In which type of research design can researchers examine differences across time and between groups?

The mixed design is the correct choice as it allows researchers to utilize both quantitative and qualitative methods, enabling a comprehensive examination of differences across time and between groups. This approach often combines elements from both experimental and non-experimental designs, facilitating a more nuanced understanding of research questions by leveraging the strengths of different methodologies.

In a mixed design, researchers can implement longitudinal elements to observe changes over time while also comparing distinct groups at specific points. This dual capacity is crucial in program evaluation, where understanding how interventions impact various populations over different periods is essential. The flexibility of mixed design thus supports a deeper analysis of complex research questions, accommodating diverse data sources and types to enrich the findings.

Other research designs may focus more narrowly on specific elements (such as experimental groups analyzing changes due to isolation of variables, or single-case designs that look closely at individual cases over time). While these have their merits, they do not typically provide the broad comparative and temporal analysis that a mixed design facilitates.