In a controlled experiment testing light color on seed germination, which item would be a controlled variable?

• A. Soil moisture kept constant across all groups.
• B. Germination rate as the outcome.
• C. Different seed varieties.
• D. Different light colors.

Answer: (A)

In experiments, a controlled variable is a condition kept the same across all groups so it can’t influence the outcome. Here, the aim is to see how light color affects seed germination, so the researcher changes the light color (independent variable) and measures germination rate (the outcome). To ensure any differences in germination come from the light color and not other factors, you keep other conditions constant. Soil moisture is one such factor that can affect germination, so keeping it the same across all groups makes it a controlled variable.

The germination rate is the outcome being measured, not something kept constant. Different seed varieties would introduce genetic differences that could influence germination, so they wouldn’t be kept constant. Different light colors are the variable being tested, not controlled.