In determining the effect of a new treatment, what should the language abilities of treated children be compared to?

The appropriate comparison for determining the effect of a new treatment on the language abilities of treated children is to a control group of children who did not receive treatment. This comparison is crucial because it allows researchers to evaluate the treatment's impact more effectively, isolating the variable of interest (the treatment itself) from other factors that might influence outcomes, such as developmental changes that occur naturally over time.

By comparing the language abilities of treated children to those of untreated children, researchers can assess whether any observed improvements are indeed due to the treatment rather than developmental progress or environmental influences that would affect both groups equally. In this context, a control group serves as a benchmark, helping to establish a causal relationship between the treatment and language development.

Other options, such as comparing to younger children or children receiving a different treatment, do not provide a clear evaluation of the treatment’s effectiveness because they may introduce confounding variables and differences in baseline abilities.