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Sufficient statistics. The Poisson and the exponential can be summarized by (n, ).

More Chapter 4. Sufficient statistics. The Poisson and the exponential can be summarized by (n, ). So too can the normal with known variance Consider a statistic S(Y)

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Sufficient statistics. The Poisson and the exponential can be summarized by (n, ).

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  1. More Chapter 4 Sufficient statistics. The Poisson and the exponential can be summarized by (n, ). So too can the normal with known variance Consider a statistic S(Y) Suppose that the conditional distribution of Y given S does not depend on , then S is a sufficient statistic for  based on Y Occurs iff the density of Y factors into a function of s(y) and  and a function of y that doesn't depend on 

  2. Example. Exponential IExp() ~ Y E(Y) =  Var(Y) = 2 Data y1,...,yn L() =  -1 exp(-yj /) l() = -nlog() - yj / yj /n is sufficient

  3. maximum

  4. =

  5. Approximate 100(1-2 )% CI for 0 Example. spring data

  6. Weibull.

  7. Note. Expected information

  8. Gamma.

  9. Example. Bernoulli Pr{Y = 1} = 1 - Pr{Y = 0} =  0   1 L() =  ^yi (1 - )^(1-yi) = r(1 - )n-r l() = rlog() + (n-r)log(1-) r =  yj R =  Yj is sufficient for , as is R/n L() factors into a function of r and a constant

  10. Score vector  [ yj / - (n-yj )/(1-)] Observed information  [yj /2 + (n-yj )/(1-)2 ] M.l.e.

  11. Cauchy. ICau() f(y;) = 1/(1+(y-)2 ) E|Y| =  Var(Y) =  L() =  1/((1+(yj -)2 ) Many local maxima l() = -log(1+(yj -)2 ) J() = 2((1-(yj -)2 )/(1+(yj -)2 )2 I() = n/2

  12. Uniform. f(u;) = 1/ 0 < u <  = 0 otherwise L() = 1/n 0 < y1 ,..., yn <  = 0 otherwise

  13. l() becomes increasingly spikey E u() = -1 i() = -

  14. Logistic regression. Challenger data Ibinomials Rj , mj , j

  15. Likelihood ratio. Model includes  dim() = p true (unknown) value 0 Likelihood ratio statistic

  16. Justification. Multinormal result If Y ~ N (,) then (Y- )T -1(Y- ) ~ p2

  17. Uses. Pr[W(0)  cp(1-2 )]  1-2 Approx 100(1-2 )% confidence region

  18. Example. exponential Spring data: 96 <  <335 vs. asymp normal approx 64 <  <273 kcycles

  19. Prob-value/P-value. See (7.28) Choose T whose large values cast doubt on H0 Pr0(T  tobs) Example. Spring data Exponential E(Y) =  H0:  = 100?

  20. Nesting : p by 1 parameter of interest : q by 1 nuisance parameter Model with params (0, ) nested within(, ) Second model reduces to first when  = 0

  21. Example. Weibull params (,) exponential when  = 1       How to examine H0 :  = 1?

  22. Spring failure times. Weibull

  23. Challenger data. Logistic regression temperature x1 pressure x2 (0 , 1 , 2 ) = exp{}/(1+exp{})  = 0 + 1 x1 + 2 x2 linear predictor loglike l(0 , 1 , 2 ) = 0  rj + 1  rj x1j + 2  rj x2j - m  log(1+exp{j }) Does pressure matter?

  24. Model fit. Are labor times Weibull? Nest its model in a more general one Generalized gamma. Gamma for =1 Weibull for =1 Exponential for ==1

  25. Likelihood results. max log likelihood: generalized gamma -250.65 gamma -251.12 Weibull -251.17 gamma vs. generalized gamma - 2 log like diff: 2(-250.65+251.12) = .94 P-value Pr0 (12 > .94) = Pr(|Z|>.969) = 2(.166) = .332

  26. Chi-squared statistics. Pearson's chi-squared categories 1,...,k count of cases in category i: Yi Pr(case in i) = i 0 < i < 1 1ki =1 E(Yi ) = ni var(Yi ) = i(1 - i )n cov(Yi ,Yj ) = -ij n i j E.g. k=2 case cov(Y,n-Y) = -var(Y) = -n12  = { (1 ,...,k ): 1ki = 1, 0<1 ,...,k <1} dimension k-1

  27. Reduced dimension possible? model i () dim() = p log like general model: 1k-1 yi log i + yk log[1-1 -...-k-1], 1k yi = n log like restricted model: l() = 1k-1 yi log i() + yk log[1-1()-...-k-1()]

  28. likelihood ratio statistic: if restricted model true The statistic is sometimes written W = 2  Oi log(Oi /Ei )  (Oi - Ei )2/Ei

  29. Pearson's chi-squared.

  30. Example. Birth data. Poisson? Split into k=13 categories [0,7.5), [7.5,8.5),...[18.5,24] hours O(bserved) 6 3 3 8 ... E(xpected) 5.23 4.37 6.26 8.08 ... P = 4.39 P-value Pr(112 > 4.39) = .96

  31. Two way contingency table. r rows and c columns n individuals Blood groups A, B, AB, O A, B antigens - substance causing body to produce antibodies group count model I model II O = 1 - A - B

  32. Question. Rows and columns independent? W = 2  yij log nyij / yi.y.j with yi. = j yij ~ k-1-p2 = (r-1)c-1)2 with k=rc p=(r-1)+(c-1) P =  (yij - yi. y.j /n)2 / (yi. y.j /n) ~ (r-1)(c-1)2

  33. Model 1 W = 17.66 Pr(12> 17.66) = Pr(|Z| > 4.202) = 2.646E-05 P = 15.73 Pr(12> 15.73) = Pr(|Z| > 3.966) = 7.309E-05 k-1-p = 4-1-2 = 1 Model 2 W = 3.17 Pr(|Z| > 1.780) = .075 P = 2.82 Pr(|Z|>1.679) = .093

  34. Incorrect model. True model g(y), fit f(y;)

  35. Example 1. Quadratic, fit linear

  36. Example 2. True lognormal, but fit exponential

  37. Large sample distribution.

  38. Model selection. Various models: non-nested Ockham's razor. Prefer the simplest model

  39. Formal criteria. Look for minimum

  40. Example. Spring failure Model p AIC BIC M1 12 744.8* 769.9* M2 7 771.8 786.5 M3 2 827.8 831.2 M4 2 925.1 929.3 6 stress levels M1: Weibull - unconnected ,  at each stress level

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