Experiment Basics: Variables Psych 231: Research Methods in Psychology Lab reminders Print out the Class experiment exercise (from the Lab web page) and bring it to labs this week Group project introduction sections due this week Many kinds of Variables Independent variables (explanatory) Dependent variables (response) Extraneous variables Control variables Random variables Confound variables 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). Floor and ceiling effects Demand characteristics Experimenter bias Reactivity 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 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? Experimenter Bias Experimenter bias (expectancy effects) The experimenter may influence the results (intentionally and unintentionally) ., 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 Jus