STAT 218
Week 2
Chapters 3, 4
Questions from homework or previous class?
CHAPTER 3
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WE WILL SKIP SECTION 3.9
SAMPLING
SIMPLE RANDOM SAMPLE
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A sample of size n from a population of size N is a simple random sample if it
was selected in such way that each of the possible observations are equally likely
o
Every member of the population has equal chance of being picked
o
The members of the sample are chosen independent of one another
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A random sample is supposed to resemble the population
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But it doesn’t always resemble the population exactly
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Chance error due to sampling
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We do not want SAMPLING BIAS  a systemic tendency to collect some values
of the population more often than we should
Example – Picking Students in this class
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Numbers in a hat – One number for each student
Other Methods for selecting a simple random sample
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10 sided dice
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Table of random digits – Table 1
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Find random numbers between 0 and 9, or 1 and 10
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Find Random number between 10 and 99, or between 1 and 100
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Find Random number between 1 and 1000
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Excel Rand Function
Example
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Use Table 1 to pick random numbers between 1 and 40
INTRO TO PROBABILITY
Probability
1
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Important to understand the main concept in order to understand statistics
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A probability is a numerical quantity that expresses the likelihood of an event.
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Grossly speaking, it is defined as the
(number of favorable outcomes divided by
the total number of possible outcomes)
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With coins, Prob(Head) = ½
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With dice,
Prob “4” = 1/6
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When probability is low, it is unlikely something will happen
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E.g:
Winning the lottery
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When probability is high, it is likely to happen
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E.g:
It will be sunny tomorrow
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More Examples
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Cards:
P (Ace of Spades),
P (a Jack)
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Coin Flip:
Toss three Nickels
P (3 heads) ????
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Dice:
P(2,3,4, or 5),
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Cards:
P (Heart greater than 9)
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Other:
Prob. of rain tomorrow
Basic Concepts
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PROBABILITY – a numerical quantity that expresses the likelihood of an event
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Notation:
P (E),
PROB (E)
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A Chance operation – either the event occurs or not
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Probability Rules
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Always a number between 0.0 and 1.0
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The sum of possible outcomes must equal 1.0.
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 Spring '08
 staff
 Normal Distribution, Probability, Standard Deviation, Simple random sample, PROBABILITY Probability

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