Worked example
A team asks if a salt solution changes the time a dish of water takes to freeze. The treatment is the assigned solution. Time to freeze is the response. Compare the salt solution with plain water in identical dishes, with the same volume and freezer setting. Comparing times now addresses the question about dissolved salt.
Contrast and common error
An empty dish makes a poor comparison: it has no liquid to freeze. An unsalted dish on a colder shelf also changes temperature. A comparison may receive the same handling without the active ingredient. The untreated comparison and the fixed background conditions play different roles.
Transfer check
A researcher adds one drop of a nutrient solution to a yeast culture. What comparison tests the nutrient if it is dissolved in water? Check: use the same number of drops of water without the nutrient in an otherwise matched culture. Compare the same measured response after the same elapsed time.
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