Experiments and variables Extraneous variables Extraneous variables We

Experiments and variables Extraneous variables

Extraneous variables We already know what an IV and a An extraneous variable is anything DV are. that affects the DV but is not the IV. IV: the thing that the investigator manipulates DV: the thing that the investigator measures Examples will illustrate this

Extraneous variables IV: Sunlight or not DV: Growth of plant An extraneous variable is anything that affects the DV but is not the IV. Possible extraneous variables: • Species of plant (fast growing, used to dark conditions) • Size of seed (bigger seeds lead to stronger plants) • Receiving the same amount of water? (the outside plant may get rain etc) • Quality of the soil • Type of soil

Extraneous variables need to be controlled for accurate experiments • Same species of plant • Same size of seed • Same amount of water • Same soil • Same quality NOW WE ARE MORE CERTAIN THAT IT IS THE SUNLIGHT THAT IS AFFECTING THE PLANTS GROWTH An extraneous variable is anything that affect the DV but is not the IV. Possible extraneous variables: • Species of plant (fast growing, used to dark conditions) • Size of seed (bigger seeds lead to stronger plants) • Receiving the same amount of water? (the outside plant may get rain etc) • Quality of the soil • Type of soil

Extraneous variables IV: Chinese school or not DV: Exam success An extraneous variable is anything that affect the DV but is not the IV. Possible extraneous variables: • High academic level amongst one class • Studying outside of school • Prior knowledge of subjects such as Mandarin • Novelty factor of new foreign teachers • Quality of Chinese teachers (they taught in elite schools in China) • Difficulty understanding accents

Extraneous variables need to be controlled for accurate experiments • Randomly assign students to each group or do IQ test to make sure groups are same ability • Hard to control but could introduce rules • Hard to control but could give questionnaire beforehand • Hard to control The greater the control of extraneous variables the more certain we can be that the Chinese style teaching was superior An extraneous variable is anything that affect the DV but is not the IV. Possible extraneous variables: • High academic level amongst one class • Studying outside of school • Prior knowledge of subjects such as Mandarin • Novelty factor of new foreign teachers • Quality of Chinese teachers (they taught in elite schools in China) • Difficulty understanding accents

Bickman 1974 Opportunity sample, 153, Brooklyn NY. “pick this bag up for me” “this man is overparked at the meter but doesn’t have any change – give him a dime” “do you know you have to stand on the other side of the pole? The sign says ‘No standing’” IV: the uniform of the experimenters DV: the obedience rate of the passing pedestrians

Bickman 1974 – (on a street in Brooklyn, NY) Possible extraneous variables • “street variables” such as crowding, noise, weather • Possible variations in pedestrians on the street • Opportunity sampling • All the experimenters were male