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№1 слайд
Содержание слайда: Lecture 4 Rescaling, Sum and difference of random variables: simple algebra for mean and standard deviation
(X+Y)2=X2 + Y2 + 2 XY
E (X+Y)2 = EX2 + EY2 + 2 EXY
Var (X+Y) = Var (X) + Var (Y) if independence
Demonstrate with Box model (computer simulation)
Two boxes : BOX A ; BOX B
Each containing “infinitely” many tickets with numeric values (so that we don’t have to worry about the estimation problem now; use n)
№2 слайд
Содержание слайда: Change of scale
Inch to centimeter: cm= inch times 2.54
pound to kilogram: kg=lb times 2.2
Fahrenheit to Celsius oC= ( oF-32)/1.8
Y= X+a
E Y = E X + a
SD (Y) = SD (X) ; SD(a) =0
Y= c X
E Y = c E X
SD (Y)= |c| SD(X); Var (Y)= c2Var (X)
Y=cX + a
EY= c E X + a
SD (Y) =| c| SD (X); Var (Y)= c2 Var(X)
Var X= E (X-)2= E X2 - (EX)2 (where = E X)
№3 слайд
№4 слайд
Содержание слайда: Two Boxes A and B ; independence
№5 слайд
Содержание слайда: E (X+ Y) = E X + E Y; always holds
E ( X Y) = ( E X ) ( EY) ; holds under independence assumption (show this! Next)
Without independence assumption E(XY) is in general not equal to EX times EY ; it holds under a weaker form of independence called “uncorrelatedness” (to be discussed )
№6 слайд
Содержание слайда: Combination
Var (a X + b Y) = a2 Var X + b2 Var Y if X and Y are independent
Var (X-Y) = Var X + Var Y
Application : average of two independent measurement is more accurate than one measurement : a 50% reduction in variance
Application : difference for normal distribution
№7 слайд
№8 слайд
Содержание слайда: Example
Phone call charge : 40 cents per minute plus
a fixed connection fee of 50 cents
Length of a call is random with mean 2.5 minutes and a standard deviation of 1 minute.
What is the mean and standard deviation of
the distribution of phone call charges ?
What is the probability that a phone call costs
more than 2 dollars?
What is the probability that two independent phone calls in total cost more than 4 dollars?
What is the probability that the second phone call costs more than the first one by least 1 dollar?
№9 слайд
Содержание слайда: Example
Stock A and Stock B
Current price : both the same, $10 per share
Predicted performance a week later: same
Both following a normal distribution with
Mean $10.0 and SD $1.0
You have twenty dollars to invest
Option 1 : buy 2 shares of A portfolio mean=?, SD=?
Option 2 : buy one share of A and one share of B
Which one is better? Why?
№10 слайд
Содержание слайда: Better? In what sense?
What is the prob that portfolio value will be higher than 22 ?
What is the prob that portfolio value will be lower than 18?
What is the prob that portfolio value will be between18 and 22?
(draw the distribution and compare)