CAUCHY RANDOM NUMBERS FOR I = 1 1 10000
LET Y4 = DOUBLE EXPONENTIAL RANDOM NUMBERS FOR I = 1 1 10000
LET A1 = BIWEIGHT LOCATION Y1
LET A2 = BIWEIGHT LOCATION Y2
LET A3 = BIWEIGHT LOCATION Y3
LET A4 = BIWEIGHT LOCATION Y4
LET B1 = MEAN Y1
LET B2 = MEAN Y2
LET B3 = MEAN Y3
LET B4 = MEAN Y4
LET C1 = MEDIAN Y1
LET C2 = MEDIAN Y2
LET C3 = MEDIAN Y3
LET C4 = MEDIAN Y4
PRINT "BIWEIGHT LOCATION ESTIMATE FOR NORMAL      RANDOM NUMBERS = ^A1"
PRINT "MEAN              ESTIMATE FOR NORMAL      RANDOM NUMBERS = ^B1"
PRINT "MEDIAN            ESTIMATE FOR NORMAL      RANDOM NUMBERS = ^C1"
PRINT " "
PRINT "BIWEIGHT LOCATION ESTIMATE FOR LOGISTIC    RANDOM NUMBERS = ^A2"
PRINT "MEAN              ESTIMATE FOR LOGISTIC    RANDOM NUMBERS = ^B2"
PRINT "MEDIAN            ESTIMATE FOR LOGISTIC    RANDOM NUMBERS = ^C2"
PRINT " "
PRINT "BIWEIGHT LOCATION ESTIMATE FOR CAUCHY      RANDOM NUMBERS = ^A3"
PRINT "MEAN              ESTIMATE FOR CAUCHY      RANDOM NUMBERS = ^B3"
PRINT "MEDIAN            ESTIMATE FOR CAUCHY      RANDOM NUMBERS = ^C3"
PRINT " "
PRINT "BIWEIGHT LOCATION ESTIMATE FOR DOUBLE EXPO RANDOM NUMBERS = ^A4"
PRINT "MEAN              ESTIMATE FOR DOUBLE EXPO RANDOM NUMBERS = ^B4"
PRINT "MEDIAN            ESTIMATE FOR DOUBLE EXPO RANDOM NUMBERS = ^C4"

Dataplot generates the following output:
    BIWEIGHT LOCATION ESTIMATE FOR NORMAL      RANDOM NUMBERS = 0.001006
MEAN              ESTIMATE FOR NORMAL      RANDOM NUMBERS = 0.005167
MEDIAN            ESTIMATE FOR NORMAL      RANDOM NUMBERS = -0.01028

BIWEIGHT LOCATION ESTIMATE FOR LOGISTIC    RANDOM NUMBERS = -0.0074
MEAN              ESTIMATE FOR LOGISTIC    RANDOM NUMBERS = 0.000867
MEDIAN            ESTIMATE FOR LOGISTIC    RANDOM NUMBERS = 0.016679

BIWEIGHT LOCATION ESTIMATE FOR CAUCHY      RANDOM NUMBERS = -0.00439
MEAN              ESTIMATE FOR CAUCHY      RANDOM NUMBERS = 3.70155
MEDIAN            ESTIMATE FOR CAUCHY      RANDOM NUMBERS = -0.01582

BIWEIGHT LOCATION ESTIMATE FOR DOUBLE EXPO RANDOM NUMBERS = -0.00203
MEAN              ESTIMATE FOR DOUBLE EXPO RANDOM NUMBERS = -0.00723
MEDIAN            ESTIMATE FOR DOUBLE EXPO RANDOM NUMBERS = -0.00557

Program 2:

SKIP 25
TITLE AUTOMATIC
XLIMITS 1 10
MAJOR XTIC MARK NUMBER 10
MINOR XTIC MARK NUMBER 0
XTIC OFFSET 1 1
X1LABEL BATCH
Y1LABEL BIWEIGHT LOCATION OF DIAMETER
BIWEIGHT LOCATION PLOT DIAMETER BATCH Program 3:

MULTIPLOT 2 1
MULTIPLOT CORNER COORDINATES 0 0 100 100
LET Y = CAUCHY RANDOM NUMBERS FOR I = 1 1 1000
TITLE AUTOMATIC
BOOTSTRAP BIWEIGHT LOCATION PLOT Y
X1LABEL B025 = ^B025, B975 = ^B975
TITLE BOOTSTRAP OF BIWEIGHT LOCATION: CAUCHY RANDOM NUMBERS
HISTOGRAM YPLOT
END OF MULTIPLOT NIST is an agency of the U.S. Commerce Department.

Date created: 11/20/2001
Last updated: 11/02/2015