The basics of experiments

8/12/2019 The basics of experiments

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Independent and Dependent Variables

Operational Definitions

Evaluating Operational Definitions

Planning the Method Section


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What is an independent variable?


An independent variable (IV) is the variable(antecedent condition) an experimenterintentionally manipulates.

Levelsof an independent variable are thevalues of the IV created by the experimenter.

An experiment requires at least two levels.


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Explain confounding.


An experiment is confounded when the valueof an extraneous variable systematicallychanges along with the independent variable.

For example, we could confound our experimentif we ran experimental subjects in the morningand control subjects at night.


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What is a dependent variable?


A dependent variable is the outcome measurethe experimenter uses to assess the change inbehavior produced by the independent variable.

The dependent variable depends on the valueof the independent variable.


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What is an operational definition?


An operat ional def ini t ion specifies the exactmeaning of a variable in an experiment bydefining it in terms of observable operations,

procedures, and measurements.


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What is an operational definition?


An experimental operat ional def ini t ionspecifies the exact procedure for creatingvalues of the independent variable.

A measured operat ional def in i t ionspecifiesthe exact procedure for measuring thedependent variable.


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What are the properties of a nominal scale?


A nom inal scale assigns items to two or moredistinct categories that can be named using ashared feature, but does not measure their

magnitude.

Example: you can sort canines into friendly andshy categories.


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What are the properties of an interval scale?


An interval scale measures the magnitudeof the dependent variable using equal intervalsbetween values with no absolute zero point.

Example: degrees Celsius or Fahrenheit andSarnoff and Zimbardos (1961) 0-100 scale.


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What are the properties of a ratio scale?


A rat io scale measures the magnitude of thedependent variable using equal intervalsbetween values and an absolute zero.

This scale allows us to state that 2 meters aretwice as long as 1 meter.

Example: distance in meters or time in seconds.


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What does reliability mean?


Reliabil i tyrefers to the consistency ofexperimental operational definitions andmeasured operational definitions.

Example: a reliable bathroom scale shoulddisplay the same weight if you measureyourself three times in the same minute.


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Explain interrater reliability.


In terrater rel iab i l i ty is the degree to whichobservers agree in their measurementof the behavior.

Example: the degree to which three observersagree when scoring the same personal essaysfor optimism.


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Explain test-retest reliability.


Test-retest reliab il i ty means the degree towhich a person's scores are consistent acrosstwo or more administrations of a measurement

procedure.Example: highly correlated scores on theWechsler Adult Intelligence Scale-Revisedwhen it is administered twice, 2 weeks apart.


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Explain interitem reliability.


In teri tem rel iabi l i ty measures the degree towhich different parts of an instrument(questionnaire or test) that are designed to

measure the same variable achieve consistentresults.


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What does validity mean?


Validitymeans the operational definitionaccurately manipulates the independentvariable or measures the dependent variable.


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What is face validity?


Face validity is the degree to which the validityof a manipulation or measurement technique isself-evident. This is the least stringent form of

validity.For example, using a ruler to measure pupil size.


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What is content validity?


Content val id i ty means how accurately ameasurement procedure samples the contentof the dependent variable.

Example: an exam over chapters 1-4 that onlycontains questions about chapter 2 has poorcontent validity.


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What is predictive validity?


Predic t ive val id i ty means how accurately ameasurement procedure predicts futureperformance.

Example: the ACT has predictive validity if thesescores are significantly correlated with collegeGPA.


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What is construct validity?


Construct valid i ty is how accurately anoperational definition represents a construct.

Example: a construct of abusive parents mightinclude their perception of their neighbors asunfriendly.


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Explain internal validity.


In ternal val idity is the degree to which changesin the dependent variable across treatmentconditions were due to the independent variable.

Internal validity establishes a cause-and-effectrelationship between the independent anddependent variables.


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Explain the problem of confounding.


Confoundingoccurs when an extraneousvariable systematically changes across theexperimental conditions.

Example: a study comparing the effects ofmeditation and prayer on blood pressure wouldbe confounded if one group exercised more.


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Explain history threat.


History threat occurs when an event outsidethe experiment threatens internal validity bychanging the dependent variable.

Example: subjects in group A were weighedbefore lunch while those in group B wereweighed after lunch.


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Explain maturation threat.


Matu rat ion th reat is produced when physicalor psychological changes in the subject threateninternal validity by changing the DV.

Example: boredom may increase subject errorson a proofing task (DV).


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Explain testing threat.


Testing threat occurs when prior exposure to ameasurement procedure affects performance onthis measure during the experiment.

Example: experimental subjects used a bloodpressure cuff daily, while control subjects onlyused one during a pretest measurement.


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Explain instrumentation threat.


Ins trum entat ion threat is when changes inthe measurement instrument or measuringprocedure threatens internal validity.

Example: if reaction time measurementsbecame less accurate during the experimentalthan the control conditions.


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Explain statistical regression threat.


Stat ist ical regress ion threat occurs whensubjects are assigned to conditions on the basisof extreme scores, the measurement procedure

is not completely reliable, and subjects areretested using the same procedure to measurechange on the dependent variable.


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Explain selection threat.


Select ion th reat occurs when individualdifferences are not balanced across treatmentconditions by the assignment procedure.

Example: despite random assignment, subjectsin the experimental group were more extrovertedthan those in the control group.


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Explain subject mortality threat.


Subject mortal i ty threat occurs when subjectsdrop out of experimental conditions at differentrates.

Example: even if subjects in each group startedout with comparable anxiety scores, drop outcould produce differences on this variable.


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Explain selection interactions.


Select ion in teract ions occur when a selectionthreat combines with at least one other threat(history, maturation, statistical regression,

subject mortality, or testing).


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What is the purpose of the Method section of an

APA report?


The Method sect ion of an APA research reportdescribes the Participants, Apparatus orMaterials, and Procedure of the experiment.

This section provides the reader with sufficientdetail (who, what, when, and how) to exactlyreplicate your study.


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When is an Apparatus section needed?


An Apparatus sect ion of an APA researchreport is appropriate when the equipment usedin a study was unique or specialized, or when we

need to explain the capabilities of more commonequipment so that the reader can better evaluateor replicate the experiment.

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The basics of experiments