Chapter 20
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Find the Expected Value
How to find expected value in a sample of 25000 people
10000 republicans
15000 democrats
Sample size = 100
Find the EV of republicans (not box)
To find the expected value you have to find the average of the box
EV = (Number of draw x average of box)
Solution:
ave of republicans = 10000/25000 =.04
EV = 100 (#draws) x .04 (Ave) = 40
Find the SD
Using SD ShortCut (289)
Formula:
Big# - Small# x [Square Root of (fraction of Big#) x (fraction of Small#)]
Can only be used with two numbers
5,1,1,1
Example
(5-1) x
example 1,0
25000
10000 republicans
15000 democrats
1s, 0s
SD = (1-0) x [Square root of (10000/25000 x 15000/25000)] = 4.9
Finding the SE for Precentage
SE for Precentage = SE of sample/#Draws) x 100
How to find expected value in a sample of 25000 people
10000 republicans
15000 democrats
Sample/draws = 100
SE for Number (Republicans)
Find SD = 4.9 (or about 5)
SE = 10 x .5 = 5
Now use percentage formula
(.5/100) x 100% = 5%
Using Box Model
If there are more than 2 numbers use box model to Consisting of 1 & 0
example:
out of a 100000 samples
20% have shoes
50% are broke
40% have bad credit
10% are on crack
Draws =1000
Isolate the Variable it ask for into 1s & 0s
find expected value (EV) of people who have bad credit 40% or 40,000
find the SE (need SD... this is where the box model shines)
40% bad credit and the 60% good credit
SD = Square Root of (.4 x .6) = .49
SE = Square Root of (1000draws) x (.49 SD)
SE = 15
Using the Normal Curve
out of a 100000 samples
20% have shoes
50% are broke
40% have bad credit
10% are on crack
Draws =1000
Find percentage between 39% and 40% have bad credit
Need to convert into standard units
EV = 40%
SE = 15
Convert to percent 15/1000 = 1.5%
39-40 = 1, 1/1.5 = +/- .67
Now look a normal curve to find percentage
Answer = 48%
Card Set Information
Author:
damea134
ID:
306338
Filename:
Chapter 20
Updated:
2015-08-13 06:22:13
Tags:
Chance Error
Folders:
Statistics
Description:
Chance error in sampling
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