Computer Science Canada Probability Help |

Author: | Raknarg [ Tue Oct 22, 2013 12:41 pm ] |

Post subject: | Probability Help |

I'm hoping someone here understands statistics pretty well.. I've seen this come up a few times: E (3X + 1) Var (3X + 1) That's expected value and variance. But I have no idea what i'm supposed to do with those transformations to x, or what it means. Anyone know? |

Author: | DemonWasp [ Tue Oct 22, 2013 1:04 pm ] |

Post subject: | RE:Probability Help |

Expected value and variance have properties that will let you manipulate them. See: http://en.wikipedia.org/wiki/Expected_value#Properties http://en.wikipedia.org/wiki/Variance#Properties In your example, because E ( X + c ) for c some constant is E ( X ) + c, we know that E ( 3X + 1 ) = E ( 3X ) + 1. Similarly, we know that E ( aX ), for some constant a, is a * E ( X ), so E ( 3X + 1 ) = 3 * E ( X ) + 1. The rules for variance are different, but similar. You should find that Var ( 3X + 1 ) = 9 * Var ( X ), if I'm doing it correctly. |

Author: | Raknarg [ Tue Oct 22, 2013 1:12 pm ] |

Post subject: | RE:Probability Help |

Wat's the point, is it to scale between different sample sizes? |

Author: | Tony [ Tue Oct 22, 2013 2:22 pm ] |

Post subject: | RE:Probability Help |

The point is that if you know the distributions for a given random variable (X in your examples), you could figure out the expected outcomes of more complex systems. If I propose a bet where you flip a coin 3 times and get paid $1 + $1 for each heads. How much would you expect to win? E (3X + 1) where X depends on the fairness of the coin. |

Author: | Raknarg [ Tue Oct 22, 2013 4:05 pm ] |

Post subject: | RE:Probability Help |

Ohhh that makes sense. |

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