The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education Chapter Six: The Basics...

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

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)

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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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Extraneous Variables Extraneous Variables (confounders)(confounders)• Extraneous variables

• Uncontrolled Variables that can cause unintended changes between groups.

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.

• 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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Dependent VariablesDependent Variables

• Dependent Variable (DV)

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Recording or Measuring Recording or Measuring the DVthe DV• Correctness

• Only correct responses are counted.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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 Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency• Degree or Amount

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Recording or Measuring Recording or Measuring the DVthe DV• Correctness• Rate or Frequency• Degree or Amount• Latency or Duration

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Characteristics of a Good Characteristics of a Good DVDV• A DV is valid when it measures what the experimental

hypothesis says it should measure.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination

• A control technique whereby extraneous variables are completely removed from an experiment.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination • Constancy

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination • Constancy• Balancing

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination • Constancy• Balancing

• A control procedure that achieves group equality by distributing extraneous variables equally to all groups.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination • Constancy• Balancing• Counterbalancing

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination • Constancy• Balancing• Counterbalancing

• A procedure for controlling order effects by presenting

different treatment sequences.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

Basic Control TechniquesBasic Control Techniques

• Randomization• Elimination • Constancy• Balancing• Counterbalancing

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Within-Subject counterbalancing

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Within-Subject counterbalancing• Presentation of different treatment sequences to the same

participant.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Within-Subject counterbalancing• Presentation of different treatment sequences to the same

participant.• Within-Group counterbalancing

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:

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.

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.

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing

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.

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).

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Within-Subject counterbalancing• Within-Group counterbalancing• Complete counterbalancing• Incomplete counterbalancing

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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Sequence or Order Effects

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 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.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Carryover Effects

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Carryover Effects• The effects of one treatment persist or carry over and

influence responses to the next treatment.

The Psychologist as Detective, 4e by Smith/Davis © 2007 Pearson Education

CounterbalancingCounterbalancing

• Differential Carryover

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.