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Likelihood Theory and Classical Calibration
1 · Prerequisites
- Absolute and Conditional Convergence; Rearrangement; Products
- Algebraic Closure, Embeddings, and Separability
- Algebraic Extensions, Extension Degree, and Finite Fields
- Areas of Elementary Plane Figures
- Binary Operations, Monoids, Groups and Subgroups
- Central Limit Theorems
- Characteristic Functions Inversion and Continuity
- Compactness
- Compactness in Metric Spaces
- Complete Metrizability, Čech-Completeness, and Baire Category
- Completeness, Completion, and Uniform Continuity
- Complex Lp Spaces and Test-Function Conventions
- Composition Series, the Jordan–Hölder Theorem and Solvable Groups
- Congruences, the Integers Modulo n and the Chinese Remainder Theorem
- Consistency and Limiting Distributions
- Construction of the Natural Numbers
- Construction of the Real Numbers via Cauchy Sequences
- Construction of the Real Numbers via Dedekind Cuts
- Continuity, IVT, EVT, and Uniform Continuity
- Cosets, Index and Lagrange's Theorem
- Countability and Uncountability
- Countability Axioms and Cardinal Functions
- Cyclic Groups and Direct Products
- Darboux, L'Hôpital, and Taylor's Theorem
- Density Separability and Convolution in Lᵖ
- Determinants of Matrices over a Commutative Ring
- Diagonalisation and the Minimal Polynomial
- Divisibility, Euclidean Domains, Principal Ideal Domains and Unique Factorisation
- Divisibility, Greatest Common Divisors and Bézout's Identity
- Domination and Likelihood Ratios
- Dual Spaces, Bilinear and Quadratic Forms, and Sylvester's Law of Inertia
- Eigenvalues, Eigenvectors and the Characteristic Polynomial
- Filters and Ultrafilters
- Finite Counting, Factorials and Binomial Coefficients
- Finite Probability and the Probabilistic Method
- Foundations of the Real Numbers for Analysis
- Fourier Transform Convolution and Approximate Identities
- Fubini and Change of Variables
- Fundamental Trigonometric Identities
- Gaussian Elimination, Elementary Matrices and Reduced Row Echelon Form
- Gaussian Orthogonal Decomposition and Quadratic Forms
- Group Actions, Orbits, Stabilisers and Cayley's Theorem
- Group Homomorphisms and the Isomorphism Theorems
- Hausdorff via the Diagonal
- Ideals, Quotient Rings and the Isomorphism Theorems for Rings
- Improper and Parameter-Dependent Multiple Integrals
- Improper Integrals
- Independence Borel Cantelli and Zero One Laws
- Infinite Product Measures and Kolmogorov Extension
- Inner Product Spaces, Gram-Schmidt, Projections and Adjoints
- Lebesgue Measure on Euclidean Space
- Limits of Real Functions
- limsup, liminf, and Subsequential Limits
- Linear Independence, Bases and Dimension
- Linear Transformations, Rank-Nullity and Quotient Spaces
- Matrices, the Matrix of a Linear Map, and Change of Basis
- Measurable Functions and Simple Approximation
- Measures and Their Basic Properties
- Metric Spaces
- Mixed Partials, Taylor Formulae, and Extrema
- Modes of Convergence Egorov and Lusin
- Modes of Convergence for Random Variables
- Monotone Functions, Discontinuities, and Continuity Sets
- Monotone Sequences, Bolzano-Weierstrass, and Cauchy Completeness
- Normal Subgroups and Quotient Groups
- Order, Zorn's Lemma, and the Axiom of Choice
- Outer Measure and the Caratheodory Extension Theorem
- Partitions of Unity and Paracompactness
- Pointwise Extended-Real Function Products
- Polynomial Rings, the Division Algorithm and Roots
- Power Series and Real-Analytic Functions
- Primes, Euclid's Lemma and the Fundamental Theorem of Arithmetic
- Probability Spaces Random Variables and Expectation
- Product Measures and the Fubini Tonelli Theorems
- Properties of the Integral and the Working FTC
- Radon Measures and the Riesz Markov Kakutani Theorem
- Relations, Functions, and Quotients
- Rings, Subrings, Integral Domains and Fields
- Rⁿ as a Normed Space; Vector-Valued Functions
- Roots, Rational Powers, and Classical Inequalities
- Scores and Information Bounds
- Separation Axioms: the Hierarchy
- Sequences and Limits
- Sequences and Series of Functions; Uniform Convergence
- Series: Convergence and the Nonnegative Tests
- Sigma Algebras and Borel Sets
