Worked example
One group of seedlings gets a new growth lamp in a warm room. The other gets the usual lamp in a cooler room. The warm group grows more. Because room temperature and lamp type move together, either could explain the result. Compare both lamps in each temperature setting and compare growth within each setting.
Contrast and common error
Identical pot size controls one condition, but it does not resolve room temperature. Adding ten seedlings in each original room provides more measurements while keeping the same design problem. Once both treatments are represented in each room, a difference that appears only in one room may tell you the lamp effect depends on conditions.
Transfer check
A club compares two brands of rain gauge. One is always under trees, the other is always on open ground. How can it compare brands fairly? Check: compare both brands side by side under trees and both side by side in the open. Read the gauges over the same rain event.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.
Sources & authorship
Original OpenExam explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
OpenStax Biology 2e 1.1 The Science of Biology