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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
Chapter Six: Chapter Six: The Basics of The Basics of
Experimentation I: Experimentation I: Variables and Variables and
ControlControl
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
The Nature of VariablesThe Nature of Variables
• Variable• A variable is an event or behavior that can assume at least
two values.• Bridgman (1927) suggested that researchers should
define their variables in terms of the operations needed to produce them.
• Such definitions allow others to replicate your research
and are called operational definitions.
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Independent VariablesIndependent Variables
• Independent Variables (IVs)
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
Independent VariablesIndependent Variables
• Independent Variables (IVs)• IVs are those variables that the experimenter purposely
manipulates.
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Independent Variables
• Independent Variables (IVs)• IVs are those variables that the experimenter purposely
manipulates. • The IV constitutes the reason the research is being
conducted; the experimenter is interested in determining what effect the IV has.
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Types of IVsTypes of IVs
• Physiological IV• The physiological state of the participant that the
experimenter manipulates.• Experience IV
• Manipulation of the amount or type of training or learning.• Stimulus or environmental IV
• An aspect of the environment that the experimenter manipulates.
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Participant characteristicsParticipant characteristics
• Aspects of the participant, such as age, sex or personality traits, that are treated as if they are IVs.
• They are not IVs because they cannot be manipulated by the experimenter.
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Extraneous Variables Extraneous Variables (confounders)(confounders)• Extraneous Variables
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
Extraneous Variables Extraneous Variables (confounders)(confounders)• Extraneous variables
• Uncontrolled Variables that can cause unintended changes between groups.
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Extraneous Variables Extraneous Variables (confounders)(confounders)• Extraneous variables
• Uncontrolled Variables that can cause unintended changes between groups.
• Confounding
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Extraneous Variables Extraneous Variables (confounders)(confounders)• Extraneous Variables
• Uncontrolled Variables that can cause unintended changes between groups.
• Confounding• A situation in which the results of an experiment can be
attributed to either the operation of an IV or an extraneous variable.
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Dependent VariablesDependent Variables
• Dependent Variable (DV)
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Dependent VariablesDependent Variables
• Dependent Variable (DV)• A response or behavior that is measured. It is desired that
changes in the DV are directly related to manipulation of the IV.
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness
• Only correct responses are counted.
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency
• Rate of responding determines how rapidly responses are made during a specified time period.
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency
• Rate of responding determines how rapidly responses are made during a specified time period.
• The number of responses or events that occur within a specified time period is the frequency.
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency• Degree or Amount
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Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency• Degree or Amount• Latency or Duration
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Recording More than One Recording More than One DVDV• If you have the measurement capabilities, there is
nothing to prohibit the recording of more than one DV.
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Should You Record More Should You Record More than One DV?than One DV?• If you have the measurement capabilities, there is
nothing to prohibit the recording of more than one DV. • If recording an additional DV makes a meaningful
contribution to your understanding of the phenomenon under study, then you should give it serious consideration.
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Characteristics of a Good Characteristics of a Good DVDV• A DV is valid when it measures what the experimental
hypothesis says it should measure.
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Characteristics of a Good Characteristics of a Good DVDV• A DV is valid when it measures what the experimental
hypothesis says it should measure.• A good DV must be directly related to the IV and must
measure the effects of the IV manipulation as the experimental hypothesis predicts it will.
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Characteristics of a Good Characteristics of a Good DVDV• A DV is valid when it measures what the experimental
hypothesis says it should measure.• A good DV must be directly related to the IV and must
measure the effects of the IV manipulation as the experimental hypothesis predicts it will.
• A good DV is also reliable.
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Nuisance VariablesNuisance Variables
• Nuisance Variables
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Nuisance VariablesNuisance Variables
• Nuisance Variables• Unwanted variables that can cause the variability of scores
within groups to increase.
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Nuisance VariablesNuisance Variables
• Nuisance Variables• Unwanted variables that can cause the variability of scores
within groups to increase.• Nuisance variables increase the spread of scores within a
distribution; they do not cause a distribution to change its location.
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Controlling Extraneous Controlling Extraneous VariablesVariables• The experimenter must exercise control over both
extraneous variables and nuisance variables so the results of the experiment are as meaningful (no extraneous variables present) and clear (minimal influence of nuisance variables) as possible.
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Basic Control TechniquesBasic Control Techniques
• Randomization
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Basic Control TechniquesBasic Control Techniques
• Randomization• A control technique that ensures that each participant has
an equal chance of being assigned to any group in an experiment.
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination
• A control technique whereby extraneous variables are completely removed from an experiment.
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy
• A control technique by which an extraneous variable is reduced to a single value that is experienced by all participants.
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy• Balancing
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy• Balancing
• A control procedure that achieves group equality by distributing extraneous variables equally to all groups.
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy• Balancing• Counterbalancing
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy• Balancing• Counterbalancing
• A procedure for controlling order effects by presenting
different treatment sequences.
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Basic Control TechniquesBasic Control Techniques
• Randomization• Elimination • Constancy• Balancing• Counterbalancing
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CounterbalancingCounterbalancing
• Within-Subject counterbalancing
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CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Presentation of different treatment sequences to the same
participant.
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CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Presentation of different treatment sequences to the same
participant.• Within-Group counterbalancing
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CounterbalancingCounterbalancing
• Within-Group counterbalancing• Presentation of different treatment sequences to different
participants.• Three basic requirements:
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Within-Group counterbalancing• Presentation of different treatment sequences to different
participants.• Three basic requirements:
• Each treatment must be presented to each participant an equal number of times.
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Within-Group counterbalancing• Presentation of different treatment sequences to different
participants.• Three basic requirements:
• Each treatment must be presented to each participant an equal number of times.
• Each treatment must occur an equal number of times at each testing or practice session.
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Within-Group counterbalancing• Presentation of different treatment sequences to different
participants.• Three basic requirements:
• Each treatment must be presented to each participant an equal number of times.
• Each treatment must occur an equal number of times at each testing or practice session.
• Each treatment must precede and follow each of the other treatments an equal number of times.
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CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing
• All possible treatment sequences are presented.
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing
• All possible treatment sequences are presented.• You can calculate the number of sequences by using the
formula n! (n factorial).
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CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing• Incomplete counterbalancing
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing• Incomplete counterbalancing
• Only a portion of all possible sequences are presented.
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CounterbalancingCounterbalancing
• Sequence or Order Effects
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Sequence or Order Effects• Sequence or order effects are produced by the
participant’s being exposed to the sequential presentation of the treatments.
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Sequence or Order Effects• Sequence or order effects are produced by the
participant’s being exposed to the sequential presentation of the treatments.
• The sequence or order effect depends on where in the sequential presentation of treatments the participant’s performance is evaluated, not which treatment is experienced.
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
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• Carryover Effects
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
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• Carryover Effects• The effects of one treatment persist or carry over and
influence responses to the next treatment.
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Differential Carryover
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The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education
CounterbalancingCounterbalancing
• Differential Carryover• The response to one treatment depends on which
treatment was administered previously.