Transcript PowerPoint on Regression Analysis

Equations in Simple
Regression Analysis
The Variance
x
sx 
n 1
2
2
The standard deviation
Sx 
s
2
x
The covariance
 xy
sxy 
n 1
The Pearson product moment
correlation
rxy 
sxy
sx s y
The normal equations (for the
regressions of y on x)
 xy
bxy 
x
sxy

sx
2
2
a  Y - byx X
The structural model (for an
observation on individual i)
Yi  a  byx X i  ei
The regression equation
Y  a  b yx X
 (Y  b yx X )  b yx X
 Y  b yx ( X  X )
 Y  b yx X
Partitioning a deviation score, y
y YY
 Y  (Y  Y )  (Y  Y )  Y


 (Y  Y )  (Y  Y )
Partitioning the sum of squared
deviations (sum of squares, SSy)
y
2
 (Y  Y )
  [(Y  Y )  (Y  Y )]
  (Y  Y )   (Y  Y )

2
2
2
 SS reg  SS res
2
Calculation of proportions of sums of
squares due to regression and due to
error (or residual)
y
y
2
2

1
SS reg
y
2
SS reg
y
2
SS res

y

SS res
y
2
2
Alternative formulas for computing the
sums of squares due to regression
 (Y  Y )
  (Y  bx  Y )
  (bx )
b x
(  xy )

x

( x )
(  xy )

x
xy


xy

x
 b xy
SS reg 
2
2
2
2
2
2
2 2
2
2
2
2
Test of the regression coefficient, byx,
(i.e. test the null hypothesis that byx = 0)
First compute the variance of estimate
s
2
y x
 est ( )
2
y
2

(Y  Y )


N k1
SS res

N k1
Test of the regression coefficient, byx,
(i.e. test the null hypothesis that byx = 0)
Then obtain the standard error of estimate
s y x 
2
s y x
Then compute the standard error of the regression
coefficient, Sb
sb 
s
2
y x
( x ) / (n  1)
2

s y x
( x ) / ( N  1)
2
The test of significance of the regression
coefficient (byx)
The significance of the regression coefficient is tested
using a t test with (N-k-1) degrees of freedom:
t

b yx
sb
b yx
S y x
Sx n  1
Computing regression using
correlations
The correlation, in the
population, is given by
 xy

N x  y
The population correlation
coefficient, ρxy, is estimated by
the sample correlation
coefficient, rxy
 zx z y
rxy 
N
sxy

sx s y
 xy

2
2
x
y
 
Sums of squares, regression (SSreg)
Recalling that r2 gives the proportion of variance of Y
accounted for (or explained) by X, we can obtain
SS reg  r 2  y 2
SS res  (1  r ) y
2
2
or, in other words, SSreg is that portion of SSy
predicted or explained by the regression of Y on X.
Standard error of estimate
From SSres we can compute the variance of
estimate and standard error of estimate as
(1  r ) y

N  k 1
 s y x
2
2
s y x
s y x
2
(Note alternative formulas were given earlier.)
Testing the Significance of r
The significance of a correlation coefficient,
r, is tested using a t test:
t
r N 2
1 r
2
With N-2 degrees of freedom.
Testing the difference between
two correlations
To test the difference between two
Pearson correlation coefficients, use
the “Comparing two correlation
coefficients” calculator on my web site.
Testing the difference between
two regression coefficients
This, also, is a t test:
b1  b2
t
S b21  S b22
Where
S
2
b
was given earlier. When the variances, Sb2 , are
unequal, used the pooled estimate given on page
258 of our textbook.
Other measures of correlation
Chapter 10 in the text gives several
alternative measures of correlation:
Point-biserial correlation
Phi correlation
Biserial correlation
Tetrachoric correlation
Spearman correlation
Point-biserial and Phi correlation
These are both Pearson Product-moment
correlations
The Point-biserial correlation is used when on
variable is a scale variable and the other
represents a true dichotomy.
For instance, the correlation between an
performance on an item—the dichotomous
variable—and the total score on a test—the
scaled variable.
Point-biserial and Phi correlation
The Phi correlation is used when both
variables represent a true dichotomy.
For instance, the correlation between two
test items.
Biserial and Tetrachoric
correlation
These are non-Pearson correlations.
Both are rarely used anymore.
The biserial correlation is used when one
variable is truly a scaled variable and
the other represents an artificial
dichotomy.
The Tetrachoric correlation is used when
both variables represent an artificial
dichotomy.
Spearman’s Rho Coefficient and
Kendall’s Tau Coefficient
Spearman’s rho is used to compute the
correlation between two ordinal (or
ranked) variables.
It is the correlation between two sets of
ranks.
Kendall’s tau (see pages 286-288 in the
text) is also a measure of the
relationship between two sets of ranked
data.