Which axis is used to display the dependent variable in a three-dimensional plot where the axes correspond to two independent factors and one response?

• A. X-axis
• B. Y-axis
• C. Z-axis
• D. None

Answer: (C)

When you plot two independent factors in three dimensions, those factors sit on the two horizontal directions, and the dependent variable is shown by how high the surface rises above that base plane. The vertical dimension encodes the outcome, so the axis that represents the response is the Z-axis. For example, if temperature and time are the factors, you place temperature on one horizontal axis and time on the other, and the measured response (like yield) appears as height on the vertical axis. The horizontal axes carry the input variables, not the output, so they don’t convey the dependent variable. Therefore, the dependent variable is displayed along the vertical Z-axis.