Which of the following is crucial for experimental design?

The emphasis on testing only one variable while having a control group is vital for experimental design because it allows researchers to isolate the effect of that specific variable on the outcome of the experiment. This approach helps to establish a cause-and-effect relationship, making it clear whether the changes observed in the dependent variable are directly attributable to the manipulation of the independent variable.

A control group serves as a baseline for comparison, enabling scientists to see the effects of the experimental treatment in context. By controlling other variables and only testing one at a time, researchers can draw more reliable conclusions about the relationship between the independent and dependent variables. This methodical approach is fundamental to the scientific method and ensures that the results can be replicated and verified by others.

In contrast, using multiple variables can lead to confounding results, as it's difficult to determine which variable is responsible for any observed changes. Making assumptions and ignoring external factors can lead to unreliable data, as scientific inquiry relies on evidence-based conclusions. Therefore, focusing on a single variable and incorporating a control group is essential for designing robust experiments that yield valid insights.