Graduate Econometrics Lecture Notes
Michael Creel
Version 0.4, 06 Nov. 2002, copyright (C) 2002 by Michael Creel
Contents
1 License, availability and use
10
1.1 License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2 Obtaining the notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Economic and econometric models 12
3 Ordinary Least Squares 14
3.1 The classical linear model . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Estimation by least squares . . . . . . . . . . . . . . . . . . . . . . . 15
3.3 Estimating the error variance . . . . . . . . . . . . . . . . . . . . . . 16
3.4 Geometric interpretation of least squares estimation . . . . . . . . . . 17
3.4.1 In X Y Space . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Dept. of Economics and Economic History, Universitat Autònoma de Barcelona.

1
3.4.2 In Observation Space . . . . . . . . . . . . . . . . . . . . . . 17
3.4.3 Projection Matrices . . . . . . . . . . . . . . . . . . . . . . . 19
3.5 Influential observations and outliers . . . . . . . . . . . . . . . . . . 20
3.6 Goodness of fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.7 Small sample properties of the least squares estimator . . . . . . . . . 25
3.7.1 Unbiasedness . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.7.2 Normality . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.7.3 Efficiency (Gauss-Markov theorem) . . . . . . . . . . . . . . 26
4 Maximum likelihood estimation 28
4.1 The likelihood function . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2 Consistency of MLE . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.3 The score function . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.4 Asymptotic normality of MLE . . . . . . . . . . . . . . . . . . . . . 33
4.5 The information matrix equality . . . . . . . . . . . . . . . . . . . . 37
4.6 The Cramér-Rao lower bound . . . . . . . . . . . . . . . . . . . . . 39
5 Asymptotic properties of the least squares estimator 43
5.1 Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
5.2 Asymptotic normality . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.3 Asymptotic efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6 Restrictions and hypothesis tests 47
6.1 Exact linear restrictions . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.1.1 Imposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6.1.2 Properties of the restricted estimator . . . . . . . . . . . . . . 52
6.2 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2
6.2.1 t-test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.2.2 F test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.2.3 Wald-type tests . . . . . . . . . . . . . . . . . . . . . . . . . 58
6.2.4 Score-type tests (Rao tests, Lagrange multiplier tests) . . . . . 59
6.2.5 Likelihood ratio-type tests . . . . . . . . . . . . . . . . . . . 62
6.3 The asymptotic equivalence of the LR, Wald and score tests . . . . . . 63
6.4 Interpretation of test statistics . . . . . . . . . . . . . . . . . . . . . . 68
6.5 Confidence intervals . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.6 Bootstrapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.7 Testing nonlinear restrictions . . . . . . . . . . . . . . . . . . . . . . 71
7 Generalized least squares 76
7.1 Effects of nonspherical disturbances on the OLS estimator . . . . . . 77
7.2 The GLS estimator . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7.3 Feasible GLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7.4 Heteroscedasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
7.4.1 OLS with heteroscedastic consistent varcov estimation . . . . 84

7.4.2 Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.4.3 Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
7.5 Autocorrelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.5.1 Causes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.5.2 AR(1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
7.5.3 MA(1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
7.5.4 Asymptotically valid inferences with autocorrelation of un-
known form . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7.5.5 Testing for autocorrelation . . . . . . . . . . . . . . . . . . . 104
3
7.5.6 Lagged dependent variables and autocorrelation . . . . . . . . 105
8 Stochastic regressors 107
8.1 Case 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
8.2 Case 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
8.3 Case 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
8.4 When are the assumptions reasonable? . . . . . . . . . . . . . . . . . 112
9 Data problems 114
9.1 Collinearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
9.1.1 A brief aside on dummy variables . . . . . . . . . . . . . . . 116
9.1.2 Back to collinearity . . . . . . . . . . . . . . . . . . . . . . . 116
9.1.3 Detection of collinearity . . . . . . . . . . . . . . . . . . . . 118
9.1.4 Dealing with collinearity . . . . . . . . . . . . . . . . . . . . 118
9.2 Measurement error . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
9.2.1 Error of measurement of the dependent variable . . . . . . . . 123
9.2.2 Error of measurement of the regressors . . . . . . . . . . . . 124
9.3 Missing observations . . . . . . . . . . . . . . . . . . . . . . . . . . 126
9.3.1 Missing observations on the dependent variable . . . . . . . . 126
9.3.2 The sample selection problem . . . . . . . . . . . . . . . . . 129
9.3.3 Missing observations on the regressors . . . . . . . . . . . . 130
10 Functional form and nonnested tests 132