- Signed and Complex Measures Hahn and Jordan
- Simple Field Extensions and the Construction of the Complex Numbers
- Sine, Cosine, and the Definition of Pi
- Splitting Fields
- Statistical Experiments, Models and Identifiability
- Subspaces, Products, and Quotients
- Suprema and Infima
- Sylow's Theorems, p-Groups and Nilpotent Groups
- Symmetric Groups, Cycle Decomposition and the Sign Homomorphism
- The Cantor Set, Baire Category, and Measure Zero in ℝ
- The Complex Exponential and Euler's Formula
- The Derivative and the Mean Value Theorems
- The Determinant of a Linear Operator, Cofactors and Cramer's Rule
- The Exponential Function
- The Fundamental Theorem of Algebra
- The Fundamental Theorem of Finite Abelian Groups
- The Galois Correspondence
- The Inverse and Implicit Function Theorems
- The Lebesgue and Riemann Integrals Compared
- The Lebesgue Integral and the Convergence Theorems
- The Logarithm and General Powers
- The Lᵖ Spaces Holder Minkowski and Riesz Fischer
- The Maximal Function and Lebesgue Differentiation
- The Radon Nikodym Theorem and Lebesgue Decomposition
- The Riemann Integral in Rᵐ and Jordan Content
- The Riemann Integral: Definition and Integrability
- The Spectral Theorem, Positive Operators and Singular Value Decomposition
- The Topology of Euclidean Space
- The Total Derivative in ℝᵐ → ℝⁿ
- The ZFC Axioms and the Basic Set Constructions
- Topological Spaces and Continuity
- Topology of ℝ
- Triangularisation, Generalised Eigenspaces and Jordan Canonical Form
- Urysohn's Lemma and the Tietze Extension Theorem
- Vector Spaces, Linear Subspaces, Span and Direct Sums
- Weak Convergence Tightness and Representation
- Weak Laws and Series of Independent Random Variables
2 · Summary
Quadratic-mean differentiability gives an notion of smoothness for a dominated model, with a score vector and Fisher information that remain meaningful when pointwise derivatives are unavailable. Under a correct iid model, an interior true parameter, nonsingular information, a consistent measurable MLE, and a square-integrable local log-density Lipschitz envelope, the MLE has the usual asymptotic linear representation and covariance .
For a regular interior equality null whose local tangent has codimension , the Wald, efficient-score, and likelihood-ratio statistics share the same first-order quadratic form and converge to . The calibration is asymptotic and uses the stated local likelihood and score expansions. It does not apply when the null has a boundary tangent cone; the examples page works that distinction out in a normal location model.
3 · Logical flowchart
4 · Definitions, theorems and proofs
Quadratic-mean differentiability at a parameter
Definition
Let be a parametric model, let be open in , and let a supplied sigma-finite measure dominate every (Parametric statistical models, Dominated statistical experiments). Supply as part of the setup a family of nonnegative finite-valued measurable functions satisfying for every (The measure with density relative to ). Thus each integrates to one. The density family is supplied, rather than selected from domination alone; no choice principle, joint measurability in , or common exceptional null set is asserted. A supplied extended-valued probability density can be made finite-valued by setting it to zero on its measurable infinite-value set, which has measure zero because its integral is one. The QMD condition is unchanged by replacing any supplied density by an almost-everywhere equal version: for each fixed , the norm in the condition below changes only on the union of two null sets. At an interior point , the model is quadratic-mean differentiable (QMD) with score if is measurable, , and
as with . Values of the score on do not affect this condition. The Fisher information associated with this QMD score is ; it may be singular. QMD is an condition and does not assert pointwise differentiability of or local mutual absolute continuity of and .
A finite-dimensional QMD score has mean zero
Statement
Let a dominated finite-dimensional parametric model be quadratic-mean differentiable, with the supplied density family of Quadratic-mean differentiability at a parameter, at an interior point , with score . Then .
