part 17: regression residuals 17-1/38 statistics and data analysis professor william greene stern...
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Part 17: Regression Residuals17-1/38
Statistics and Data Analysis
Professor William Greene
Stern School of Business
IOMS Department
Department of Economics
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Statistics and Data Analysis
Part 17 – The Linear Regression Model
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Regression Modeling
Theory behind the regression model Computing the regression statistics Interpreting the results Application: Statistical Cost Analysis
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A Linear Regression
Predictor: Box Office = -14.36 + 72.72 Buzz
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Data and Relationship
We suggested the relationship between box office sales and internet buzz is Box Office = -14.36 + 72.72 Buzz
Box Office is not exactly equal to -14.36+72.72xBuzz How do we reconcile the equation with the data?
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Modeling the Underlying Process
A model that explains the process that produces the data that we observe: Observed outcome = the sum of two parts (1) Explained: The regression line (2) Unexplained (noise): The remainder.
Internet Buzz is not the only thing that explains Box Office, but it is the only variable in the equation.
Regression model The “model” is the statement that part (1) is the
same process from one observation to the next.
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The Population Regression
THE model: (1) Explained:
Explained Box Office = α + β Buzz (2) Unexplained: The rest is “noise, ε.”
Random ε has certain characteristics Model statement
Box Office = α + β Buzz + ε Box Office is related to Buzz, but is not exactly
equal to α + β Buzz
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The Data Include the Noise
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What explains the noise?What explains the variation in fuel bills?
ROOMS
FUEL
BIL
L
111098765432
1400
1200
1000
800
600
400
200
Scatterplot of FUELBILL vs ROOMS
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Noisy Data?What explains the variation in milk production other
than number of cows?
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Assumptions
(Regression) The equation linking “Box Office” and “Buzz” is stable
E[Box Office | Buzz] = α + β Buzz
Another sample of movies, say 2012, would obey the same fundamental relationship.
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Model Assumptions
yi = α + β xi + εi α + β xi is the “regression function” εi is the “disturbance. It is the unobserved
random component The Disturbance is Random Noise
Mean zero. The regression is the mean of yi.
εi is the deviation from the regression. Variance σ2.
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We will use the data to estimate and β
Sample : a + b Buzz
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We also want to estimate 2 =√E[εi2]
Sample : a + b Buzz
e=y-a-bBuzz
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Standard Deviation of the Residuals
Standard deviation of εi = yi-α-βxi is σ
σ = √E[εi2] (Mean of εi is zero)
Sample a and b estimate α and β Residual ei = yi – a – bxi estimates εi
Use √(1/N-2)Σei2 to estimate σ.
N N2 2i i ii=1 i=1
e
e (y - a -bx )s = =
N- 2 N- 2
Why N-2? Relates to the fact that two parameters (α,β) were estimated. Same reason N-1 was used to compute a sample variance.
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Residuals
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Summary: Regression Computations
N
ii 1
N
ii 1
N2 2x ii 1
N2 2y ii 1
The same 5 statistics (with N) are still needed:
N = 62 complete observations.
1y = y = 20.721
N1
x = x = 0.48242N
1Var(x) = s = (x x) = 0.02453
N-11
Var(y) = s = (y y) = 305N-1
xy
N
i ii 1
.985
Cov(x,y) = s
1 = (x x)(y y) = 1.784
N-1
xy
2x
2 2 2y x
e
2 22 x
2y
sb = = 72.72
s
a = y - bx = -14.36
(N-1)(s -b s )s = = 13.386
N- 2(for later...),
b sR = = 0.424
s
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Using se to identify outliersRemember the empirical rule, 95% of observations will lie within mean ± 2 standard deviations? We show (a+bx) ± 2se below.)
This point is 2.2 standard deviations from the regression.
Only 3.2% of the 62 observations lie outside the bounds. (We will refine this later.)
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Linear Regression
Sample Regression Line
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Results to Report
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The Reported Results
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Estimated equation
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Estimated coefficients a and b
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S = se = estimated std. deviation of ε
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Square of the sample correlation between x and y
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N-2 = degrees of freedom
N-1 = sample size minus 1
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Sum of squared residuals, Σiei
2
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S2 = se2
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N 2ii=1
Total Variation
= (y - y)
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2
N2
N
2ii=1
2ii=1
Coefficient of Determination R
b (x - x)= =
(y - y)
RegressionSS
TotalSS
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The Model
Constructed to provide a framework for interpreting the observed data What is the meaning of the observed relationship
(assuming there is one) How it’s used
Prediction: What reason is there to assume that we can use sample observations to predict outcomes?
Testing relationships
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A Cost Model
Electricity.mpj
Total cost in $Million
Output in Million KWH
N = 123 American electric utilities
Model: Cost = α + βKWH + ε
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Cost Relationship
Output
Cost
80000700006000050000400003000020000100000
500
400
300
200
100
0
Scatterplot of Cost vs Output
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Sample Regression
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Interpreting the Model
Cost = 2.44 + 0.00529 Output + e Cost is $Million, Output is Million KWH. Fixed Cost = Cost when output = 0
Fixed Cost = $2.44Million Marginal cost
= Change in cost/change in output= .00529 * $Million/Million KWH= .00529 $/KWH = 0.529 cents/KWH.
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Summary
Linear regression model Assumptions of the model Residuals and disturbances
Estimating the parameters of the model Regression parameters Disturbance standard deviation
Computation of the estimated model