Maximum likelihood estimationMaximum likelihood estimation

Nuno VasconcelosUCSD

Bayesian decision theory• recall that we have

– Y – state of the world– X – observations– g(x) – decision function– L[g(x),y] – loss of predicting y with g(x)

• Bayes decision rule is the rule that minimizes the risk

[ ]),( YXLERisk YX=• for the “0-1” loss

[ ]),(,YX

⎧ ≠)(1

• optimal decision rule is the maximum a posteriori⎩⎨⎧

=≠

=yxgyxg

yxgL)(,0)(,1

]),([

2

• optimal decision rule is the maximum a-posteriori probability rule

MAP ruleh h th t it b i l t d i f th• we have shown that it can be implemented in any of the

three following ways– 1) )|(maxarg)( |

* xiPxi XY=)

– 2)

)|(maxarg)( | xiPxi XYi

[ ])()|(maxarg)( |* iPixPxi YYX=)

– 3)

[ ]|i

[ ])(log)|(logmaxarg)( |* iPixPxi YYX +=)• by introducing a “model” for the class-conditional

distributions we can express this as a simple equation

[ ])(g)|(gg)( | YYXi

distributions we can express this as a simple equation– e.g. for the multivariate Gaussian

⎬⎫

⎨⎧ Σ )()(11)|( 1TP

3⎭⎬⎫

⎩⎨⎧ −Σ−−

Σ= − )()(

21exp

||)2(1)|( 1| ii

Ti

idYX

xxixP µµπ

The Gaussian classifierdi i i t

• the solution is

[ ]iii xdxi αµ += ),(minarg)(*discriminant:PY|X(1|x ) = 0.5

with

[ ]iiii

xdxi αµ +),(minarg)(

)()(),( 1 yxyxyxd iT

i −Σ−=−

)(log2)2log( iPYid

i −Σ= πα

• the optimal rule is to assign x to the closest class• closest is measured with the Mahalanobis distance di(x,y)• can be further simplified in special cases

4

• can be further simplified in special cases

Geometric interpretation• for Gaussian classes, equal covariance σ2I

µµ −

( )Yji

ji

iP

w

σµµσµµ

+

=

)(l2

2

( )jiY

Y

ji

ji

jPx µµ

µµ

µµ−

−−=

)()(log

2 20

σ

w

µi

µjσ

x0 )(1 iP

5)()(log1

2

2 jPiP

Y

Y

ji

σµµ −

Geometric interpretation• for Gaussian classes, equal but arbitrary covariance

( )w µµΣ−1( )

( ) ( ) ( )jiY

Tji

ji

jPiPx

w

µµµµµµ

µµ

µµ

−Σ

−+

=

−Σ=

− )()(log1

2 10 ( ) ( ) Yjiji jPµµµµ −Σ− )(2

w

µi

µjx0

6

Bayesian decision theory• advantages:

– BDR is optimal and cannot be beaten– Bayes keeps you honest– models reflect causal interpretation of the problem, this is how

we think– natural decomposition into “what we knew already” (prior) and

“what data tells us” (CCD)– no need for heuristics to combine these two sources of infono need for heuristics to combine these two sources of info– BDR is, almost invariably, intuitive– Bayes rule, chain rule, and marginalization enable modularity,

and scalability to very complicated models and problemsand scalability to very complicated models and problems

• problems:– BDR is optimal only insofar the models are correct.

7

Implementation• we do have an optimal solution

( )jiw µµ −Σ= −1

f( ) ( ) ( )jiY

Y

jiT

ji

ji

jPiPx µµ

µµµµµµ

−−Σ−

−+

=− )(

)(log12 10

• but in practice we do not know the values of the parameters µ, Σ, PY(1)– we have to somehow estimate these values– this is OK, we can come up with an estimate from a training set– e.g. use the average value as an estimate for the mean

( )( )

( )Yjiji

iPx

w

µµµµ

µµ

ˆˆ)(ˆ

log1ˆˆ

ˆˆˆ 1

+=

−Σ= −

8( ) ( ) ( )jiYjiTji jP

x µµµµµµ )(ˆ

logˆˆˆˆ2 10

−−Σ−

−=−

Important • warning: at this point all optimality claims for the BDR

cease to be valid!!• the BDR is guaranteed

to achieve the minimumloss when we use theloss when we use thetrue probabilities

• when we “plug in” the b bilit ti tprobability estimates,

we could be implementing p ga classifier that is quite distant from the optimal– e.g. if the PX|Y(x|i) look like the example above

I could never approximate them well by parametric models (e g

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– I could never approximate them well by parametric models (e.g. Gaussian)

Maximum likelihood• this seems pretty serious

– how should I get these probabilities then?

