In a research study of classrooms, when is it appropriate for investigators to collect data on reading comprehension?

The appropriate time for investigators to collect data on reading comprehension in a research study of classrooms is when varying the time of day reading is taught systematically. This choice is valid because it allows researchers to examine the impact of timing on students' reading comprehension. By systematically varying the time at which reading instruction occurs, researchers can observe whether different times of the day lead to different outcomes in comprehension skills. This approach can highlight the influence of external factors, such as students' attentiveness or energy levels throughout the day, on their reading performance.

In contrast, the collection of data following the surveying of teachers about teaching methods doesn't directly relate to the comprehension outcomes for students but rather focuses on the opinions and practices of teachers. A one-week observation period may not provide a sufficiently robust dataset to draw conclusions about reading comprehension, as learning and retention may require longer observation to reflect the true effects. Similarly, conducting assessments during a single semester might limit the understanding of comprehension, as it does not control for variations that may occur over multiple time points or different instructional environments. Thus, systematic variation in teaching timing stands out as the most effective way to gather meaningful data on reading comprehension.