Diffusion (or imitation) of treatments

Has the control group been contaminated in some way? Sometimes the treatment and control group participants are able to communicate with each other. The danger is that some aspects of the experimental stimulus (i.e., the intervention) are passed on from the treatment group to the control group (e.g., methods, materials, perspectives, etc.); that is, there is an exchange of information between the groups, which influences the behaviour of the control group. If this is the case, the control group may not be an actual control group. As a result, the scores on the dependent variable that is being measured are more likely to be similar between the groups. This is known as diffusion or imitation of treatments, resulting from an exchange of information between groups.

There are many factors that can lead to cross-contamination between the treatment and control group (e.g., an increased time between the pre-test and post-test; greater homogeneity between the sample/participants that may result in their interacting outside of the research process, such as their sharing similar social groups, their being geographical closeness between participants, and so forth).

Compensatory rivalry and internal validity

In experimental and quasi-experiment research designs where there is a treatment and control group, participants can sometimes become competitive when not included in the treatment group. As a result, they exert additional effort, which may improve the score on the dependent variable for the control group compared with normal conditions (i.e., compared with what is typical or expected for such a group). This can even happen when there are two treatment groups and no control group, so long as one of the groups is receiving a less attractive treatments/intervention. It is known as compensatory rivalry (or compensatory equalization of treatments). Imagine the following scenarios:

Study #5
The introduction of new work methods designed to increase employee performance

The John Henry effect concerns a steel worker, John Henry, who was aware that his output was going to be compared to a steam drill. As a result, he put in extra effort, but whilst managing to outperform the steam drill, the overexertion led to his death (Cook and Campbell, 1979, p. 55). Similarly, you could be examining how a new process in an organisation could reduce the requirement for manual labour. In comparing the treatment group using the new process with the control group, it is possible that the control group would work additionally hard for the period of the experiment in order to reduce the risk of losing their jobs. This could occur in a number of potential change situations.

In such cases, members of the control group try to compensate for the fact that they are missing out on the experiment treatment by working/trying/concentrating harder.

Demoralization and internal validity

Demoralization (or resentful demoralization; and in some cases, compensatory demoralization) can happen in experimental research when participants are assigned to the control group rather than the treatment group. This is not always the case, especially where there are no negative outcomes associated with control group membership. However, there are instances where being assigned to the control group can be viewed to be negative, leading to feelings of anger, demoralization, resentment, neglect, amongst other negative feelings.

Demoralization is a threat to internal validity when it:

Imagine the following study:

Study #6
The impact of team away days on employee motivation

The experiment:
A firm wants to understand the impact of team away days (i.e., the independent variable) on employee motivation (i.e., the dependent variable). Team away days, which are basically 'team building' exercises, ranging from paintballing to creative challenges, and even wine tasting, are expensive, so the firm wants to know if they are worthwhile. To test the impact of team away days of employee motivation, the firm randomly assigns employees into two groups. One group does not go on the team away day (i.e., the control group), whilst the other group goes on the away day (i.e., the treatment group). The pre-test consists of a questionnaire that employees from both groups take, which measures employees? level of motivation. When the employees that go on the team away day return, they re-take the employee motivation survey. This provides the post-test scores on the dependent variable (i.e., employee motivation). The aim is to compare the scores between the two groups on the dependent variable to see if there are any differences. However, irrespective of the results of the experiment, a threat to internal validity has entered the experiment: demoralization. After all, to avoid selection bias [see the section: Selection biases and internal validity], employees are randomly assigned to the treatment group and control group.

However, since employees are assigned to the treatment and control groups randomly rather than based on their performance; that is, employees are not selected as a reward (or bonus) because they have performed well in their jobs, this can cause a problem. It can cause a problem if employees in the control group see selection for the team away day (i.e., the treatment group) as being a privilege (or bonus), since they may resent not receiving the privilege/bonus (i.e., why this threat to internal validity is sometimes called resentful demoralization or compensatory demoralization).

Compensation and internal validity

Sometimes you will choose (or need) to compensate participants to encourage them to take part in your research. Broadly, such compensation can be viewed as either general compensation or control group compensation:

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