You are trying to determine the effect of raising water temperature from 25.5°C to 27°C on the survival rate of zebrafish in a tank that contained 1000 fish. Water temperature is kept constant at 27°C for a period of 4 weeks. What is the independent variable?

• A. The temperature of the water in the tank
• B. The total number of fish in the tank at the beginning of the experiment
• C. The number of fish remaining at the end of the experiment
• D. The number of times the tank is cleaned during the 4 weeks of the experiment

Answer: (A)

Understanding how scientists determine cause and effect in experiments hinges on identifying what is deliberately changed versus what is observed. The key idea here is the variable that you alter to see its effect on an outcome. In this scenario, the factor you modify is the water temperature: you raise it from 25.5°C to 27°C and then keep it at 27°C for four weeks to watch how many zebrafish survive. Because temperature is what you control and vary to test its impact on survival, it serves as the independent variable. The outcome you measure—the number or proportion of fish that remain alive after the period—is the dependent variable, since it depends on the temperature change. The initial number of fish is a starting condition, and how often the tank is cleaned is a procedural detail that should be kept constant to avoid confounding the effect of temperature.