Facts & Assumptions
Given: The dominated model, supplied density family, and QMD expansion at the interior point .
Quadratic-mean differentiability at a parameter: QMD means , with square-integrable score and supplied nonnegative measurable densities satisfying .
Cauchy-Schwarz inequality for : for square-integrable , .
Integrating against a density agrees with integrating the product: integration of a nonnegative function against equals integration of its product with against ; applying this to positive and negative parts gives the same identity for integrable real functions.
The Lebesgue integral is linear on : finite linear combinations of integrable real functions are integrable and their integrals are linear.
Proof
Write and . By [F1], . By [F3], each is a probability square-root density, so . Since is a probability and the score is square-integrable, [F2] with the constant function shows its coordinates are integrable. All products in the following norm expansion are integrable by [F2], since , , and belong to . Thus [F4] permits integrating the finite expansion term by term, and [F3] transfers the score term to expectation under . Expanding gives . Cauchy–Schwarz [F2] bounds the inner product by ; square-integrability of the score and make the last term . Thus .
For each coordinate vector , set with small enough that . Divide the relation from step 1.1 by and let to obtain . This holds for all , hence the score vector has mean zero.
Wald, score, and likelihood-ratio statistics for regular nulls
Definition
Assume AC for the supplied density representatives in Score and Fisher information and The Axiom of Choice. Let be iid from a dominated parametric model on an open , and let . For any nested null for which the maxima exist, the likelihood-ratio statistic is
For the Wald and score statistics, specialize to a regular equality null of codimension at an interior point : this is a null set which, in a local parameter chart on an open neighborhood of , , is near . Its local tangent is the -coordinate subspace. Assume the QMD information is positive definite. Let maximize over the full parameter space and maximize it over the null , so the general likelihood-ratio definition is .
For the score statistic, assume is differentiable in the local chart at the parameter values where it is evaluated, and write . Require explicitly that this pointwise derivative at agrees -almost surely with the QMD score; this compatibility is an assumption, not a consequence of QMD alone. The pointwise score and its information-matrix convention are those in Score and Fisher information.
Partition the score and information in coordinates: and The matrix is the information for the tested coordinates after accounting for the nuisance coordinates. If , interpret empty-block products as zero, so and below is the empty matrix. If , then is positive definite; for , the quadratic form of at equals . Thus is positive definite in either case. Put
For a consistent estimator of and a consistent estimator of , when set . The score statistic is . For a consistent covariance estimator of the -block of , when the Wald statistic is , where is the tested coordinate of . Set when and when , without evaluating coordinates or scores outside the chart. When a displayed inverse does not exist, set that statistic to zero. Under the regularity conditions used in the later calibration theorem, consistency places both maximizers in and the estimated inverses exist with probability tending to one. This definition gives the statistics and their normalizations; it asserts no finite-sample or asymptotic distribution.
Asymptotic linearity and normality of a regular MLE
Statement
Assume the Axiom of Choice. Let be iid with law in a dominated parametric model , where is open in , , and . Let be densities with respect to one supplied sigma-finite measure. Suppose the model is QMD at with score and positive-definite information . Assume there are a neighborhood of and a nonnegative measurable with such that, on one -full set,
Suppose a measurable exact MLE exists and is consistent, in the sense that for every . Then Under the same local hypotheses, every consistent measurable exact likelihood maximizer over a possibly -dependent subset that contains is also root- bounded in probability. The conclusion is asymptotic; it gives no exact finite-sample normal law.
Facts & Assumptions
Given: The iid dominated model, QMD score and information, local envelope, consistent measurable exact MLE, and Axiom of Choice in the statement.
Parametric statistical models: a parametric model is the supplied map on an experiment.
Dominated statistical experiments: domination means every is absolutely continuous with respect to one supplied sigma-finite measure.
Quadratic-mean differentiability at a parameter: QMD means the square-root densities have the stated first-order expansion with score .
Stochastic order and convergence notation for finite-dimensional random vectors: is convergence in probability to zero, and is boundedness in probability in finite-dimensional Euclidean norm.
Finite-dimensional stochastic-order calculus and tightness: finite-dimensional convergence in distribution implies boundedness in probability; products of a vanishing factor and a bounded factor vanish in probability.
