Chapter Five

Smith/Davis (c) 2005 Prentice Hall

Chapter Five

The Basics of Experimentation I: Variables and Control

PowerPoint Presentation created by Dr. Susan R. BurnsMorningside College


The Nature of Variables

Variable is an event or behavior that can assume at least two values.


Operationally Defining Variables

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 Variables

Independent Variables (IV’s) 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 IV’s

Physiological IV– Physiological IVs are used when the participants

in an experiment are subjected to conditions that alter or change their normal biological state.

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 characteristics

Aspects of the participant, such as age, sex or personality traits, that are treated as if they are IV’s.

They are not IV’s because they cannot be manipulated by the experimenter.


Psychological Detective

Although many researchers may treat participant characteristics as if they are IVs, they really are not. Why?


Extraneous Variables (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.


Confounder Influences Figure A demonstrates the

situation when there is no confounder present.

Figure B demonstrates the difference between two groups when a confounder is present and has moved the groups closer together.

Figure C demonstrates the difference between two groups when a confounder is present and has moved the groups farther apart.


Dependent Variables

Dependent Variable (DV) is a response or behavior that is measured. It is desired that changes in the DV are directly related to manipulation of the IV.


Selecting the DV

Because psychology often is defined as the science of behavior, the DV typically consists of some type of behavior or response. However, when the researcher administers the IV, it is likely that several responses will occur. Which one should the researcher select?

Assuming you have stated your hypothesis in general implication form, the “then” portion of the hypothesis will give you an idea of the general nature of your DV.


Recording or Measuring the DV

Correctness– Only correct responses are counted.

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. Degree or Amount Latency or Duration


Should You Record More 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.

If recording and recording another DV does not make a substantive contribution, then it is probably not worth the added time and trouble.

Previous research can be used as a guide concerning whether you should consider recording more than one DV.


Characteristics of a Good DV

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.


Nuisance Variables

Nuisance Variables are either characteristics of the participants or unintended influences of the experimental situation that make the effects of the IV more difficult to see or determine.– Nuisance variables increase the spread of scores

within a distribution; they do not cause a distribution to change its location.


Nuisance Variables

Figure A demonstrates the spread of scores within a group when a nuisance variable is not operating.

Figure B demonstrates the spread of scores within a group when a nuisance variable is operating.


Nuisance Variables with Two Groups

Figure A demonstrates two groups without nuisance variables operating.

Figure B demonstrates a comparison of two groups when a nuisance variable is operating.


Controlling Extraneous Variables

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 Techniques

Randomization– The most widely used technique, randomization is a

control technique that ensures that each participant has an equal chance of being assigned to any group in an experiment.


Psychological Detective

Even though it is the most widely used control procedure, randomization has one major drawback. What is it?



Elimination – A control technique whereby extraneous variables are

completely removed from an experiment. Constancy

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

Balancing– A control procedure that achieves group equality by

distributing extraneous variables equally to all groups.


Balanced extraneous variables


Balancing to Eliminate Confounding in Teaching Two Methods of Psychological Statistics



Counterbalancing– A procedure for controlling order effects by

presenting different treatment sequences.


Counterbalancing

Within-Subject counterbalancing– Attempts to control the sequence effect within each

participant. 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 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.


Counterbalancing


Counterbalancing

Complete counterbalancing– All possible treatment sequences are presented.– You can calculate the number of sequences by

using the formula n! (n factorial). Incomplete counterbalancing

– Only a portion of all possible sequences are presented.


Counterbalancing


Counterbalancing

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.


Counterbalancing


Counterbalancing

Carryover Effects– The effects of one

treatment persist or carry over and influence responses to the next treatment.


Counterbalancing

Differential Carryover

– The response to one treatment depends on which treatment was administered previously.


Internal Validity: Evaluating Your Experiment from the Inside

Internal Validity– The concept of internal validity revolves around

the question of whether your IV actually caused any change that you observe in your DV.

If you use adequate control techniques, your experiment should be free from confounding and you can, indeed, conclude that your IV caused the change in your DV.


Threats to Internal Validity

History– History refers to events that occur between the DV

measurements in a repeated measures design. Maturation

– Maturation refers to changes in participants that occur over time during an experiment.

Testing– Testing is a threat to internal validity that occurs because

measuring the DV causes a change in the DV. Campbell (1957) noted that if you take the same test more

than once, scores on the second test may vary systematically from the first scores simply because you took the test a second time.



Practice Effect– A practice effect is a beneficial effect on a DV measurement

caused by previous experience with the DV. Reactive Measures

– Reactive measures are DV measurements that actually change the DV being measured.

Many attitude questionnaires are reactive measures. If we ask you a number of questions about how you feel about people of different racial groups, or about women’s rights, or about the President’s job performance, you can probably figure out that your attitude is being measured.



Instrumentation (Instrument Decay)– Instrumentation is a threat to internal validity that occurs if the

equipment or human measuring the DV changes the measuring criterion over time.

Statistical Regression– Statistical regression occurs when low scorers improve or high

scorers fall on a second administration of a test due solely to statistical reasons.

Selection– If we choose participants in such a way that our groups are not

equal before the experiment, we cannot be certain that our IV caused any difference we observe after the experiment.



Mortality– Mortality can occur if experimental participants from different groups

drop out of the experiment at different rates. Interactions with Selection

– Interactions with selection can occur when the groups we have selected show differences on another variable (i.e., maturation, history, or instrumentation) that vary systematically by groups.

Diffusion or Imitation of Treatments– Diffusion or imitation of treatments can occur if participants in one

treatment group become familiar with the treatment of another group and copy that treatment.


Protecting Internal Validity

Two Approaches– You can (and should) implement the various

control procedures discussed in this chapter.– Use a standard procedure

Experimenters used standard procedures called experimental designs to help ensure internal validity.


Protecting Internal Validity

How Important is Internal Validity?– It is the most important property of any

experiment. If you do not concern yourself with the internal validity of

your experiment, you are wasting your time.

Chapter Five

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