Experimental Design · Lesson 2

Control groups and why they matter

A control is a trial or group that gets no treatment, or the standard condition, so the treated groups have a baseline to be measured against. Without it you can see that coated nails rusted 21 mg, but you cannot tell whether the coating helped, because you don’t know how much an uncoated nail would have rusted under the same conditions.

A good control matches the treated groups in every way except the treatment: same nails, same liquid, same temperature, same time. Then the difference between a treated group and the control can only come from the treatment. That is what the control rules out: rust (or growth, or heating) that would have happened anyway.

Pick the control by its setup, never by its result. The group with the smallest or largest number is not the control unless it is also the untreated one. And to measure an effect, subtract: difference between the treated result and the matching control result (for a reduction, control minus treated), using the control that matches.

Coated Steel Nails

Steel rusts when the iron in it reacts with oxygen and water, and dissolved salt can speed the reaction. Students tested whether coatings protect steel nails from rust.

Students cleaned 25 identical steel nails and divided them into 5 groups of 5 nails. The nails in Groups Q, R and S were each dipped in a coating; the nails in Groups P and T were left uncoated. Each nail was placed in its own open jar with 100 mL of liquid: 3% salt water for Groups P, Q, R and S, and distilled water for Group T. All jars sat at 22 °C for 14 days. The students then scraped the rust off each nail, weighed it, and averaged the 5 masses for each group (see Table 1).

Table 1
GroupCoatingLiquidAverage rust (mg)
PNoneSalt water48
QWaxSalt water21
RVegetable oilSalt water30
SPaintSalt water9
TNoneDistilled water14
Table 1 lists each group’s coating, liquid and average mass of rust after 14 days: Group P (no coating, salt water) 48 mg, Q (wax, salt water) 21 mg, R (vegetable oil, salt water) 30 mg, S (paint, salt water) 9 mg, and T (no coating, distilled water) 14 mg.

Worked example Easy

Show the whole passage

Coated Steel Nails

Steel rusts when the iron in it reacts with oxygen and water, and dissolved salt can speed the reaction. Students tested whether coatings protect steel nails from rust.

Students cleaned 25 identical steel nails and divided them into 5 groups of 5 nails. The nails in Groups Q, R and S were each dipped in a coating; the nails in Groups P and T were left uncoated. Each nail was placed in its own open jar with 100 mL of liquid: 3% salt water for Groups P, Q, R and S, and distilled water for Group T. All jars sat at 22 °C for 14 days. The students then scraped the rust off each nail, weighed it, and averaged the 5 masses for each group (see Table 1).

Table 1
GroupCoatingLiquidAverage rust (mg)
PNoneSalt water48
QWaxSalt water21
RVegetable oilSalt water30
SPaintSalt water9
TNoneDistilled water14
Table 1 lists each group’s coating, liquid and average mass of rust after 14 days: Group P (no coating, salt water) 48 mg, Q (wax, salt water) 21 mg, R (vegetable oil, salt water) 30 mg, S (paint, salt water) 9 mg, and T (no coating, distilled water) 14 mg.

Which group served as the control for judging how well the coatings protect nails in salt water?

  1. Group P, uncoated nails in salt waterAnswer
  2. Group S, the nails with the least rust
  3. Group T, uncoated nails in distilled water
  4. Group R, nails coated with vegetable oil

How to solve it

  1. The coated groups (Q, R and S) all sat in salt water. Their control must match them in everything but the coating: uncoated, in salt water.
  2. In Table 1, Group P is the only uncoated group in salt water. Its 48 mg of rust is the baseline each coating is compared with.
  3. Group T is uncoated but sat in distilled water, so it differs from the coated groups in two ways. The answer is A.

Why each choice is right or wrong

  • A. Correct. Group P had no coating and sat in 3% salt water, matching Groups Q, R and S in every way except the coating. Its 48 mg of rust (Table 1) is the baseline.
  • B. Incorrect. Group S (9 mg) had the least rust, but a control is chosen by its setup, not its result. Group S was painted, so it received a treatment.
  • C. Incorrect. Group T had no coating, but it sat in distilled water while every coated group sat in salt water, so it differs from them in two ways.
  • D. Incorrect. Group R was coated with vegetable oil, so it is one of the treated groups being tested, not the baseline.

Worked example Medium

Show the whole passage

Coated Steel Nails

Steel rusts when the iron in it reacts with oxygen and water, and dissolved salt can speed the reaction. Students tested whether coatings protect steel nails from rust.

Students cleaned 25 identical steel nails and divided them into 5 groups of 5 nails. The nails in Groups Q, R and S were each dipped in a coating; the nails in Groups P and T were left uncoated. Each nail was placed in its own open jar with 100 mL of liquid: 3% salt water for Groups P, Q, R and S, and distilled water for Group T. All jars sat at 22 °C for 14 days. The students then scraped the rust off each nail, weighed it, and averaged the 5 masses for each group (see Table 1).

Table 1
GroupCoatingLiquidAverage rust (mg)
PNoneSalt water48
QWaxSalt water21
RVegetable oilSalt water30
SPaintSalt water9
TNoneDistilled water14
Table 1 lists each group’s coating, liquid and average mass of rust after 14 days: Group P (no coating, salt water) 48 mg, Q (wax, salt water) 21 mg, R (vegetable oil, salt water) 30 mg, S (paint, salt water) 9 mg, and T (no coating, distilled water) 14 mg.

Compared with the control, by how much did the wax coating reduce the average mass of rust?

  1. 7 mg
  2. 21 mg
  3. 27 mgAnswer
  4. 39 mg

How to solve it

  1. Find the control: Group P, uncoated in salt water, 48 mg (Table 1).
  2. Find the wax group: Group Q, also in salt water, 21 mg.
  3. Subtract: 48 − 21 = 27 mg less rust, choice C. Using Group T (14 mg) instead would mix in the change of liquid.

Why each choice is right or wrong

  • A. Incorrect. 7 mg is 21 − 14, which compares Group Q with Group T. Group T sat in distilled water, so this difference mixes the coating with the change of liquid.
  • B. Incorrect. 21 mg is the rust on the wax-coated nails themselves (Group Q), not the reduction compared with the control.
  • C. Correct. The control, Group P, averaged 48 mg of rust and the wax group, Group Q, averaged 21 mg (Table 1): 48 − 21 = 27 mg less rust.
  • D. Incorrect. 39 mg is 48 − 9, the reduction for paint (Group S), not for wax.

Trap A “control” that differs in two ways

Not every untreated group is the right control. Group T has no coating, but it also has a different liquid, so comparing a coated group with T changes two things at once and the difference can’t be credited to the coating.

Before you accept a control, line it up with the treated group column by column. It should differ in the treatment and in nothing else.