10.1 Flexible functional forms . . . . . . . . . . . . . . . . . . . . . . . . 133
10.1.1 The translog form . . . . . . . . . . . . . . . . . . . . . . . . 135
10.1.2 FGLS estimation of a translog model . . . . . . . . . . . . . 141
10.2 Testing nonnested hypotheses . . . . . . . . . . . . . . . . . . . . . . 145
4
11 Exogeneity and simultaneity 149
11.1 Simultaneous equations . . . . . . . . . . . . . . . . . . . . . . . . . 149
11.2 Exogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
11.3 Reduced form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
11.4 IV estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
11.5 Identification by exclusion restrictions . . . . . . . . . . . . . . . . . 163
11.5.1 Necessary conditions . . . . . . . . . . . . . . . . . . . . . . 164
11.5.2 Sufficient conditions . . . . . . . . . . . . . . . . . . . . . . 167
11.6 2SLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
11.7 Testing the overidentifying restrictions . . . . . . . . . . . . . . . . . 179
11.8 System methods of estimation . . . . . . . . . . . . . . . . . . . . . 185
11.8.1 3SLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
11.8.2 FIML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
12 Limited dependent variables 195
12.1 Choice between two objects: the probit model . . . . . . . . . . . . . 195
12.2 Count data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
12.3 Duration data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
12.4 The Newton method . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
13 Models for time series data 208
13.1 Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
13.2 ARMA models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
13.2.1 MA(q) processes . . . . . . . . . . . . . . . . . . . . . . . . 211
13.2.2 AR(p) processes . . . . . . . . . . . . . . . . . . . . . . . . 211
13.2.3 Invertibility of MA(q) process . . . . . . . . . . . . . . . . . 222
5

14 Introduction to the second half 225
15 Notation and review 233
15.1 Notation for differentiation of vectors and matrices . . . . . . . . . . 233
15.2 Convergenge modes . . . . . . . . . . . . . . . . . . . . . . . . . . 234
15.3 Rates of convergence and asymptotic equality . . . . . . . . . . . . . 238
16 Asymptotic properties of extremum estimators 241
16.1 Extremum estimators . . . . . . . . . . . . . . . . . . . . . . . . . . 241
16.2 Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
16.3 Example: Consistency of Least Squares . . . . . . . . . . . . . . . . 247
16.4 Asymptotic Normality . . . . . . . . . . . . . . . . . . . . . . . . . 248
16.5 Example: Binary response models. . . . . . . . . . . . . . . . . . . . 251
16.6 Example: Linearization of a nonlinear model . . . . . . . . . . . . . 257
17 Numeric optimization methods 261
17.1 Search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
17.2 Derivative-based methods . . . . . . . . . . . . . . . . . . . . . . . . 262
17.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
17.2.2 Steepest descent . . . . . . . . . . . . . . . . . . . . . . . . 264
17.2.3 Newton-Raphson . . . . . . . . . . . . . . . . . . . . . . . . 264
17.3 Simulated Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . 269
18 Generalized method of moments (GMM) 270
18.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
18.2 Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
18.3 Asymptotic normality . . . . . . . . . . . . . . . . . . . . . . . . . . 274
18.4 Choosing the weighting matrix . . . . . . . . . . . . . . . . . . . . . 276
6
18.5 Estimation of the variance-covariance matrix . . . . . . . . . . . . . 279
18.5.1 Newey-West covariance estimator . . . . . . . . . . . . . . . 281
18.6 Estimation using conditional moments . . . . . . . . . . . . . . . . . 282
18.7 Estimation using dynamic moment conditions . . . . . . . . . . . . . 288
18.8 A specification test . . . . . . . . . . . . . . . . . . . . . . . . . . . 288

