Final answer:
A time series design with a nonequivalent control group adds multiple before and after measurements and a control group to the time series to control for threats such as history effects, while a nonequivalent control group design lacks the repeated measures.
Step-by-step explanation:
A time series design with a nonequivalent control group and a nonequivalent control group design differ in terms of their structures and the methods they use to address threats to internal validity. A time series design with a nonequivalent control group incorporates repeated measurements over time both before and after the intervention with both an experimental group and a nonequivalent control group. This allows for the comparison of trends over time between the two groups. In contrast, a stand-alone nonequivalent control group design compares the experimental group with the control group at just one point before and one point after the intervention, without the repeated measurements over time that characterize a time series.
Adding a nonequivalent control group to a simple interrupted time series design helps control for specific threats to internal validity, such as history effects. History effects refer to events that occur between the pre-test and post-test that are not part of the experimental manipulation but that can influence the outcome. By comparing the changes over time in the experimental group with changes in a nonequivalent control group, researchers can better determine whether the observed effects in the experimental group are due to the intervention or to other external factors.