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Chapter 9 Enhancing Rigor in Quantitative Research. Controlling Extraneous Variables. Controlling external, situational factors—constancy of conditions. Constancy of the environment Constancy of treatment conditions Constancy of time Constancy of communications to subjects.

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controlling extraneous variables
Controlling Extraneous Variables

Controlling external, situational factors—constancy of conditions

  • Constancy of the environment
  • Constancy of treatment conditions
  • Constancy of time
  • Constancy of communications to subjects
controlling intrinsic subject characteristics
Controlling Intrinsic Subject Characteristics
  • Randomization
  • Repeated measures
  • Homogeneity
  • Blocking
  • Matching/balancing
  • Statistical control
benefits and limitations of control methods randomization
Benefits and Limitations of Control Methods: Randomization

Benefits:

  • Controls all extraneous variables
  • Does not require advance knowledge of which variables to control

Limitations:

  • Ethical and practical constraints on manipulation
  • Possible artificiality of conditions
benefits and limitations of control methods repeated measures
Benefits and Limitations of Control Methods: Repeated Measures

Benefits:

  • If done with randomization, strongest possible approach
  • Reduces sample size requirements

Limitations:

  • Cannot be used if there are possible carry-over effects from one condition to another
benefits and limitations of control methods homogeneity
Benefits and Limitations of Control Methods: Homogeneity

Benefits:

  • Easy to achieve
  • Enhances interpretability of relationships

Limitations:

  • Limits generalizability
  • Requires knowledge of which variables to control
benefits and limitations of control methods blocking
Benefits and Limitations of Control Methods: Blocking

Benefits:

  • Enhances interpretability of relationships
  • Offers possibility of examining blocking variable as an independent variable

Limitations:

  • Manageable only with a few blocking variables
  • Requires knowledge of which variables to control
benefits and limitations of control methods matching
Benefits and Limitations of Control Methods: Matching

Benefits:

  • Enhances interpretability of relationships
  • Easy to do if there is a large pool of available comparison group subjects

Limitations:

  • Manageable only with a few matching variables
  • Requires knowledge of which variables to control
  • May be hard to find comparison group matches
benefits and limitations of control methods statistical control
Benefits and Limitations of Control Methods: Statistical Control

Benefits:

  • Enhances interpretability of relationships
  • Easy and economical
  • Can be used with a large number of extraneous variables

Limitations:

  • Requires knowledge of which variables to control
  • Requires statistical sophistication
characteristics of good research design in quantitative studies
Characteristics of Good Research Design in Quantitative Studies
  • Statistical conclusion validity
  • Internal validity
  • External validity
  • Construct validity
threats to statistical conclusion validity
Threats to Statistical Conclusion Validity
  • Low statistical power
  • Inadequate precision
  • Unreliable implementation of a treatment
  • Inadequate participation in treatment conditions
problems with treatment implementation analytic options
Problems with Treatment Implementation: Analytic Options

Analytic strategies when subjects withdraw from study or fail to get full exposure to treatment:

  • On protocol analysis
  • Intention to treat principle
  • Analysis of “dose” of treatment received
threats to internal validity
Threats to Internal Validity
  • History
  • Selection
  • Maturation
  • Testing
  • Instrumentation
  • Mortality
accessible vs target population
Accessible vs. Target Population

Accessible population:

The population available for a particular study

Target population:The total group of people in whom a researcher is interested and to whom results could be generalized

threats to external validity
Threats to External Validity
  • Inadequate sampling
  • Expectancy effects
  • Novelty effects
  • Interaction of history and treatment effects
  • Experimenter effects
  • Measurement effects
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