Vector Slutsky theorem: if and , then , with continuous maps preserving this limit.
Multivariate iid central limit theorem: under AC, iid finite-second-moment vectors satisfy the multivariate CLT, including singular covariance.
Multivariate normal law, including singular covariance: under AC, exists for each finite-dimensional symmetric positive-semidefinite covariance .
Cauchy-Schwarz inequality for : for square-integrable , .
Dominated convergence: pointwise almost-everywhere convergence under an integrable dominating function permits passing limits through the integral.
Chebyshev-Markov inequality for the integral: for measurable and , .
A finite-dimensional QMD score has mean zero: the score of a finite-dimensional parametric QMD model at an interior point has mean zero under that point's law.
The Axiom of Choice: AC is assumed for the multivariate CLT and Gaussian-law suppliers [F7]–[F8].
The score and information are those in [F3]; is positive definite.
The local log-density envelope and consistency/MLE hypotheses are as in the Statement.
Proof
Write , , and . By [F3], , and [F12] gives . On both and vanish, so the probability mass is . Expanding the QMD approximation in [F3] and applying [F9] to its cross term gives .
Normalize the absolutely continuous part on by , and put on . Since , [F3] gives , so and from . The local envelope makes square-integrable on and . Along every sequence with , the convergence of has an almost-everywhere convergent subsequence. On it, pointwise. Since and , ; dominated convergence [F10] gives convergence in along that subsequence. Every subsequence has such a subsubsequence, so convergence holds for the full sequence. Sequential compactness of the unit sphere therefore yields . Moreover, the squares are uniformly integrable because converges in along every convergent direction. Taking the logarithm expansion in expectation requires a tail check. Fix . On , the Taylor remainder is bounded by , where . Uniform integrability gives and therefore . On this tail, ; its expectation is by the envelope, Cauchy–Schwarz [F9], and the tail probability. The tail expectations of and are also by [F9] and uniform integrability. Letting proves that the expected Taylor remainder is . Taking expectations then gives the normalization term is .
The squared Hellinger distance equals by step 1.1. The affinity . On let , so . For , follows from for , since this function has derivative and its minimum is at ; integrating gives . Since for because has derivative and value at . Thus , so for some , whenever .
Let for the empirical measure . The pointwise Lipschitz envelope gives . The sample and mean processes are continuous in on compact local balls, so their suprema equal suprema over countable dense subsets and are measurable. To bound the local supremum, take an independent ghost sample . For each fixed original sample, the supremum of absolute values is convex in the ghost sample vector, so Jensen's inequality replaces by in the expected supremum. Swapping each pair with an independent sign and then using the triangle inequality bounds that supremum by twice the expected conditional Rademacher supremum. Condition on the observations for the latter. For the conditional Rademacher process , Hoeffding's exponential bound , since (integrate for and use evenness), makes each increment sub-Gaussian with variance proxy . Choose successively finer dyadic Euclidean nets of the radius- parameter ball. At level the net has at most points, each increment from its level- parent has scale at most , and the expected maximum of centered sub-Gaussian variables of scale is at most , by the exponential-moment bound and optimizing . Summing the resulting bounds uses the convergent series and gives . Symmetrization and therefore give For , step 2.1 gives , and increments have the square-integrable pointwise envelope . Fix a mesh and cover the bounded -ball by finitely many mesh- points. At each center , the variance of is , so Chebyshev and finiteness of the net make the maximum over centers . For increments within cells, form , where is the center of the cell containing . Each is pointwise bounded by . Apply conditional Rademacher chaining to the union of cell classes: at dyadic level there are at most links, each with conditional scale at most after the normalization. The exponential maximum bound and summation of give an expected supremum at most after symmetrization and . Markov's inequality then shows that these cell increments vanish in probability as . Letting and then proves . Thus, for each fixed , uniformly over , The expectation term is step 2.1; the centered term follows by writing and using the two net bounds.
Reduce if needed so the log envelope holds on the radius ball, and put . Consistency makes lie within of with probability tending to one. On the dyadic shells with , exact maximization and require . The expectation bound in step 3.1 and Markov's inequality [F11] bound the probability of this event by . Summing the geometric series and then letting proves using [F4]. The argument used only consistency, feasibility of , and the maximizing inequality; it therefore proves the same bound for any exact maximizer over a subset containing .
