Experiments in social science

Seeking causal explanations

Causality Telecommunications managers, like social

scientists, would like to be able to make causal statements--even if they only point to partial causality

“X causes Y” Incomplete or imperfect causality

• Multiple causality• Partial causality (increased likelihood)• Necessary and sufficient conditions

Three conditions for establishing a causal relationship between two concepts

1. Covariation

2. Time order

3. Elimination of alternative explanations

Experiments

The experiment is a method where the researcher manipulates one variable (independent variable) and observes its effect on another (dependent variable) under controlled conditions

Experiments Example: A researcher may expose a group

of students to a movie with one ending and a second group to the same movie with a different ending (both under laboratory conditions), then measure their emotional response to the movie

Features of the experiment

Independent variable Dependent variable Subjects Control

Independent variable

The independent variable is the ‘cause’ or ‘causal variable’ in the hypothesis to be tested

The researcher manipulates the independent variable and subsequently measures subjects on the dependent variable

A factor in an experiment is an independent variable whose levels are set by the experimenter • http://www.stat.yale.edu/Courses/1997-98/101/expdes.htm

Independent variable

The levels of the factor that are introduced into the experiment are called the ‘treatments’

If a group is measured on the dependent variable but is not exposed to a non-zero treatment, it is called a ‘control group’ • Some consider this a zero-level treatment,

others say it is not a treatment

Independent variable

The factor in an experiment must be represented by at least two treatments—experimental and control treatments or two experimental treatments

Stronger experiments include multiple treatment levels

Examples: • exposure to a video game v. non-exposure • exposure to different executions of a creative

idea • having one half of a class use a website as part

of the course and the other half not use it• Exposing one group of subjects to one hour of

rap music, another to two hours, another to three, and another to four hours

Dependent variable (effect) The outcome of interest in the study All groups are measured on the DV Represents the ‘effect’ Examples:

• liking for a show or a television personality• recall of information from a website • time spent at a website• purchase of cell phones • political activity

Subjects

People who are assigned to experimental conditions and measured on the dependent variable

They should be members of the target market/audience

They are often a ‘convenience sample’—especially students in lower-level psychology classes• Nonrandom sampling

Control

Any procedures used to see that the only thing that varies for the subject groups is the independent variable• The goal is to isolate impact of the independent

variable on the dependent variable

Forms of control

Control of the environment• Minimize distraction from noise, lights, action

other than exposure to the independent variable• Keep the environment the same across groups

Random assignment of subjects to treatments (randomization)• Trying to make subject groups equivalent in

terms of personalities, experiences, demographics

Forms of control (continued)

Identical presentation of treatment and measure of dependent variable among groups• Placebo• Timing

Statistical control• Statistically remove the influence of

demographics, prior experience, etc.• Requires measuring all variables you will use

as controls

Forms of control (cont’d)

Experimental Design• Blocking

Basic experimental design

R X O

R O

Goal

The ultimate goal of the research is to determine whether the independent variable ‘causes’ the dependent variable under specified conditions

This sounds simpler than it is

Strengths of the experimental method Strong claims to ‘internal’ validity Strongest ability to infer causality Relatively low cost Straightforward interpretation Scientific aura

Weaknesses of the experimental method Troubles with ‘external’ validity

• Artificial setting• Demand characteristics• Hawthorne effect• Forced exposure• Multiple influences controlled

• Non-representative samples• Measures divorced from concepts

• Kicking a Bobo doll