18.9 Other estimators interpreted as GMM estimators . . . . . . . . . . . . 291
18.9.1 OLS with heteroscedasticity of unknown form . . . . . . . . 291
18.9.2 Weighted Least Squares . . . . . . . . . . . . . . . . . . . . 293
18.9.3 2SLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
18.9.4 Nonlinear simultaneous equations . . . . . . . . . . . . . . . 296
18.9.5 Maximum likelihood . . . . . . . . . . . . . . . . . . . . . . 297
18.10Application: Nonlinear rational expectations . . . . . . . . . . . . . . 300
18.11Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
19 Quasi-ML 306
19.0.1 Consistent Estimation of Variance Components . . . . . . . . 309
20 Nonlinear least squares (NLS) 312
20.1 Introduction and definition . . . . . . . . . . . . . . . . . . . . . . . 312
20.2 Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314
20.3 Consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
20.4 Asymptotic normality . . . . . . . . . . . . . . . . . . . . . . . . . . 316
20.5 Example: The Poisson model for count data . . . . . . . . . . . . . . 318
20.6 The Gauss-Newton algorithm . . . . . . . . . . . . . . . . . . . . . . 320
20.7 Application: Limited dependent variables and sample selection . . . . 322
20.7.1 Example: Labor Supply . . . . . . . . . . . . . . . . . . . . 322
7
21 Examples: demand for health care 326
21.1 The MEPS data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326
21.2 Infinite mixture models . . . . . . . . . . . . . . . . . . . . . . . . . 331
21.3 Hurdle models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
21.4 Finite mixture models . . . . . . . . . . . . . . . . . . . . . . . . . . 341
21.5 Comparing models using information criteria . . . . . . . . . . . . . 347
22 Nonparametric inference 348
22.1 Possible pitfalls of parametric inference: estimation . . . . . . . . . . 348
22.2 Possible pitfalls of parametric inference: hypothesis testing . . . . . . 352
22.3 The Fourier functional form . . . . . . . . . . . . . . . . . . . . . . 354

22.3.1 Sobolev norm . . . . . . . . . . . . . . . . . . . . . . . . . . 358
22.3.2 Compactness . . . . . . . . . . . . . . . . . . . . . . . . . . 359
22.3.3 The estimation space and the estimation subspace . . . . . . . 359
22.3.4 Denseness . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
22.3.5 Uniform convergence . . . . . . . . . . . . . . . . . . . . . . 362
22.3.6 Identification . . . . . . . . . . . . . . . . . . . . . . . . . . 363
22.3.7 Review of concepts . . . . . . . . . . . . . . . . . . . . . . . 363
22.3.8 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 364
22.4 Kernel regression estimators . . . . . . . . . . . . . . . . . . . . . . 365
22.4.1 Estimation of the denominator . . . . . . . . . . . . . . . . . 366
22.4.2 Estimation of the numerator . . . . . . . . . . . . . . . . . . 369
22.4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
22.4.4 Choice of the window width: Cross-validation . . . . . . . . . 371
22.5 Kernel density estimation . . . . . . . . . . . . . . . . . . . . . . . . 371
22.6 Semi-nonparametric maximum likelihood . . . . . . . . . . . . . . . 372
8
23 Simulation-based estimation 378
23.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
23.1.1 Example: Multinomial and/or dynamic discrete response models378
23.1.2 Example: Marginalization of latent variables . . . . . . . . . 381
23.1.3 Estimation of models specified in terms of stochastic differen-
tial equations . . . . . . . . . . . . . . . . . . . . . . . . . . 383
23.2 Simulated maximum likelihood (SML) . . . . . . . . . . . . . . . . . 385
23.2.1 Example: multinomial probit . . . . . . . . . . . . . . . . . . 386
23.2.2 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388
23.3 Method of simulated moments (MSM) . . . . . . . . . . . . . . . . . 389
23.3.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
23.3.2 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
23.4 Efficient method of moments (EMM) . . . . . . . . . . . . . . . . . 392
23.4.1 Optimal weighting matrix . . . . . . . . . . . . . . . . . . . 395

23.4.2 Asymptotic distribution . . . . . . . . . . . . . . . . . . . . 397
23.4.3 Diagnotic testing . . . . . . . . . . . . . . . . . . . . . . . . 398
23.5 Application I: estimation of auction models . . . . . . . . . . . . . . 399
23.6 Application II: estimation of stochastic differential equations . . . . . 401
23.7 Application III: estimation of a multinomial probit panel data model . 403
24 Thanks 404
25 The GPL 404
9
1 License, availability and use
1.1 License
These lecture notes are copyrighted by Michael Creel with the date that appears above.
They are provided under the terms of the GNU General Public License, which forms
Section 25 of the notes. The main thing you need to know is that you are free to modify
and distribute these notes in any way you like, as long as you do so under the terms of
the GPL. In particular, you must make available the source files in editable form for
your version of the notes.
1.2 Obtaining the notes
These notes are part of the OMEGA (Open-source Materials for Econometrics, GPL
Archive) project at pareto.uab.es/omega. They were prepared using L
Y
X www.lyx.org.
L
Y
X is a free
1
“what you see is what you mean” word processor. It (with help from
other applications) can export your work in T
E
X, HTML, PDF and several other forms.
It will run on Unix, Windows, and MacOS systems. The source file is the L

