Random Assignment
An important procedure in conducting research is random assignment.  What is random assignment?  Below is a definition of random assignment:

Random Assignment Definition

Random assignment is a procedure in conducting experiments in which each participant has the same probability of being assigned to a particular condition of the experiment.

Random Assignment Example

Imagine that a researcher was interested in the influence of music on job motivation.  Some participants would be assigned to a condition in which they would hear music while working.  Other participants would be assigned to a condition in which they would not hear music while working.  A coin could be flipped to determine which condition each participant is assigned to.  This would reflect a random process.

The Importance of Random Assignment

Why would a researcher randomly assign participants to conditions?  Random assignment can allow researchers to make causal conclusions about the effect of one variable on another variable.  Random assignment allows the researchers to rule out explanations based on personality and individual difference variables.  Random assignment makes it equally likely that a participant with a certain characteristic is in each condition of the experiment.