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STAT3023 Two-sided tests Now consider testing H0 : θ = θ0 vs. H1 : θ ̸= θ0. It can be shown (proof omitted) that if the family has monotone likelihood ratio then the power functions of the UMP 1-sided tests are strictly monotone. Each of the 1-sided UMP test works well for their stated 1-sided H1. However, for the 2-sided H1, there exists θ such that the tests are biased in the sense that Eθ [δ0(X)] < Eθ0 [δ0(X)]. 1 Simple vs. composite: UMPU tests An unbiased test has power function no small than α for all θ under H1. For 1-parameter exponential families with PDF fθ(x) = h(x)eθT(x)−A(θ), a UMP-Unbiased (UMPU) test exists. Theorem: For a 1-parameter exponential family with sufficient statistic T(X), a UMPU test at level α for testing H0 : θ = θ0 vs. H1 : θ ̸= θ0 exists and is given by: δ(X) = 1, T(X) > c2 or T(X) < c1 γi, T(X) = ci, i = 1, 2 0, c1 < T(X) < c2 where ci,γi, i = 1, 2, are chosen such that Eθ0 [δ(X)] = α and Eθ0 [T(X)δ(X)] = Eθ0 [T(X)]Eθ0 [δ(X)] = αEθ0 [T(X)]. 2 Simple vs. composite: UMPU tests Example: Suppose we can observe a sample X ∼ exp(θ), with PDF fθ(x) = 1θ e −x/θ, x > 0. Determine the UMPU test for testing H0 : θ = 1 vs. H1 : θ ̸= 1. 3 Simple vs. composite: UMPU tests Example: Suppose X ∼ exp(θ), fθ(x) = 1θ e−x/θ, x > 0. Determine the UMPU test for testing H0 : θ = 1 vs. H1 : θ ̸= 1. 4 Simple vs. Composite We have mainly presented methods in this case applicable to 1-parameter exponential families. The general properties of MLE also lead to generally applicable testing methods. Suppose we have X = (X1, . . . ,Xn) iid with common fθ(·) for the parametric family {fθ(·) : θ ∈ Θ}, Θ ∈ R. Recall the most power test for H0 : θ = θ0 vs. H1 : θ = θ1 is given by the Neyman-Pearson likelihood ratio (NPLR) test statistic ∏ni=1 fθ1(Xi) ∏ni=1 fθ0(Xi) , which requires knowing both θ0 and θ1. If we are testing H0 : θ = θ0 vs. H1 : θ ∈ Θ\{θ0}, we could try to first “estimate” a θ1 value and plug it into the NPLR statistic. 5 Simple vs. Composite: GLRT Definition: The Generalised likelihood ratio test (GLRT) for testing H0 : θ = θ0 vs. H1 : θ ∈ Θ\{θ0} uses the statistic: ∏ni=1 fθˆ(Xi) ∏ni=1 fθ0(Xi) , where θˆ = argmaxθ∈Θ∏ n i=1 fθ(Xi). 6 Simple vs. Composite: GLRT The limiting distribution of the log-GLRT statistic under H0 is 12χ 2 1 (under some regularity conditions). Sketch proof: 7 Simple vs. Composite: GLRT (Proof continued) 8 Simple vs. Composite: GLRT (Proof continued) 9 Composite vs. Composite Suppose X ∼ fθ(x) for a 1-parameter family {fθ(·) : θ ∈ Θ}, Θ ∈ R. We are testing H0 : θ ∈ Θ0 vs. H1 : θ ∈ Θ\Θ0, Θ0 ⊆ Θ. For certain composite H0, optimal tests exist. Proposition: If the family has monotone likelihood ratio in a statistic T(X), then the UMP test of H0 : θ ≤ θ0 vs. H1 : θ > θ0 is of the same form as for H0 : θ = θ0 vs. H1 : θ > θ0: δ(X) = 1, T(X) > c γ, T(X) = c 0, T(X) < c, where c,γ are chosen such that Eθ0 [δ(X)] = α. 10 Composite vs. Composite Proposition: For a 1-parameter exponential family: fθ(x) = h(x) exp(w(θ)T(x)−A(θ)) where w(θ) is strictly increasing in θ. We are testing H0 : θ ≤ θ1 or θ ≥ θ2 (θ1 < θ2) vs. H1 : θ1 < θ < θ2. The UMPU test exists and is of the form: δ(X) = 1, c1 < T(X) < c2 γi, T(X) = γi, i = 1, 2 0, T(X) < c1 or T(X) > c2, where ci,γi, i = 1, 2, are selected such that Eθ1 [δ(X)] = Eθ2 [δ(X)] = α. (Such tests are of interest when trying to show a new drug is “effectively equivalent” to some standard.) 11 Multivariate case Suppose we have a family of distributions indexed by more than 1 parameter. The GLRT provides a useful general method of testing H0 : θ ∈ Θ0 vs. H1 : θ ∈ Θ\Θ0. Compute the test statistic: log ( L(θˆ;X) L(θˆ0;X) ) where θˆ = argmax θ∈Θ L(θ;X) is the “unrestricted” MLE, and θˆ0 = argmax θ∈Θ0 L(θ;X). 12 Multivariate case Many commonly used statistical tests are equivalent to the GLRT. Example: 1-way ANOVA F-test. Consider Xij ∼ N(µi, σ2), for i = 1, . . . , g being indexes for groups and j = 1, . . . ,ni, and all Xij are independent. The total sample size is denoted by N = ∑gi=1 ni. Consider testing H0 : µ1 = µ2 = · · · = µg vs. H1 : µi’s are not all equal. 13 Multivariate case (Example continued) 14 Multivariate case (Example continued) 15 Multivariate case (Example continued) 16 Multivariate case Example: one-sided t-test. Let X1, . . . ,Xn be iid N(µ, σ2), and Θ = {(µ, σ2) : µ ≥ 0, σ2 > 0}. Consider testing H0 : µ = 0 vs.