Y
X file
notes.lyx,
which is available at pareto.uab.es/omega/Project_001. There you will
find the L
Y
X source file, as well as PDF, HTML, T
E
X and zipped HTML versions of
the notes.
1.3 Use
You are free to use the notes as you like, for study, preparing a course, etc. I find
that a hard copy is of most use for lecturing or study, while the html version is useful
for quick reference or answering students’ questions in office hours. I would greatly
1
”Free” is used in the sense of ”freedom”, but L
Y
X is also free of charge.
10
appreciate that you inform me of any errors you find. I’d also welcome contributions
in any area, especially in the areas of time series and nonstationary data.
1.4 Sources
The following is a partial list of the sources that have been used in preparing these
notes.
References
[Amemiya (1985)] Amemiya, T. (1985) Advanced Econometrics,
Harvard Univ. Press.
[Davidson and MacKinnon (1993)] Davidson, R. and J.G. MacKinnon (1993) Esti-
mation and Inference in Econometrics, Oxford
Univ. Press.

[Gallant (1987)] Gallant, A.R. (1985) Nonlinear Statistical Mod-
els, Wiley.
[Gallant (1997)] Gallant, A.R. (1997) An Introduction to Econo-
metric Theory, Princeton Univ. Press.
[Hamilton (1994)] Hamilton, J. (1994) Time Series Analysis, Prince-
ton Univ. Press
[Hayashi (2000)] Hayashi, F. (2000) Econometrics, Princeton Univ.
Press.
[Judge (1985)] Judge et. al. (1985) The Theory and Practice of
Econometrics, Wiley.
11
2 Economic and econometric models
Economic theory tells us that demand functions are something like:
x
i
x
i
p
i
m
i
z
i
x
i
is G 1 vector of quantities demanded
p
i
is G 1 vector of prices
m

i
is income
z
i
is a vector of individual characteristics related to preferences
Suppose we have a sample consisting of one observation on n individuals’ demands at
time period t (this is a cross section, where i 1 2 n indexes the individuals in the
sample). The model is not estimable as it stands, since:
The form of the demand function is different for all i
Some components of z
i
may not be observable to an outside modeler For ex-
ample, people don’t eat the same lunch every day, and you can’t tell what they
will order just by looking at them. Suppose we can break z
i
into the observable
components w
i
and a single unobservable component ε
i
.
A step toward an estimable (e.g., econometric) model is
x
i
β
0
p
i
β
p

m
i
β
m
w
i
β
w
ε
i
We have imposed a number of restrictions on the theoretical model:
12
The functions x
i
which may differ for all i have been restricted to all belong
to the same parametric family.
Of all parametric families of functions, we have restricted the model to the class
of linear in the variables functions.
There is a single unobservable component, and we assume it is additive.
These are very strong restrictions, compared to the theoretical model. Furthermore,
these restrictions have no theoretical basis. In addition, we still need to make more
assumptions in order to determine how to estimate the model. The validity of any
results we obtain using this model will be contingenton these restrictions being correct.
For this reason, specification testing will be needed, to check that the model seems to
be reasonable. Only when we are convinced that the model is at least approximately
correct should we use it for economic analysis. In the next sections we will obtain
results supposing that the econometric model is correctly specified. Later we will
examine the consequences of misspecification and see some methods for determining
if a model is correctly specified.
13

3 Ordinary Least Squares
3.1 The classical linear model
The classical linear model is based upon several assumptions.
1. Linearity: the model is a linear function of the parameter vector β
0
:
y
t
x
t
β
0
ε
t
or in matrix form,
y Xβ
0
ε
where y is n 1 X
x
1
x
2
x
n
where x
t
is K 1 and β
0
and ε are

conformable. The subscript “0” in β
0
means this is the true value of the unknown
parameter. It will be suppressed when it’s not necessary for clarity. Linear
models are more general than they might first appear, since one can employ
nonlinear transformations of the variables:
ϕ
0
z
t ϕ
1
w
t
ϕ
2
w
t
ϕ
p
w
t
β
0
ε
t
(The φ
i
are known functions). Defining y
t
ϕ