Put and . The multivariate CLT [F7] and [F5] make ; step 4.1 makes . For , Uniform convergence on bounded balls in step 3.1, feasibility of bounded local because is interior, and exact MLE maximization therefore force .
The CLT [F7] gives ; the limiting law exists by [F8]. Since is invertible, by the continuous linear map. Combining this with step 5.1 by [F6] proves both the asymptotic linear representation and the stated normal limit. AC is used only through [F7]–[F8].
Wald, score, and likelihood-ratio asymptotic chi-square calibration
Statement
Assume AC. Let be iid from in a correctly specified dominated model, with an interior point of the full open parameter set . In a local parameter chart, suppose the null is , so its local null set is a linear subspace of codimension ; all score and information coordinates below use this chart. Assume QMD at with positive-definite information
and write . If , interpret nuisance blocks as empty, so and products involving are zero. Let . Assume the hypotheses of Asymptotic linearity and normality of a regular MLE hold for the full model. Assume the constrained MLE is measurable, exact, consistent, and maximizes over a set containing ; it is root- bounded by that theorem's localization conclusion. For every finite the following likelihood and score expansions hold uniformly over :
Assume the covariance, score-projection, and efficient-information estimators in Wald, score, and likelihood-ratio statistics for regular nulls converge in probability to their stated limits. Partition , put , and set . Then, under the null,
Here is the law defined in Chi-square laws, including zero degrees. This is an asymptotic calibration; no exact finite-sample chi-square law is asserted.
Facts & Assumptions
Given: The iid correctly specified model and the local-null, QMD, positive-information, consistency, local-expansion, estimator, and covariance hypotheses in the Statement. Assume AC.
Quadratic-mean differentiability at a parameter: QMD supplies the score and information matrix .
Wald, score, and likelihood-ratio statistics for regular nulls: in local coordinates , uses the tested MLE coordinates and their covariance, uses the consistent estimated efficient score and information, and is twice the unrestricted-minus-restricted log likelihood.
Asymptotic linearity and normality of a regular MLE: under its local envelope and consistency hypotheses, the full MLE has the stated asymptotic linear representation, and any consistent exact restricted maximizer containing is root- bounded.
Stochastic order and convergence notation for finite-dimensional random vectors: boundedness in probability and convergence in probability use the finite-dimensional Euclidean norm.
Finite-dimensional stochastic-order calculus and tightness: a vector converging in distribution is bounded in probability, and products of bounded and vanishing vectors vanish in probability.
Vector Slutsky theorem: a continuous map of a weakly convergent vector and a vector converging in probability to a constant has the corresponding weak limit.
Multivariate iid central limit theorem: under AC, the normalized sum of iid finite-second-moment vectors converges to a multivariate normal with their covariance.
Multivariate normal law, including singular covariance: under AC, has covariance and its linear maps are multivariate normal.
Chi-square laws, including zero degrees: for , is the law of when has independent standard-normal coordinates.
The local likelihood and normalized score expansions are assumed uniformly on each bounded local parameter set as stated; QMD alone is not used to infer them here.
is positive definite and ; when , the nuisance blocks are empty and their products are interpreted as zero.
The Axiom of Choice: AC is inherited from the density-representative convention in [F2], the MLE theorem [F3], and the probability-law suppliers [F7]–[F9].
Proof
If , then is positive definite. If , is a positive-definite principal block. For nonzero , evaluate the quadratic form of at ; the result is , so is positive definite in either case. Normalizing in the QMD expansion [F1], write with . Expanding the squared norm, whose value is , gives . Taking and dividing by proves each score coordinate has mean zero. Since is square integrable by [F1], the iid CLT [F7] gives under AC. Define . Its covariance, by block multiplication, is Thus the linear-map case of Slutsky [F6] gives ; its covariance is the block calculation above.
