PSYC 275 Introduction to Research Methods

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PSYC 275Introduction to Research Methods

• SCC Chapter Two
• Samples, Good and Bad

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SC C Chapter TwoSamples, Good and Bad

• The Town Talk takes an opinion poll
• Should local ambulance company be the only
  service provider in the town?
• Collected data via telephone call in
• 3763 calls
• (638 calls came from the ambulance company!)
• Voluntary response and the same people responding
  repeatedly biased the poll.

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How to Sample Badly

• Biased Sampling Methods
• The design of a statistical study is biased if it
  systematically favors certain outcomes.
• Any time one type of individual is oversampled
  (more likely to be selected) relative to another,
  the sample is biased.

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Biased Sampling Methods Convenience Sampling

• Convenience Sampling involves selection of the
  easiest to reach individuals.
• These are samples that are collected at malls and
  on sidewalks.
• Select only those individuals who appear friendly
  and unhurried.

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Biased Sampling Methods Voluntary Response

• Voluntary Response Sample chooses itself by
  responding to a general appeal.
• Newspaper/TV call/write in polls
• Respondents are typically strongly opinionated
  about the subject, sometimes responding multiple
  times or getting their equally opinionated
  friends to respond as well. Ex. Ann Landers /
  Dear Abby polls.

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Internet Voluntary Response

• www.misterpoll.com

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Simple Random Samples

• Simple Random Sample - (SRS) of size n consists
  of n individuals from the population chosen in
  such a way that every set of n individuals has an
  equal chance to be the sample actually selected.
• (The n represents the number of individuals
  included in the sample.)

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whats your n?
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Choosing an SRS

• Choose an SRS in two steps
• Step 1 Label. Assign a numerical label to every
  individual in the population. Make sure that all
  labels have the same number of digits.
• Step 2 Table of Random Digits. Use random digits
  to select labels at random.
• http//www.whfreeman.com/scc/con_index.htm?99spt

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Choosing an SRS

• Example

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Choosing an SRS Example

• A PSYC 275 student wants to understand the
  attitudes of Psychology Department Faculty toward
  the USD WebAdvisor system.
• Beginning at line 115, use the Table of Random
  Digits (Pages 545 546) to choose 5 faculty
  members to be interviewed.

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USD Psychology Dept. Faculty

• Peterson
• Quevillon
• Reyes
• Schieber
• Simons
• Skillman
• Straub

Struckman-Johnson Todd-Bazemore Wang Yutrzenka

• Berkhout
• Caraway
• Daniels
• Elliott
• French
• Granaas
• Jacobs

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USD Psychology Dept. Faculty

• Peterson
• Quevillon
• Reyes
• Schieber
• Simons
• Skillman
• Straub

Struckman-Johnson Todd-Bazemore Wang Yutrzenka
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• Berkhout
• Caraway
• Daniels
• Elliott
• French
• Granaas
• Jacobs

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09
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Randomization and Software

• Research Randomizer Choose an SRS
• www.randomizer.org

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Can You Trust a Sample?

• Depends on how the sample was collected
• Simple random sample?
• Will be developing other questions in Chapter 4.

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End Chapter Two