While there are many possible ways to structure your evaluation, the following designs are the most common:

• One-Group, Post-Only Design

  IMPLEMENT PROGRAM

  ASSESS TARGET GROUP AFTER PROGRAM

  In this design, you would administer a post-test (e.g., survey) to your target group after participants have received an intervention. This design is common and relatively inexpensive, but it does not allow you to statistically measure changes from baseline (before the intervention), nor does it allow you to measure change in relation to other groups of people who did not take part in the intervention.

• One-Group, Pre- and Post-Program Design

  ASSESS TARGET GROUP BEFORE PROGRAM

  IMPLEMENT PROGRAM

  ASSESS TARGET GROUP AFTER PROGRAM

  In this design, you assess your target group both before and after program implementation. The strength of this design is that it provides baseline information that you can compare with your post-test data. For this design to work, you must administer the same instrument in the same way both before and after the program. This design can tell you whether your target group made improvements, but cannot assure you that your program was responsible for the outcomes. Alternative explanations are still possible (e.g., change occurred because participants matured over time).

• Pre- and Post-Program with Comparison Group Design

  BEFORE PROGRAM	IMPLEMENT PROGRAM	ASSESS TARGET GROUP AFTER PROGRAM
  ASSESS COMPARISON GROUP BEFORE PROGRAM		ASSESS COMPARISON GROUP AFTER PROGRAM

  In this design, you assess both your target group and another similar group that does not receive the program, both before and after implementation. The addition of a comparison group helps you determine whether or not your target group would have improved over time even if they had not experienced your program. The more similar the two groups are with respect to variables that may affect program outcomes (e.g., gender, race or ethnicity, socioeconomic status, and education), the more confident you can be that your program contributed to any detected changes. This design also helps control for test effects (e.g., improvements on the post-test due to participants' experience with the pre-test).

  However, this design does increase both the expense and complexity of your evaluation. It also leaves room for alternative explanations, since the program and comparison groups may differ in some important ways.

• Pre- and Post-Program with Control Group Design

  RANDOMLY ASSIGN PEOPLE FROM THE SAME TARGET POPULATION TO GROUP A OR GROUP B	TARGET GROUP A	ASSESS TARGET GROUP A	IMPLEMENT PROGRAM WITH TARGET GROUP A	ASSESS TARGET GROUP A
  	CONTROL GROUP B	ASSESS CONTROL GROUP B		ASSESS CONTROL GROUP B

  This design offers the greatest opportunity to attribute evaluation outcomes to program activities. By adding a control group to your pre- and post-program design, you introduce the element of random assignment. When you randomly assign individuals to either a target or a control group, all members of the target population have an equal chance of winding up in either group. This fact should ensure that members of the target and control groups are equivalent with respect to many key variables that could affect their performance on the pre- and post-tests. Of the four designs discussed here, this is the most complex and expensive to conduct, but it also provides the highest level of certainty that it was your program that caused any changes detected by your evaluation.

  Below are some important issues to consider when selecting a design:

  • Complex evaluation designs are most costly, but allow for greater confidence in a study's findings.

  • Complex evaluation designs are more difficult to implement, and so require higher levels of expertise in research methods and analysis.

  • Be prepared to encounter stakeholder resistance to the use of comparison or control groups, such as a parent wondering why his or her child will not receive a potentially beneficial intervention. Click here to see some suggestions for dealing with objections to random assignment.

  • No evaluation design is immune to threats to its validity; there is a long list of possible complications associated with any evaluation study. However, your evaluator will help you maximize the quality of your evaluation study.