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-Correlation does not equal causation -necessary but not sufficient -Where there’s smoke there may be fire Experimental approach attempts to create conditions where third variable effects and issues of directionality can be eliminated. But what conditions would define a causal relationship? Mills’ Canons of Causality conditions that must be met if a relationship is causal • Where “A” is presumed cause and “B” is presumed effect. • Method of agreement • Method of disagreement • Method of concomitant variation • Method of multiple causality Method of Agreement • If “A” is present then so should be “B” Method of Disagreement • If “A” is not present then neither should be “B.” Method of concomitant Variation • If “A” is varied in intensity then “B” should show corresponding variation How do we set up experimental conditions to test Mills assumptions The Experimental approach • Systematic manipulation (variation) of the presumed causal variable • Systematic measurement of the presumed affected variable • Under controlled conditions Terminology • The presumed causal variable = the independent variable (IV) – The manipulated variable – The administered variable The presumed affected variable = the dependent variable (DV) – The measured variable Identifying variables by “Titles.” • “The effects of Stress on attention.” • The Effects of Alcohol on social interaction.” • Marijuana increases attention to drug cues.” • Cognitive processing is diminished by nicotine.” • The relationship of reported drug use to political affiliation.” Identifying Correlational studies vs Experimental studies. There should be clear inference of cause or effect in Experimental studies • • • • The effects of peanut butter on… Peanut butter increases… Brain size is increased… Typing speed is related to…. Manipulation of IV to test Mills assumptions • Minimally, the IV must have two Levels or conditions • Present/absent or High vs low • IV may have more than two levels– Concomitant variation The Experimental and Control Conditions • The control condition is both a point of comparison and • Helps control for possible unwanted effects of extraneous (confounding) variables. BASIC EXPERIMENTAL DESIGNS • Between group designs • Within group designs Between vs Within • Both use groups of subjects • Between groups test different groups at same time. Each group exposed to different level of IV • Within groups use same subjects tested repeatedly across different conditions of IV. Design decision making Fewer subjects More time The Experimental approach • Systematic manipulation (variation) of the presumed causal variable • Systematic measurement of the presumed affected variable • Under controlled conditions Extraneous variables Extraneous variables may become confounding variables Confounding variables: An extraneous variable that varies with the IV. Confounding variables • “ a variable that should be extraneous, but becomes systematically varied along with the IV. • Eg…”Effects of marijuana on memory.” • MJ accidentally administered only to experienced users but not to naïve users. Experimental Control • Procedures to minimize CVs • By elimination, constancy or specific testing of suspected confounds • The Control group or condition • = point of comparison and.. • A fundamental control procedure