Experimental study designsThere are so many types of experimental design that not all of them can be considered within the scopeof this book. This section, therefore, is confined to describing those most commonly used in the socialsciences, the humanities, public health, marketing, education, epidemiology, social work, and so on.These designs have been categorised as:the after-only experimental design;the before-and-after experimental design;the control group design;the double-control design;the comparative design;the ‘matched control’ experimental design;the placebo design.FIGURE 8.8 The after-only designThe after-only experimental designIn an after-only design the researcher knows that a population is being, or has been, exposed to anintervention and wishes to study its impact on the population. In this design, information on baseline(pre-test or before observation) is usually ‘constructed’ on the basis of respondents’ recall of thesituation before the intervention, or from information available in existing records – secondary sources(Figure 8.8). The change in the dependent variable is measured by the difference between the ‘before’(baseline) and ‘after’ data sets. Technically, this is a very faulty design for measuring the impact of anintervention as there are no proper baseline data to compare the ‘after’ observation with. Therefore, oneof the major problems of this design is that the two sets of data are not strictly comparable. Forexample, some of the changes in the dependent variable may be attributable to the difference in the waythe two sets of data were compiled. Another problem with this design is that it measures total change,including change attributable to extraneous variables; hence, it cannot identify the net effect of anintervention. However, this design is widely used in impact assessment studies, as in real life manyprogrammes operate without the benefit of a planned evaluation at the programme planning stage(though this is fast changing) in which case it is just not possible to follow the sequence strictly –collection of baseline information, implementation of the programme and then programme evaluation.An evaluator therefore has no choice but to adopt this design.