Experiment Basics: Variables Psych 231: Research Methods in Psychology

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Print out the Class experiment exercise (from the Lab web page) and bring it to labs this week Turn in your data sheets (pass to front) I will analyze the data and the results will be discussed in labs this week Turn in your consent forms Class Experiment

So you want to do an experiment? You’ve got your theory. What behavior you want to examine Identified what things (variables) you think affects that behavior So you want to do an experiment?

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 Theory & Hypotheses: What you try to memorize & how you try to memorize it will impact memory performance. Concrete vs. abstract things Depth of processing Recall of things from memory So you want to do an experiment?

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 choices So you want to do an experiment?

Constants 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 situation Variables 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 … Constants vs. Variables

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 manipulated Conceptual How we memorize (Levels of processing) Kinds of things Memory Operational Has an ‘a’ ‘Related to ISU’ Words rated as abstract or concrete Memory test Variables

Many kinds of Variables Independent variables (explanatory) Dependent variables (response) Extraneous variables Control variables Random variables Confound variables Correlational designs have similar functions Many kinds of Variables

Many kinds of Variables Independent variables (explanatory) Dependent variables (response) Extraneous variables Control variables Random variables Confound variables Many kinds of Variables

Independent 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 experiment Independent Variables

Independent Variables Condition 1 Condition 2 Factor A 1 factor, 2 levels Cond 1 Factor A Cond 3 Cond 2 1 factor, 3 levels Cond 1 Factor B Cond 3 Cond 2 Factor A Cond 4 Cond 6 Cond 5 2 factors, 2 x 3 levels Independent Variables

Manipulating 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-experiment Abstract vs. concrete words Has an “a” vs. “ISU related” Manipulating your independent variable

Choosing 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 range Choosing your independent variable

Identifying 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 effects Identifying potential problems

Demand 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: Having participants see lists of words and pictures and then later testing to see if pictures or words are remembered better Biased or leading questions: Don’t you think it’s bad to murder unborn children? Demand characteristics

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 Experimenter Bias

Reactivity 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-cooperative Cooperative “You seem like a nice person: I’ll help you get the right results” Defensive “I don’t want to look stupid/evil. I’ll do what a smart/good person is expected to do (rather than what I normally would do).” Noncooperative “This experiment is annoying. Let me screw up the results.” Reactivity

Floor 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. Floor 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 middle level performance in your DV Ceiling effects

Range effects Floor: 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: 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 middle level performance in your DV Range effects