0
z
t
x
t1
ϕ
1
w
t
etc. leads to
a model in the form of equation (??). For example, the Cobb-Douglas model
z Aw
β
2
2
w
β
3
3
exp
ε
can be transformed logarithmically to obtain
lnz lnA β
2
lnw
2
β
3
lnw
3

ε
14
2. IID mean zero errors:
E ε 0
Var ε E εε σ
2
0
I
n
3. Nonstochastic, linearly independent regressors
(a) X has rank K
(b) X is nonstochastic
(c) lim
n ∞
1
n
X X Q
X
a finite positive definite matrix.
4. Normality (Optional): ε is normally distributed
3.2 Estimation by least squares
The objective is to gain information about the unknown parameters β
0
and σ
2
0
The
ordinary least squares estimator is defined as the value that minimizes the sum of the
squared errors:
ˆ

β argmins β
n
∑
t
1
y
t
x
t
β
2
s β y Xβ y Xβ
y y 2y Xβ β X Xβ
y Xβ
2
This last expression makes it clear how the OLS estimator chooses
ˆ
β : it minimizes the
Euclidean distance between y and Xβ
15
To minimize the criterion s β take the f.o.n.c. and set them to zero:
D
β
s
ˆ
β
2X y 2X X
ˆ
β 0
so

ˆ
β
X X
1
X
y
To verify that this is a minimum, check the s.o.s.c.:
D
2
β
s
ˆ
β
2X X
Since ρ X K this matrix is positive definite, since it’s a quadratic form in a
p.d. matrix (identity matrix of order n , so
ˆ
β is in fact a minimizer.
The fitted values are in the vector ˆy X
ˆ
β
The residuals are in the vector
ˆ
ε y X
ˆ
β
Note that
y Xβ ε
X
ˆ

β
ˆ
ε
3.3 Estimating the error variance
The OLS estimator of σ
2
0
is
σ
2
0
1
n K
ˆ
ε
ˆ
ε
16
3.4 Geometric interpretation of least squares estimation
3.4.1 In X Y Space
Figure 1 shows a typical fit to data, with a residual. The area of the square is that
residual’s contribution to the sum of squared errors. The fitted line is chosen so as to
minimize this sum.
Figure 1: Fitted Regression Line
x
x
x
x
x
x

x
e_i
The fitted line and a residual
contribution of e_i to the sum of squared errors
x
y
3.4.2 In Observation Space
If we want to plot in observation space, we’ll need to use only two or three observa-
tions, or we’ll encounter some limitations of the blackboard. Let’s use two. With only
17
two observations, we can’t have K 1
Figure 2: The fit in observation space
Observation 2
Observation 1
x
y
S(x)
x*beta=P_xY
e = M_xY
We can decompose y into two components: the orthogonal projection onto the
K dimensional space spanned by X, X
ˆ
β and the component that is the or-
thogonal projection onto the n K subpace that is orthogonal to the span of X
ˆ
ε
Since
ˆ
β is chosen to make
ˆ

ε as short as possible,
ˆ
ε will be orthogonal to the space
spanned by X Since X is in this space, X
ˆ
ε
0 Note that the f.o.c. that define
the least squares estimator imply that this is so.
18
3.4.3 Projection Matrices
We have that X
ˆ
β is the projection of y on the span of X or
X
ˆ
β X X X
1
X y
Therefore, the matrix that projects y onto the span of X is
P
X
X X X
1
X
since
X
ˆ
β P
X
y

ˆ
ε is the projection of y off the space spanned by X (that is onto the space that is
orthogonal to the span of X
We have that
ˆ
ε
y X
ˆ
β
y X X X
1
X
y
I
n
X X X
1
X
y
So the matrix that projects y off the span of X is
M
X
I
n
X X X
1
X
I
n
P

X
We have
ˆ
ε
M
X
y
19
Therefore
y P
X
y M
X
y
X
ˆ
β
ˆ
ε
Note that both P
X
and M
X
are symmetric and idempotent.
– A symmetric matrix A is one such that A A
– An idempotent matrix A is one such that A AA
– The only nonsingular idempotent matrix is the identity matrix.
3.5 Influential observations and outliers
The OLS estimator of the i
th

element of the vector β
0
is simply
ˆ
β
i
X X
1
X
i
y
c
i
y
This is how we define a linear estimator - it’s a linear function of the dependent
variable. Since it’s a linear combination of the observations on the dependent vari-
able, where the weights are detemined by the observations on the regressors, some
observations may have more influence than others. Define
h
t
P
X
tt
e
t
P
X
e
t
P

