Transcription of Joint Distribution - Example - Duke University
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Lecture 17: Joint DistributionsStatistics 104 Colin RundelMarch 26, 2012 Section Distributions of Discrete RVsJoint Distribution - ExampleDraw two socks at random, without replacement, from a drawer full oftwelve colored socks:6 black, 4 white, 2 purpleLetBbe the number of Black socks,Wthe number of White socksdrawn, then the distributions ofBandWare given by:012P(B=k)612511=15662612611=366661251 1=1566P(W=k)812711=28662412811=326641231 1=666 Note -B HyperGeo(12,6,2) =(6k)(62 k)(122)andW HyperGeo(12,4,2) =(4k)(82 k)(122)Statistics 104 (Colin Rundel)Lecture 17 March 26, 20121 / 32 Section Distributions of Discrete RVsJoint Distribution - Example , the number of Black socks andWthe number of White socksdrawn, then the Joint Distribution ofBandWis given by:W01201668666661566B112662466036662156 6001566286632666666666P(B=b,W=w) = 1/66 If b=0,w=08/66 If b=0,w=16/66 If b=0,w=212/66 If b=1,w=024/66 If b=1,w=10/66 If b=1,w=215/66 If b=2,w=00/66 If b=2,w=10/66 If b=2,w=2P(B=b,W=w) =(6b)(4w)(22 b w)(122), for 0 b,w 2 andb+w 2 Statistics 104 (Colin Rundel)Lecture 1
Depending on which range de nition you choose it makes life easier when evaluating the marginal densities. f X(x) = Z 1 1 f(x;y) dy = Z 3 x 0 2 9 dy = 2 9 (3 x) for x 2(0;3) f Y (y) = Z 1 1 f(x;y) dy = Z 3 y 0 2 9 dx = 2 9 (3 y) for y 2(0;3) Are X and Y independent? Statistics 104 (Colin Rundel) Lecture 17 March 26, 2012 19 / 32
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