• we rely on the maximum likelihood (ML) principle• this has three steps:

1) we choose a parametric model for all probabilities– 1) we choose a parametric model for all probabilities– to make this clear we denote the vector of parameters by Θ and

the class-conditional distributions by

note that this means that Θ is NOT a random variable (otherwise

);|(| ΘixP YX– note that this means that Θ is NOT a random variable (otherwise

it would have to show up as subscript)– it is simply a parameter, and the probabilities are a function of this

parameter

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parameter

Maximum likelihood• three steps:

– 2) we assemble a collection of datasetsD(i) = {x (i) x (i)} set of examples drawn independently fromD(i) = {x1(i) , ..., xn(i)} set of examples drawn independently from class i

3) we select the parameters of class i to be the ones that– 3) we select the parameters of class i to be the ones that maximize the probability of the data from that class

( )ΘΘ ;|maxarg )( iDP i( )( )Θ=

Θ=ΘΘ

;|logmaxarg

;|maxarg

)(|

)(|

iDP

iDP

iYX

YXi

– like before, it does not really make any difference to maximize

( )Θ

;|gg |YX

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, y yprobabilities or their logs

Maximum likelihood• since

– each sample D(i) is considered independentlyt ti t d l f l (i)– parameter Θi estimated only from sample D(i)

• we simply have to repeat the procedure for all classes• so from now on we omit the class variable• so, from now on we omit the class variable

( )Θ=ΘΘ

;maxarg* DPX

( )Θ=Θ

Θ

;logmaxarg DPX

• the function PX(D;Θ) is called the likelihood of the parameter Θ with respect to the data

i l h lik lih d f i12

• or simply the likelihood function

Maximum likelihood• note that the likelihood

function is a function of the parameters Θof the parameters Θ

• it does not have the same shape as the

fdensity itself• e.g. the likelihood

function of a Gaussianfunction of a Gaussian is not bell-shaped

• the likelihood isdefined only after we have a sample

⎫⎧ 2)(1 µd

13⎭⎬⎫

⎩⎨⎧ −−=Θ 22 2

)(exp)2(

1);(σµ

σπddPX

Maximum likelihood• given a sample, to obtain ML estimate we need to solve

( )Θ=Θ ;maxarg* DP

• when Θ is a scalar this is high-school calculus

( )Θ=ΘΘ

;maxarg DPX

when Θ is a scalar this is high school calculus

• we have a maximum whenfirst derivative is zero

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– first derivative is zero– second derivative is negative

The gradient• in higher dimensions, the generalization of the derivative

is the gradient

∇f

• the gradient of a function f(w) at z is

Tff ⎞⎜⎛ ∂∂ ∇f

• the gradient has a nice geometrici t t ti

n

zwfz

wfzf ⎟

⎠

⎞⎜⎜⎝

⎛∂∂

∂∂

=∇−

)(,),()(10

L

interpretation– it points in the direction of maximum

growth of the functionf(x,y)

– which makes it perpendicular to the contours where the function is constant

),( 00 yxf∇

15

),( 00 yf

),( 11 yxf∇

maxThe gradient

max

• note that if ∇f = 0– there is no direction of growthg– also -∇f = 0, and there is no direction of

decrease– we are either at a local minimum or maximum

“ ddl ” i tor “saddle” point• conversely, at local min or max or saddle

pointdi ti f th d min– no direction of growth or decrease

– ∇f = 0• this shows that we have a critical point if

d l if ∇f 0saddle

and only if ∇f = 0• to determine which type we need second

order conditions

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The Hessian• the extension of the second-order derivative is the

Hessian matrix⎤⎡

⎥⎥⎥⎤

⎢⎢⎢⎡

∂∂∂

∂∂

=∇−

)()(

)(10

2

20

2

2

xxxfx

xf

xfn

M

L

⎥⎥⎥

⎦⎢⎢⎢

⎣ ∂∂

∂∂∂

=∇

−−

)()(

)(

21

2

01

2

xx

fxxx

fxf

nn

L

M

– at each point x, gives us the quadratic function

⎦⎣

xxfx t )(2∇

that best approximates f(x)

xxfx )(∇

17

The Hessian• this means that, when gradient is

zero at x, we have max– a maximum when function can be

approximated by an “upwards-facing” quadratic

– a minimum when function can be approximated by a “downwards-facing” quadratic

– a saddle point otherwise

saddlemin

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The Hessian max• for any matrix M, the function