X
e
t
2
e
t
2
1
20
h
t
is the t
th
element on the main diagonal of P
X
( e
t
is a n vector of zeros with a 1 in
the t
th
position). So 0
h
t
1 and
TrP
X
K h K n
So, on average, the weight on the y
t
’s is K n. If the weight is much higher, then the

observation is influential. However, an observation may also be influential due to the
value of y
t
, rather than the weight it is multiplied by, which only depends on the x
t
’s.
To account for this, consider estimation of β without using the t
th
observation (des-
ignate this estimator as
ˆ
β
t
One can show (see Davidson and MacKinnon, pp. 32-5
for proof) that
ˆ
β
t
ˆ
β
1
1 h
t
X X
1
X
t
ˆ
ε
t

so the change in the t
th
observations fitted value is
X
t
ˆ
β
X
t
ˆ
β
t
h
t
1 h
t
ˆ
ε
t
While and observation may be influential if it doesn’t affect its own fitted value, it
certainly is influential if it does. A fast means of identifying influential observations is
to plot
h
t
1 h
t
ˆ
ε
t
as a function of t.

After influential observations are detected, one needs to determine why they are
influential. Possi causes include:
data entry error, which can easily be corrected once detected. Data entry errors
are very common.
special economic factors that affect some observations. These would need to
be identified and incorporated in the model. This is the idea behind structural
change: the parameters may not be constant across all observations.
21
pure randomness may have caused us to sample a low-probability observation.
There exist robust estimation methods that downweight outliers.
3.6 Goodness of fit
The fitted model is
y X
ˆ
β
ˆ
ε
Take the inner product:
y
y
ˆ
β
X X
ˆ
β 2
ˆ
β X
ˆ
ε
ˆ

ε
ˆ
ε
But the middle term of the RHS is zero since X
ˆ
ε
0, so
y y
ˆ
β
X X
ˆ
β
ˆ
ε
ˆ
ε
The uncentered R
2
u
is defined as
R
2
u
1
ˆ
ε
ˆ
ε
y y

ˆ
β X X
ˆ
β
y y
P
X
y
2
y
2
cos
2
φ
where φ is the angle between y and the span of X (show with the one regressor, two
observation example).
The uncentered R
2
changes if we add a constant to y since this changes φ.
Another, more common definition measures the contribution of the variables,
other than the constant term, to explaining the variation in y Thus it measures
22
Figure 3: Uncentered R
2
23
the ability of the model to explain the variation of y about its unconditional sam-
ple mean.
Let ι 1 1 1 a n -vector. So
M
ι

I
n
ι ι ι
1
ι
I
n
ιι n
M
ι
y just returns the vector of deviations from the mean.
The centered R
2
c
is defined as
R
2
c
1
ˆ
ε
ˆ
ε
y M
ι
y
1
ESS
TSS
Supposing that X contains a column of ones (i.e., there is a constant term),

X
ˆ
ε
0
∑
t
ˆ
ε
t
0
so M
ι
ˆ
ε
ˆ
ε
In this case
y M
ι
y
ˆ
β X M
ι
X
ˆ
β
ˆ
ε
ˆ
ε

So
R
2
c
RSS
TSS
Supposing that a column of ones is in the space spanned by X (P
X
ι ι then
one can show that 0 R
2
c
1
24
3.7 Small sample properties of the least squares estimator
3.7.1 Unbiasedness
For
ˆ
β we have
ˆ
β
X X
1
X
y
X X
1
X
Xβ ε
β

0
X X
1
X
ε
E
ˆ
β β
0
So the OLS estimator is unbiased.
For
ˆ
σ
2
we have
σ
2
0
1
n K
ˆ
ε
ˆ
ε
1
n K
ε
Mε
E σ
2

0
1
n K
E
Trε Mε
1
n K
E TrMεε
1
n K
TrE Mεε
1
n K
σ
2
0
TrM
1
n K
σ
2
0
n TrX X X
1
X
1
n K
σ
2
0

n Tr X X
1
X X
σ
2
0
25