Chapter 2 - Chapter 2 - Samples, Good and Bad Goal Gain...

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Chapter 2 - Samples, Good and Bad Goal – Gain information about the whole by examining only a part “You don’t have to eat the whole ox to know that the meat is tough.” – Samuel Johnson Ex. Opinion polls – try to estimate the opinion of the U.S. with a small sample How to sample badly? Convenience sampling : selecting individuals that are easiest to reach Ex. Acceptance Sampling – units on top; Customer interviews conducted at a mall Listed phone numbers Voluntary response sampling : allowing individuals to choose to be in the sample (usually, sample chooses itself by responding to questions asked by mail or during TV or radio broadcast) Ex. Write-in and call-in opinion polls Both of these techniques are biased -- systematically favor certain outcomes Bias : sampling method consistently and repeatedly differs from the truth about the population in the same direction How do we avoid biased samples? Goal – eliminate bias by using chance
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Chapter 2 - Chapter 2 - Samples, Good and Bad Goal Gain...

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