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 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.


Independent VariablesIndependent Variables

• Independent Variables (IVs)


Independent VariablesIndependent Variables

• Independent Variables (IVs)• IVs are those variables that the experimenter purposely

manipulates.


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.


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.


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.


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


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

• Uncontrolled Variables that can cause unintended changes between groups.


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


• Confounding


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


• Confounding• A situation in which the results of an experiment can be

attributed to either the operation of an IV or an extraneous variable.


Dependent VariablesDependent Variables

• Dependent Variable (DV)


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.


Recording or Measuring Recording or Measuring the DVthe DV• Correctness


Recording or Measuring Recording or Measuring the DVthe DV• Correctness

• Only correct responses are counted.


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.



• 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.


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


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


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.


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.


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

hypothesis says it should measure.



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.



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.


Nuisance VariablesNuisance Variables

• Nuisance Variables



• Nuisance Variables• Unwanted variables that can cause the variability of scores

within groups to increase.



• 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.


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.


Basic Control TechniquesBasic Control Techniques

• Randomization



• Randomization• A control technique that ensures that each participant has

an equal chance of being assigned to any group in an experiment.



• Randomization• Elimination



• Randomization• Elimination

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



• Randomization• Elimination • Constancy



• Randomization• Elimination • Constancy

• A control technique by which an extraneous variable is reduced to a single value that is experienced by all participants.



• Randomization• Elimination • Constancy• Balancing



• Randomization• Elimination • Constancy• Balancing

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



• Randomization• Elimination • Constancy• Balancing• Counterbalancing




• A procedure for controlling order effects by presenting

different treatment sequences.


CounterbalancingCounterbalancing

• Within-Subject counterbalancing



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

participant.



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

participant.• Within-Group counterbalancing



• 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 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.



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




• All possible treatment sequences are presented.




• All possible treatment sequences are presented.• You can calculate the number of sequences by using the

formula n! (n factorial).



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



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

• Only a portion of all possible sequences are presented.



• Sequence or Order Effects



• Sequence or Order Effects• Sequence or order effects are produced by the

participant’s being exposed to the sequential presentation of the treatments.



• 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.



• Carryover Effects



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

influence responses to the next treatment.



• Differential Carryover



• Differential Carryover• The response to one treatment depends on which

treatment was administered previously.

the psychologist as detective, 4e by smith/davis © 2007 pearson education chapter six: the basics...

Documents

nature of variables

pearson education chapter

control slide

confounding slide

environmental iv

unintended changes

control chapter

basics of experimentation