Experiment Basics: VariablesExam 1So you want to do an experiment?Slide 4Slide 5Slide 6Constants vs. VariablesVariablesMany kinds of VariablesSlide 10Independent VariablesSlide 12Manipulating your independent variableChoosing your independent variableIdentifying potential problemsDemand characteristicsExperimenter BiasFloor effectsCeiling effectsSlide 21Dependent VariablesChoosing your dependent variableMeasuring your dependent variablesSlide 25Slide 26Scales of measurementSlide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Errors in measurementReliability & ValidityReliabilitySlide 44Slide 45Slide 46ValiditySlide 48Slide 49Construct ValidityFace ValidityExternal ValiditySlide 53Internal ValidityThreats to internal validitySlide 56Control your extraneous variable(s)Extraneous VariablesSlide 59Confound VariablesSlide 61“Debugging your study”Experiment Basics: VariablesPsych 231: Research Methods in PsychologyExam 1Mean: 81.7%Range: 60-96% ResultsIf you want to go over your exam set up a time to see meSo you want to do an experiment?From the creators of American Idol and So You Think You can DanceMon & Wed, 1c on FOXSo you want to do an experiment?What behavior you want to examineIdentified what things (variables) you think affects that behaviorYou’ve got your theory.So you want to do an experiment?You’ve got your theory. Next you need to derive predictions from the theory. These should be stated as hypotheses.In terms of conceptual variables or constructs•Conceptual variables are abstract theoretical entities Consider our class experimentHypotheses: •What you try to memorize & how you try to memorize it will impact memory performance.So you want to do an experiment?You’ve got your theory. Next you need to derive predictions from the theory. Now you need to design the experiment.You need to operationalize your variables in terms of how they will be:•Manipulated•Measured•ControlledBe aware of the underlying assumptions connecting your constructs to your operational variables•Be prepared to justify all of your choicesConstants vs. Variables Characteristics of the psychological situationsConstants: have the same value for all individuals in the situationVariables: have potentially different values for each individual in the situationVariables in our experiment:• Levels of processing• Type of words• Memory performance• time for recall• kind of filler task given• pacing of reading the words on the list• …VariablesConceptual vs. OperationalConceptual variables (constructs) are abstract theoretical entitiesOperational variables are defined in terms within the experiment. They are concrete so that they can be measured or manipulatedConceptualHow we memorize(Levels of processing)Kinds of thingsMemoryOperationalHas an ‘a’‘Related to ISU’Words rated as abstract or concreteMemory testMany kinds of VariablesIndependent variables (explanatory)Dependent variables (response)Extraneous variablesControl variablesRandom variables Confound variablesCorrelational designs have similar functionsMany kinds of VariablesIndependent variables (explanatory)Dependent variables (response)Extraneous variablesControl variablesRandom variables Confound variablesIndependent VariablesThe variables that are manipulated by the experimenter (sometimes called factors)Each IV must have at least two levelsRemember the point of an experiment is comparisonCombination of all the levels of all of the IVs results in the different conditions in an experimentIndependent Variables1 factor, 2 levelsCondition 1 Condition 2Factor A1 factor, 3 levels2 factors, 2 x 3 levelsCond 1Factor ACond 3Cond 2Cond 1Factor BCond 3Cond 2Factor ACond 4 Cond 6Cond 5Manipulating your independent variableMethods of manipulationStraightforward•Stimulus manipulation - different conditions use different stimuli•Instructional manipulation – different groups are given different instructionsStaged•Event manipulation – manipulate characteristics of the context, setting, etc.Subject (Participant)– there are (pre-existing mostly) differences between the subjects in the different conditions •leads to a quasi-experimentHas an “a” vs. “ISU related”Abstract vs. concrete wordsChoosing your independent variableChoosing the right levels of your independent variableReview the literatureDo a pilot experimentConsider the costs, your resources, your limitations Be realistic•Pick levels found in the “real world”Pay attention to the range of the levels•Pick a large enough range to show the effect•Aim for the middle of the rangeIdentifying potential problemsThese are things that you want to try to avoid by careful selection of the levels of your IV (may be issues for your DV as well).Demand characteristicsExperimenter biasReactivityFloor and ceiling effectsDemand characteristicsCharacteristics of the study that may give away the purpose of the experimentMay influence how the participants behave in the studyExamples:•Experiment title: The effects of horror movies on mood•Obvious manipulation: Ten psychology students looking straight up•Biased or leading questions: Don’t you think it’s bad to murder unborn children?Experimenter BiasExperimenter bias (expectancy effects)The experimenter may influence the results (intentionally and unintentionally)•E.g., Clever Hans One solution is to keep the experimenter (as well as the participants) “blind” as to what conditions are being testedKnowing that you are being measuredJust being in an experimental setting, people don’t always respond the way that they “normally” would. •Cooperative•Defensive•Non-cooperativeReactivityFloor effects A value below which a response cannot be madeAs a result the effects of your IV (if there are indeed any) can’t be seen.Imagine a task that is so difficult, that none of your participants can do it.Ceiling effectsWhen the dependent variable reaches a level that cannot be exceededSo while there may be an effect of the IV, that effect can’t be seen because everybody has “maxed out”Imagine a task that is so easy, that everybody scores a 100%To avoid floor and ceiling effects you want to pick levels of your IV that result in