Total, Explained, and Residual Sum of Squares

Report
Total, Explained, and Residual Sum
of Squares




Total sum of squares: Sum of the squared difference between the actual Y and the mean
of Y, or,
 TSS = Σ(Yi - mean of Y)2
 Intuition: TSS tells us how much variation there is in the dependent varaible.
Explained sum of squares: Sum of the squared differences between the predicted Y and
the mean of Y, or,
 ESS = Σ(Y^ - mean of Y)2
 Note: Y^ = Yhat
 Intuition: ESS tells us how much of the variation in the dependent varaible our model
explained.
Residual sum of squares: Sum of the squared differences between the actual Y and the
predicted Y, or,
 RSS = Σ e2
 Intuition: RSS tells us how much of the variation in the dependent varaible our model
did not explain.
Given these definitions, it must be the case that….

TSS = ESS + RSS
The coefficient of determination or R-squared






How do we know how accurate our equation is?
The coefficient of determination or R-squared: Ratio of
the explained sum of squares to the total sum of
squares.
R-squared = Explained Sum of Squares / Total Sum of
Squares
R2 = ESS/TSS
=
R2 = 1 - RSS/TSS
R2 ranges from 0 to 1. A value of zero means our model did
not explain any of the variation in the dependent variable. A
value of 1 means the model explained everything. Neither 0
or 1 is a very good result.
The Simple Correlation Coefficient (r)

r = (r2)0.5







The above is only true when the number of independent variables is one.
Examples:


Note:
= (ESS/TSS)0.5 = (1-RSS/TSS)0.5
If
If
If
If
r = 0.9, then r2 = 0.81
r = 0.7, then r2 = 0.49
r = 0.5, then r2 = 0.25
r = 0.3, then r2 = 0.09
If X = Y then r = 1
Note: Also works vise versa
If X = -Y then r = -1
If X is not related to Y, then r = 0
Adjusted R-Squared





Adding any independent variable will increase R2.
Why? Adding more variables will not change TSS. It can either
leave RSS unchanged or lower RSS.
Unless the new variable has a coefficient of zero, RSS will fall.
To combat this problem, we often report the adjusted R2 (which
Excel provides).
For those who are interested, here is the calculation:

Adjusted R2 = 1 - [RSS/(n-K-1)] / [TSS/(n-1)]

where n = observations
K = number of coefficients


ONE SHOULD NOT PLAY THE GAME OF MAXIMIZING RSQUARED OR ADJUSTED R-SQUARED!!!!
The Standard Error of β1 in a model with two
independent variables
• SE (β1–hat) =
{[Σ(ei)2 / (n-3)] /
[Σ(X1 – mean of X )2 *(1-(r12)2)]} 0.5
• Elements
– Residual sum of squares: Σ(ei)2
– Number of observations: n
– Total sum of squares of X: Σ(X1 – mean of X )2
– Correlation coefficient squared between X1 and
X2 or the r-squared if you regressed X1 on X2.
Details of Standard Error Formula
• If n increases – the denominator will rise unambiguously (because the
TSS of X must rise with more observations), but because a higher n
increases both Σ(ei)2 (or the RSS of the model) and n (obviously), the
numerator may or may not increase.
– Result: Increase n and the standard error of the β1–hat will fall.
• What if the residual sum of squares {Σ(ei)2} rises, holding n constant?
Then the standard error will rise.
• What if the total sum of squares of the X variable{Σ(X1 – mean of X )2}
increases? Then the standard error will fall.
– In other words, the more variation in X, or the more information we have about X,
the better will be our estimate.
• What if there is strong correlation between X1 and X2? Then the
standard error will rise.
Null vs. Alternative Hypothesis


The Null Hypothesis (H0): a statement of the
range of values of the regression coefficient that
would be expected if the researcher’s theory
were NOT correct.
The Alternative Hypothesis (HA): a statement
of the range of values of the regression
coefficient that would be expected if the
researcher’s theory were correct.
Some basic language

We are trying to control for the probability of
rejecting the null hypothesis when it is in fact
true. We cannot control for the probability of
accepting the null hypothesis when it is in fact
false. Hence we do not accept the null
hypothesis, rather we cannot reject the null
hypothesis.
The t-statistic
t = (β1 - βH0) / SE(β1)
 1



Since typically the border value for the null
hypothesis is zero.
In other words, our null hypothesis is that the
coefficient has a value of zero.
Given this null…. the t-stat is generally the
coefficient / standard error. It is this value the
computer packages will report.
Judging the significance of a variable




The t-statistic: estimated coefficient / standard deviation of the
coefficient.
The t-statistic is used to test the null hypothesis (H0) that the
coefficient is equal to zero. The alternative hypothesis (HA) is that
the coefficient is different than zero.
Rule of thumb: if t>2 we believe the coefficient is statistically
different from zero. WHY?
Understand the difference between statistical significance and
economic significance.
The p-value

p value = probability value






observed or exact level of significance
exact probability of committing a Type I error
the lowest significance level at which a null
hypothesis can be rejected.
Level of significance:
Indicates the probability of
observing the estimated t-value greater than the critical tvalue if the null hypothesis were correct.
Level of confidence:
Indicates the probability that the
alternative hypothesis is correct if the null hypothesis is
rejected.
One can state either:
The coefficient has been shown
to be significant at the 10% level of significance or the 90%
level of confidence.
Limitations of t-test
The t-test does not test theoretical
validity
 The t-test does not test importance
 The t-test is not intended for tests of
the entire population

More on t-test


The t-test does not test coefficients jointly.
Because β1 and β2 are statistically different than
zero it does not tell us that β1 and β2 are jointly
different than zero.
The F-Test


A method of testing a null hypothesis that includes
more than one coefficient
It works by determining whether the overall fit of an
equation is significantly reduced by constraining the
equation to conform to the null hypothesis.
The Test of Overall Significance






H0: β1 = β2 = ....... βk = 0
The R2 is not a formal test of this hypothesis.
HA :
Ho is not true.
F = [ESS/(k)] / [RSS / (n-k-1)]
Intuition: We are testing whether or not the
variation in X1, X2, .... Xk explains more of Y than
the random forces represented by error term.
Refer to the corresponding p-value of the F-test to
answer this question.

similar documents