Experiments and variables

Extraneous variables

Extraneous variablesWe already know what an IV and a DV are.

IV: the thing that the investigator manipulatesDV: the thing that the investigator measures

An extraneous variable is anything that affects the DV but is not the IV.

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 variablesExtraneous 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 notDV: 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 variablesExtraneous 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• Hard to control• 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 1974Opportunity 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 experimentersDV: 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