Critical Aspect of Experimental Design in Photosynthesis Study

Researchers conducted an experiment to examine the effect of light intensity on photosynthetic rates in a species of aquatic plant. In this study, they placed several groups of plants in different lighting conditions: low light, medium light, and high light. Each group was exposed to these conditions for one week, and the rate of photosynthesis was measured by counting the number of oxygen bubbles released per minute. The goal was to determine how changes in light intensity affect the photosynthesis process.

According to the data collected, plants in high light conditions exhibited the highest rate of photosynthesis, while those in low light conditions showed the lowest rates. The researchers also recorded the temperature and nutrient levels, noting that these factors were consistent across all groups to ensure they did not influence the results.

Based on these findings, which aspect of the experimental design is most critical for ensuring the experiment's validity?