Mxx t

• is– upwards facing quadratic when M

Mxx

upwards facing quadratic when Mis negative definite

– downwards facing quadratic when Mis positive definite

min

saddle

is positive definite– saddle otherwise

• hence, all that matters is the positived fi it f th H idefiniteness of the Hessian

• we have a maximum when Hessianis negative definite

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is negative definite

Maximum likelihood• in summary, given a sample, we need to solve

( )ΘΘ* DPmax

• the solutions are the parameters

( )Θ=ΘΘ

;maxarg DPX

the solutions are the parameters such that

0);( =Θ∇ DP 0);( =Θ∇Θ DPXn

Xt DP ℜ∈∀≤∇Θ θθθθ ,0);(

2

• note that you always have to check the second-order condition!

XΘ ,);(

20

condition!

Maximum likelihood• let’s consider the Gaussian example

• given a sample {T1, …, TN} of independent pointsgiven a sample {T1, …, TN} of independent points• the likelihood is

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Maximum likelihood• and the log-likelihood is

• the derivative with respect to the mean is zero when

• or

• note that this is just the sample mean22

• note that this is just the sample mean

Maximum likelihood• and the log-likelihood is

• the derivative with respect to the variance is zero when

• or

• note that this is just the sample variance23

• note that this is just the sample variance

Maximum likelihood• example:

– if sample is {10,20,30,40,50}

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Homework• show that the Hessian is negative definite

nt 2

• show that these formulas can be generalized to the vector

nX

t DP ℜ∈∀≤∇Θ θθθθ ,0);(2

show that these formulas can be generalized to the vector case– D(i) = {x1(i) , ..., xn(i)} set of examples from class i

th ML ti t– the ML estimates are

∑=Σ Tii xx ))((1 )()( µµ∑ ix )(1µ

• note that the ML solution is usually intuitive

∑ −−=Σj

ijiji xxn))(( )()( µµ∑=

jji xn

)(µ

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• note that the ML solution is usually intuitive

Estimators• when we talk about estimators, it is important to keep in

mind that– an estimate is a number– an estimator is a random variable

)(ˆ XXfθ

• an estimate is the value of the estimator for a given

),,( 1 nXXf K=θ

gsample.

• if D = {x1 , ..., xn}, when we say ∑=j

jxn1µ̂

what we mean is with j

nn xXxXnXXf

===

,,1 11),,(ˆ

KKµ

∑ XXXf 1)(26

the Xi are random variables∑=j

jn XnXXf ),,( 1 K

Bias and variance• we know how to produce estimators (by ML)• how do we evaluate an estimator?• Q1: is the expected value equal to the true value?• this is measured by the bias

if– if

then

),,(ˆ 1 nXXf K=θthen

an estimator that has bias will usually not converge to the perfect

( ) [ ]θθ −= ),,(ˆ 1,,1 nXX XXfEBias n KK– an estimator that has bias will usually not converge to the perfect

estimate θ, no matter how large the sample is– e.g. if θ is negative and the estimator is

the bias is clearly non zero∑=

jjn Xn

XXf 211),,( K

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the bias is clearly non-zero j

Bias and variance• the estimators is said to be biased

– this means that it is not expressive enough to approximate the tr e al e arbitraril elltrue value arbitrarily well

– this will be clearer when we talk about density estimation

• Q2: assuming that the estimator converges to the true 2 g gvalue, how many sample points do we need?– this can be measured by the variance

( )[ ]( ){ }2)()(

ˆ

XXfEXXfE

Var =θ

– the variance usually decreases as one collects more training

[ ]( ){ }1,,1,, ),,(),,( 11 nXXnXX XXfEXXfE nn KK KK −

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y gexamples

Example• ML estimator for the mean of a Gaussian N(µ,σ2)

( ) [ ] [ ]ˆˆˆ −=−= µµµµµ XXXX EEBias ( ) [ ] [ ]1

,,,,

1

11

−⎥⎦

⎤⎢⎣

⎡=

==

∑ µ

µµµµµ

iXX

XXXX

XE

EEBias

n

nn KK

[ ]1,,1

−=

⎥⎦

⎢⎣

∑

∑

µ

µ

iXX

iiXX

XE

nnK

[ ]

[ ]1,,1

−= ∑

∑

µ

µ

iX

iiXX

XE

n nK

[ ]

0=−=

∑µµ

µ

i

iX XEn i

29• the estimator is unbiased

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