Which of the following is NOT a common control variable in experimental design?

In experimental design, control variables are factors that are kept constant to ensure that any changes in the dependent variable can be attributed solely to the independent variable being tested. While temperature, pH, and light intensity are all common control variables in experiments, population size is often not considered a control variable in the same sense because it can vary depending on the context of the study and is usually a part of the experimental conditions being assessed, rather than something that needs to be maintained constant.

Temperature, pH, and light intensity are typically controlled to prevent them from influencing the results of an experiment, allowing for a more accurate evaluation of the specific effects of the independent variable. For instance, in an experiment studying the growth of marine organisms, it is vital to keep the temperature and pH within the ranges that are normal for that species, as variations could lead to misleading conclusions about their growth dynamics. Similarly, light intensity would be controlled in experiments involving photosynthetic organisms to ensure consistent exposure.

Population size, on the other hand, is often purposefully varied to study its effects under different conditions, such as competition or resource availability. Therefore, it does not serve the same function as traditional control variables and is more aligned with the aspects being investigated in an experiment.