Which of the following issues does the posttest-only control group design help control?

The posttest-only control group design is particularly useful for controlling testing effects, which can occur when participants are exposed to a pretest before the actual measurements are taken. In a traditional pretest-posttest design, participants may alter their responses or behavior due to having completed an initial test, which can influence the results of the study. By utilizing a posttest-only design, researchers eliminate the pretest phase altogether, thereby preventing any influence that prior exposure to tests might have on the outcome. This design focuses more directly on the outcomes as they are measured after treatment rather than any possible biases introduced by prior knowledge or experience with the testing process.

The other options, such as sampling bias, attrition effects, and measurement error, address different aspects of research methodology and participant behavior. While they are important issues, they are not specifically mitigated by adopting a posttest-only control group design. This design is particularly targeted at minimizing the influence of testing effects on the integrity of the study's findings.