This design met the criteria for the nonequivalent control group design. Pretest and posttests were conducted, which allowed for comparison of the relative gains in development between the children in the experimental and control groups. In addition, students, who conducted the teaching sessions with mothers, also observed and recorded how mothers interacted with their children.

The students observed that the mothers showed improved skill in stimulating their children in beneficial ways. Measured changes in development were greater among the experimental children in comparison to the control children. Boys showed greater improvement than girls, but this was because girls scored so high on the pretest that there was less room for further improvement.

Interrupted time series design

In a time series design, there is no control group to compare with the experimental group. There is just one group for which there are multiple measures for some variable before and after some event that could affect the variable being studied. This design is a variation on the trend study described in Chapter 5. A researcher would look for a situation where there are multiple measures for some variable and where some condition or event occurs that might influence that variable. The intervening condition might be a law, such as a large increase in the tax on cigarettes.   A researcher interested in the effects of this law on the sale of cigarettes could get data on tax revenues from the sale of cigarettes for six months preceding and following implementation of the law and then see if there was any change in sale revenues after the law went into effect.

The general pattern of observations for this design is:

0 0 0 0 X 0 0 0 0

where the measurements or points of data collection are represented by the 0s and where the X represents the event - in this case the increased tax on cigarettes.

Multiple measures before and after design

This design combines features from the nonequivalent control group design with the interrupted time series design. An experimental and control group are used and multiple measurements have to be available for the variable of interest both before and after some event occurs that could cause a change in the variable. This design can be applied in school, organizational, and other social settings. To illustrate, let's say we learned that a change was planned in the methods of teaching reading in a local elementary school. We also are given access to the reading grades for children from this school for some years before and following the change. We also obtain access to reading grades for children from one or several other elementary schools with student populations similar in all major respects to the children in the school using the new method.

With these conditions, we could design a quasi- experiment. We could compare the trend in reading grades prior to the change in the school that changed methods - the experimental school - with the trend for the same period in the control schools. In making this comparison, we would expect to see approximately the same trend in reading grades in both groups in the period before the new reading instruction was started. The trend might show little change from year to year or perhaps a small increase or decrease, but it should be about the same for each group. (If there is a large difference, we would have to abandon the experiment because the two groups are not equivalent at the outset of the experiment).

Then, we note when the change was made and see if the trend in reading grades is different between the two groups following the change. If the new teaching method was effective - as the school administrators and teachers hoped it would be - then the grades for the children in the experimental school should show a greater increase than those in the control group. If this in fact happened, then it would suggest that the new teaching methods contributed to an increase in reading grades. But because other factors cannot be ruled out, as a true experiment with randomized assignment would allow, we cannot say for certain that the new teaching method alone is responsible for the increase. On the other hand, if no difference was found between the grades of the two groups, we are left with a problem. Was the failure to find a difference because the new method was not that effective; was the lack of difference due to poor measurement of the children's reading skills; or were other factors involved? With this design, we would not be able to sort out the reasons for not finding the expected result.