Gaining Control Internal Validity

What is an experiment? Comparison – Manipulation (Independent Variable) Measurement (Dependent Variable) Control

Selection Bias Occurs when differences exist between the kinds of individuals in comparison groups self-selected – Typically occurs when groups are self-selected

The Pepsi Challenge COKE Label L PEPSI Label S Dislike Like Is the difference due to the soft drink or the label that is attached to the soft drink?

The John Snow/Cholera Study Uses Water Co. A Chose Water Co. A Uses Water Co. B Chose Water Co. B NO Cholera Cholera Is the difference due to the drinking water or some other variable that these groups differ on?

Internal Validity Refers to the degree to which a study establishes a cause- and-effect relationship – The primary advantage of the experimental research strategy

Experimental Control Extraneous Variable Extraneous Variable Any variable other than IV that might influence the DV Confounding Variable Confounding Variable When two variables are linked together in a way that makes it difficult to sort out their specific effects

Threats To Internal Validity Anything that reduces the unambiguous cause-and-effect relationship Some of these include… – Co-occurrence of Systematic Changes – Selection Bias

The Pepsi Challenge COKE Label L PEPSI Label S Dislike Like Is the difference due to the soft drink or the label that is attached to the soft drink?

Holding Constant COKE Label L PEPSI Dislike Like We can see why there is a difference between these conditions when holding one variable constant.

Threats To Internal Validity Anything that reduces the unambiguous cause-and-effect relationship – Assignment Bias Some of these include… – Co-occurrence of Systematic Changes – Selection Bias

Assignment Bias Occurs when the process used to assign different participants to different treatments produce groups of individuals with noticeably different characteristics

Matching CD 1 CD 2 MM MM FM FM FM FM FM FF CD 1 CD 2 MM MM MM MM FF FF FF FF NOT Matched Variables Matched Variable

Random Assignment Participants have an equal chance of being in any condition in the experiment

Threats To Internal Validity Anything that reduces the unambiguous cause-and-effect relationship Some of these include… – Co-occurrence of Systematic Changes – Selection Bias – Assignment Bias – Experimenter Demand

Experimenter Demand Refers to the cues from the context and/or experimenter that guide behavior of participants

Controlling the Demand Placebo Control + Double-Blind Procedure Both participant & experimenter are unaware of what treatment is being administered