requires a separate, independent group of individuals for each treatment condition

are personal characteristics that can differ from one participant to another

when characteristics in one group are significantly different from the participants in the other group

when researcher has control over the assignment of individuals to groups, the groups must be

a random process is used to assign participants to a group

the group assignment process is limited to ensure predetermined characteristics for the separate groups

involves assigning individuals to groups so that a specific variable is balanced, or matched, across the groups

a measure of differences between scores for a group individuals who have all received the same treatment.

how to minimize variance within treatments

differences in attrition rates from one group to another and can threaten the internal validity of a between-subjects experiment

the spread of treatment from the experimental group to the control group, which tends to reduce the difference between the two conditions

an untreated group learns about the treatment being received by the other group, and demands the same or equal treatment

the untreated group works extra hard to show that they can perform just as well as the individuals receiving the special treatment

participants in an untreated group simply give up when they learn that the other group is receiving special treatment.

comparing only two groups of participants: the researcher manipulates one independent variable with only two levels

research design comparing more than two groups of participants representing more than two levels of the same factor