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# 15 Bivariate Change Of Variables

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### 15 Bivariate Change Of Variables

1. 1. Quiz • Pick up quiz and handout on your way in. • Start at 1pm • Finish at 1:10pm • The quiz is not for a grade, but I will be collecting it.
2. 2. Stat 310 Bivariate Change of Variables Garrett Grolemund
3. 3. 1. Review of distribution function techniques 2. Change of variables technique 3. Determining independence 4. Simulating transformations
4. 4. Lakers' final scores
5. 5. Laker's Mean 87.2 Opponent's Mean 81.2
6. 6. Let X = The Lakers' Score Y = The opponent's score U = X -Y Then the Lakers will win if U is positive, and they will lose if U is negative. How can we model U? (i.e, How can we find the CDF and PDF of U?)
7. 7. Recall from Tuesday U = X - Y is a bivariate transformation The Distribution function technique gives us two ways to model X - Y:
8. 8. 1. Begin with FX,Y(a): Compute FU(a) in terms of FX,Y(a) by equating probabilities
9. 9. 1. Begin with FX,Y(a): Compute FU(a) in terms of FX,Y(a) by equating probabilities FU(a) = P(U < a) = P(X - Y < a) = P(X < Y + a) =?
10. 10. 2. Begin with fX,Y(a) : Compute FU(a) by integrating fX,Y(a) over the region where U < a f(x,y) P(Set A) X Set A Y
11. 11. 2. Begin with fX,Y(a) : Compute FU(a) by integrating fX,Y(a) over the region where U < a f(x,y) ∞ Y+a P(Set A) FU(a) = ∫ ∫ fX,Y(a) dxdy -∞ -∞ X Set A Y
12. 12. Change of variables
13. 13. Univariate change of variables X U If U = g(X) X = h(U) Where h is the inverse of g, then fU(u) = fx(h(u)) |h'(u)| Method works for bivariate case, once we make the appropriate modifications.
14. 14. Bivariate change of variables (X,Y) (U,V) if U = g1(X, Y) X = h1(U, V) V = g2(X, Y) Y = h2(U, V) Where h is the inverse of g, then fU,V(u, v) = fx,y(h1(U, V) , h2(U, V) ) | J |
15. 15. fU(u) = fx(h(u)) |h'(u)| Since U depends on both X and Y, we replace fx(h(u)) with the joint density fx,y(h(u), * )
16. 16. fU(u) = fx,y(h(u), * ) |h'(u)| A joint density must be a function of two random variables Let X = h1(u) and Y = h2(u)
17. 17. fU(u) = fx,y(h1(u), h2(u)) |h'(u)| But for equality to hold, we must have a function of two variables on the left side as well Define V = g2(X, Y) however you like.
18. 18. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) |h'(u)| Now we have two equations to take derivatives of (h1, h2) and two variables to take the derivative with respect to, (U,V) The multivariate equivalent of h'(u) is the Jacobian, J
19. 19. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J | And we're finished
20. 20. Jacobian δx δx δu δv J= δy δy δu δv
21. 21. a b = ad - bc c d δx δx δu δv J= = δx δy - δx δy δy δy δu δv δv δu δu δv
22. 22. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J | U=X-Y What should V be?
23. 23. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J | U=X-Y What should V be? • Sometimes we want V to be something specific • Otherwise keep V simple or helpful e.g., V = Y
24. 24. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J | What should fx,y(*, *) be?
25. 25. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J | What should fx,y(*, *) be? First consider: what should fx(*) and fy(*) be?
26. 26. How would we model the Lakers score distribution?
27. 27. How would we model the Lakers score distribution? • Discrete Data
28. 28. How would we model the Lakers score distribution? • Discrete Data • Sum of many bernoulli trials
29. 29. How would we model the Lakers score distribution? • Discrete Data • Sum of many bernoulli trials Poisson?
30. 30. Lakers' scores vs. simulated Poisson (87.2) distributions
31. 31. Opponent's scores vs. simulated Poisson (81.2)
32. 32. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J | X ~ Poisson (87.2) fx(h1(u, v)) = e-87.2 (87.2)h1(u,v) h1(u, v)! Y ~ Poisson (81.2) fy(h2(u, v)) = e-81.2 (81.2)h2(u,v) h2(u, v)! ? fx,y(h1(u, v), h2(u, v)) = fx(h1(u, v)) fy(h2(u, v))
33. 33. ρ = 0.412
34. 34. fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J |
35. 35. Your Turn Calculate the Jacobian of our transformation. Let U = X - Y and V = Y. δx δx δu δv J= = δx δy - δx δy δy δy δu δv δv δu δu δv
36. 36. Your Turn Calculate fU,V(u, v) and express fU(u) as an integral (you do not need to solve that integral). Let U = X - Y and V = Y. Let X ~ Poisson(87.2) and Y ~ Poisson(81.2)
37. 37. Skellam Distribution fU,V(u, v) = e-(87.2 + 81.2) (87.2)h1(u,v)(81.2)h2(u,v) h1(u, v)! h2(u, v)! http://en.wikipedia.org/wiki/Skellam_distribution
38. 38. U values
39. 39. U values vs. Skellam Distribution
40. 40. Testing Independence
41. 41. Recall: 1. U and V are independent if fU,V(u, v) = fU(u) fV(v) 2. By change of variables: fU,V(u, v) = fx,y(h1(u, v), h2(u, v)) | J |
42. 42. Complete the proof Complete the handout to show that U = X + Y and V = X - Y are independent when X, Y ~ N(0, 1).
43. 43. Simulation
44. 44. We can also learn a lot about the distribution of a random variable by simulating it. Let Xi ~ Uniform (0,1) Let U = (X1 + X2 ) / 2 If we generate a 100 pairs of X1 and X2 and plot (X1 + X2 ) / 2 for each pair, we will have a simulation of the distribution of U
45. 45. For comparison, V1 = X1 ~ uniform(0,1) 10,000 samples
46. 46. V1 = (X1 + X2) / 2 10,000 samples
47. 47. V1 = (X1 + X2 + X3) / 3 10,000 samples
48. 48. V1 = (X1 + X2 + … + X10) / 10 10,000 samples
49. 49. V1 = Σ11000 Xi / 1000 10,000 samples
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