By [F3], the full MLE satisfies . Solving the block system shows its tested block is , including the empty-nuisance case. If , the local null contains only , so consistency makes the constrained MLE equal to with probability tending to one and there is no nuisance expansion to prove. Otherwise, [F3] makes the constrained root- error bounded. In local coordinates for , the limiting quadratic log likelihood is For general , completing the square in gives the unique maximizer and the profile up to a term independent of . At the null this gives the unique nuisance maximizer . On every distance- set from , strict positive definiteness of makes the quadratic log likelihood at least below its maximum. Uniformity of the likelihood expansion [A1], constrained root- tightness [F3], and this gap therefore give .
Consistency places both maximizers in the open chart neighborhood with probability tending to one; the off-chart zero conventions in [F2] therefore do not affect the expansions below. The upper-left block of is by solving the block system . The differentiable local chart has tested coordinate , so the Wald statistic in [F2], the expansion in step 2.1, and covariance consistency give
If , the nuisance score is empty and the score expansion [A1] at gives . If , the score expansion [A1] at the constrained MLE and give and . The estimated projection coefficient is bounded in probability by consistency, so the efficient score in [F2] is . Efficient-information consistency and [F5] imply .
The likelihood expansion in [A1] makes equal, up to , twice the maximum of over all local minus twice its maximum over the null . The unrestricted and restricted quadratic maximizers are linear functions of , hence are by step 1.1 and [F5]. Taking a bounded local ball large enough with probability arbitrarily close to one justifies using the uniform expansions for both maxima. The profile formula in step 2.1 reduces this difference to The identity follows by completing the square at . Hence .
Since and is positive definite, its transformed vector converges to . The squared Euclidean norm of that limit is by [F9]. Combining this with steps 3.1–3.3 and [F4]–[F5] proves all three limits and their common first-order approximation. AC is inherited through [F2], [F3], and [F7]–[F9], as recorded in [A3].
First-order equivalence of Wald, score, and likelihood-ratio statistics
Statement
Assume AC and the hypotheses of Wald, score, and likelihood-ratio asymptotic chi-square calibration. Under the null, the three statistics satisfy , , and . Hence they have the same first-order null calibration, namely .
Facts & Assumptions
Given: AC and the hypotheses of Wald, score, and likelihood-ratio asymptotic chi-square calibration.
Wald, score, and likelihood-ratio asymptotic chi-square calibration: all three statistics equal one common quadratic score form up to , and that form converges to .
The Axiom of Choice: AC is inherited from [F1], whose iid score CLT uses the multivariate iid CLT.
Proof
By [F1], there is a common random quadratic form with , , and . Subtracting the first two equalities and applying the triangle inequality gives ; subtracting the second and third gives ; subtracting the third and first gives .
Theorem [F1] states and each statistic has that same limit under the theorem's hypotheses. The differences proved in step 1.1 therefore express first-order equivalence, not exact finite-sample equality.
5 · Examples, counterexamples and false statements
None yet.
Sources
- Peter Bartlett, Theoretical Statistics, Stat 210B Lecture 22, PDF pp. 8–10
- Yen-Chi Chen, STAT 581 Chapter 3: M-estimation and the maximum likelihood estimator, QMD section
- Yen-Chi Chen, STAT 581 Chapter 3: M-estimation and the maximum likelihood estimator, QMD discussion
- Peter Bartlett, Theoretical Statistics, Stat 210B Lecture 26, PDF pp. 1–17
- John Duchi, Stats 300B Lecture 6, Winter 2018, complete scribe note, PDF pp. 1–5
- Yen-Chi Chen, STAT 581 Chapter 4: Hypothesis tests in likelihood models
- Peter Bartlett, Theoretical Statistics, Stat 210B Lecture 23, PDF p. 15
- Yen-Chi Chen, STAT 581 Chapter 3: M-estimation and the maximum likelihood estimator, Theorem 14 and QMD discussion
- John Duchi, Stats 300B Lecture 3, Winter 2018, complete scribe note, PDF pp. 5–6
- Peter Bartlett, Theoretical Statistics, Stat 210B Lecture 26, PDF pp. 9–17
- Yen-Chi Chen, STAT 581 Chapter 4: Hypothesis tests in likelihood models, lines 33–76
- Peter Bartlett, Theoretical Statistics, Stat 210B Lecture 26, PDF pp. 11–17
- Yen-Chi Chen, STAT 581 Chapter 4: Hypothesis tests in likelihood models, lines 73–76