 Lab #1  Intro to Experiments and Experimental Design 2

 Experiment is a study in which a treatment is introduced  The type of experiment (such as a true, randomized experiment) depends on several factors– the elements of design dictate the type of experiment you have  Non-Experiments include responses from natural groups (e.g., most surveys are non- experiments)  Just because you manipulate something, it does not make it an experiment. 3

 Because we want to test specific, directional hypotheses – not fishing expeditions looking for relationships.  Because we want to isolate cause from effect.  Because we want tight control of conditions to rule out spurious relationships.  Because we need carefully-constructed designs that can be replicated to verify findings. 4

Tuskegee Syphilis Study Milgrim Obedience Study Stanford Prison Experiment 5

 Recruitment for online surveys via extrinsic and intrinsic motivation  Studying development of trust in different types of social interactions 6 No money Gift card $15 $60

 (1) Independent Variables must be manipulated (usually by experimenter, sometimes by context)  (2) Participants must be assigned randomly to various conditions or groups 7 Courtesy

 Independent variable manipulated by experimenter:  Whether there was any peer pressure to give incorrect answer or not.  Random Assignment to Control group or Treatment Group:  In treatment group, confederates answered incorrectly and the subject answered last  In control, no pressure to give incorrect answer 8

Active independent variable(s):  The I.V. is “given” to the participants, usually for some specified time period. It is often manipulated and controlled by the investigator. Attribute independent variable(s):  A predictive, defining characteristic of individuals. Cannot be manipulated. 9

 Random Sample  System for choosing participants from a population  Generally, larger sampling population leads to better generalizability.  Random Assignment  Method for assigning participants randomly to experiment conditions 10

 If we cannot randomly assign subjects to treatment and control groups, then it is a quasi-experiment. The independent variable is active.  If we cannot randomly assign subjects to groups because the groups contain the attribute of interest, and if we give all groups the same treatment, then it is an associational non-experiment. The independent variable is not active.  True experiments protect against both time and group threats to internal validity by randomly assigning subjects to treatment and control groups. The treatment (independent variable) is active. 11

 Pre-test: allows us to check group equivalence before the intervention X is introduced.  Experimental manipulation/Treatment: An independent variable (X) that the experimenter manipulates.  Post-test: allows us to check group equivalence after intervention X has been introduced. 12 Hypothesis  Random Assignment  Measure D.V.  Treatment  Measure D.V. (pre-test)(post-test)

13 Common types of true experiments R O O XO O R X O O Pretest-Posttest Control Post-only Control R O O XO O (1) (2) XO(3) O(4) Solomon 4-group

 Question: Do individuals in the US and Japan make different choices about ‘unique’ versus ‘less unique’ items when given a choice? 14

 Independent Variables  Cultural difference: Japanese students compared to US students  Pen layout (3 pens of one color, 1 pen of another color) ▪ Everyone received the same treatment  Assignment  Subjects were not randomly assigned because they already fell into one of the two societies.  Dependent Variable:  Would they choose the ‘common’ pen or the ‘unique’ one? 15

 Question: Do increased risk-taking behaviors over time increase interpersonal trust? 16

 Independent Variable  Experiment Condition (2 conditions):  Player Cannot control amount of money to entrust to partner  Player Can control amount to entrust to partner  Assignment  Random assignment of participants to one of the 2 conditions.  Dependent Variable  Cooperation rate (i.e., whether coins were returned by the partner or not) 17