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```							Samples, Good and Bad

STA 200
Chapter 2
How Can You Goof Up Sampling?

 Bias
Geographical
Generational
Informational
Financial
There’s More!
Biased Sampling Methods

   Bias
–   Systematically favors certain outcomes
   Convenience Sampling
–   Interviewer selects most reachable individuals
   Voluntary Response Sample
–   Individuals choose themselves
Example
who said “I love you” first? Of
the 200 coupled respondents:
   63% said he did
   37% said she did
Simple Random Sampling
   Personal choice produces bias
   Impersonal chance chooses the sample
   All individuals and every possible sample
have an equal chance
   Sample size is n
Random Digits

   A table of random digits is a list of the digits
0, 1, 2, 3, 4, 5, 6, 7, 8, 9 that has the following
properties:
1.   The digit in any position in the list has the same chance
of being any one of 0 – 9
2.   The digits in different positions are independent in the
sense that the value of one has no influence on the value
of any other.
Choosing an SRS

Let’s find a sample of 5 from this class
Line 101: 19223 95034 05756 28713 96409
12531 42544 82853

Line 101: 19223 95034 05756 28713 96409
12531 42544 82853
Choose Another SRS

Let’s find a sample of 5 from this class
Line 116: 14459 26056 31424 80371 65103
62253 50490 61181

Line 116: 14459 26056 31424 80371 65103
62253 50490 61181
 Notice that this is different from before!
Kentucky Crime Survey
views on crime and punishment to help come up with
recommendations to improve the state’s criminal
justice system.
   The survey will mail questionnaires to a random
statewide sample.
   Questions will deal with attitudes regarding
punishment, fear of crime, victimization within the past
year, and how well the criminal justice system is
working.
Americans Still Prefer Boys

   If Americans could only have one child:

   42% say they would prefer a boy
   27% say they would prefer a girl
   25% say it would not matter to them either way

   The results are based on telephone interviews with a
randomly selected national sample of 1,026 adults, 18 years
and older, conducted December 2-4, 2000.
Homework

   Suggested Chapter 2 Problems: 2 & 8

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