Experimental Design · Lesson 4

Designing the next trial

Many questions ask how the researchers could test something new: a different soil, more water, a new range of a variable. The rule is the same one that built the original studies: change one variable and hold everything else.

A new trial is useful only if you can compare it with something. Either propose a whole set of trials that differ in the new variable alone, or propose a single trial that matches an existing trial in every condition except the new one. Check each choice against every row of every table: what slope, what cover, what volume of water, how long?

Two quick tests eliminate most wrong choices. Does the plan actually vary the variable in the question? (A plan that keeps it fixed tests something else.) Does it vary anything else at the same time? (Then any change in the result can’t be pinned on one cause.)

Slopes and Sediment

Students used a stream table, a tilted tray of sand, to model how rain washes soil off hillsides.

Study 1

The students packed a tray with 3 kg of damp fine sand and tilted it to different slope angles. The sand was bare, with no grass cover. For each trial, they sprayed 2 L of water evenly over the sand in 1 minute, then dried and weighed the sand that washed into a catch basin at the bottom of the tray (see Table 1).

Study 2

The students kept the tray at a 10° slope and sprayed 2 L of water in 1 minute, as in Study 1, but covered different percentages of the sand surface with strips of living grass mat (see Table 2).

In both studies, the sand was replaced with fresh sand before every trial.

Table 1
Slope angle (°)Sediment washed off (g)
542
1088
15141
20205
Table 1 (Study 1, bare sand) lists slope angle and the mass of sediment washed off: 42 g at 5°, 88 g at 10°, 141 g at 15° and 205 g at 20°. Sediment rises steeply as the slope increases.
Table 2
Grass cover (%)Sediment washed off (g)
088
2561
5037
7519
Table 2 (Study 2, 10° slope) lists grass cover and the mass of sediment washed off: 88 g at 0%, 61 g at 25%, 37 g at 50% and 19 g at 75%. Sediment falls as grass cover increases.

Worked example Easy

Show the whole passage

Slopes and Sediment

Students used a stream table, a tilted tray of sand, to model how rain washes soil off hillsides.

Study 1

The students packed a tray with 3 kg of damp fine sand and tilted it to different slope angles. The sand was bare, with no grass cover. For each trial, they sprayed 2 L of water evenly over the sand in 1 minute, then dried and weighed the sand that washed into a catch basin at the bottom of the tray (see Table 1).

Study 2

The students kept the tray at a 10° slope and sprayed 2 L of water in 1 minute, as in Study 1, but covered different percentages of the sand surface with strips of living grass mat (see Table 2).

In both studies, the sand was replaced with fresh sand before every trial.

Table 1
Slope angle (°)Sediment washed off (g)
542
1088
15141
20205
Table 1 (Study 1, bare sand) lists slope angle and the mass of sediment washed off: 42 g at 5°, 88 g at 10°, 141 g at 15° and 205 g at 20°. Sediment rises steeply as the slope increases.
Table 2
Grass cover (%)Sediment washed off (g)
088
2561
5037
7519
Table 2 (Study 2, 10° slope) lists grass cover and the mass of sediment washed off: 88 g at 0%, 61 g at 25%, 37 g at 50% and 19 g at 75%. Sediment falls as grass cover increases.

The students want to test whether the amount of water sprayed affects how much sediment washes off. Which set of trials would best do this?

  1. Bare sand at 10°, sprayed with 1, 2, 3 and 4 L of water in 1 minuteAnswer
  2. Bare sand at 5°, 10°, 15° and 20°, each sprayed with 3 L of water
  3. Sand at 10° with 0%, 25%, 50% and 75% grass cover, sprayed with 1 L
  4. Bare sand sprayed with 1, 2, 3 and 4 L at 5°, 10°, 15° and 20° in turn

How to solve it

  1. The new variable is the volume of water, so it must change from trial to trial.
  2. Everything else must stay fixed: one slope, one cover level, the same 1-minute spray. Choice A does this at 10° on bare sand.
  3. B and C keep the water fixed, and D changes slope along with water. The answer is A.

Why each choice is right or wrong

  • A. Correct. Only the volume of water changes (1 to 4 L); the slope (10°), the bare sand and the 1-minute spray stay fixed, and the 2 L trial matches the existing 10° bare-sand trial (88 g in Table 1).
  • B. Incorrect. Every trial gets 3 L, so the water volume never varies; these trials test slope again, as Study 1 did.
  • C. Incorrect. Grass cover varies and the water is fixed at 1 L, so this tests grass cover, not the amount of water.
  • D. Incorrect. Water volume and slope change together from trial to trial, so any change in sediment could come from either one.

Worked example Medium

Show the whole passage

Slopes and Sediment

Students used a stream table, a tilted tray of sand, to model how rain washes soil off hillsides.

Study 1

The students packed a tray with 3 kg of damp fine sand and tilted it to different slope angles. The sand was bare, with no grass cover. For each trial, they sprayed 2 L of water evenly over the sand in 1 minute, then dried and weighed the sand that washed into a catch basin at the bottom of the tray (see Table 1).

Study 2

The students kept the tray at a 10° slope and sprayed 2 L of water in 1 minute, as in Study 1, but covered different percentages of the sand surface with strips of living grass mat (see Table 2).

In both studies, the sand was replaced with fresh sand before every trial.

Table 1
Slope angle (°)Sediment washed off (g)
542
1088
15141
20205
Table 1 (Study 1, bare sand) lists slope angle and the mass of sediment washed off: 42 g at 5°, 88 g at 10°, 141 g at 15° and 205 g at 20°. Sediment rises steeply as the slope increases.
Table 2
Grass cover (%)Sediment washed off (g)
088
2561
5037
7519
Table 2 (Study 2, 10° slope) lists grass cover and the mass of sediment washed off: 88 g at 0%, 61 g at 25%, 37 g at 50% and 19 g at 75%. Sediment falls as grass cover increases.

The students want to know whether slope still affects erosion when half of the sand surface is covered with grass. Which plan would test this?

  1. Repeat Study 2 at 10°, but spray 4 L of water on every trial
  2. Repeat Study 1’s four slopes with 50% grass cover on every trialAnswer
  3. Run one trial at 5° with 50% cover and a second at 20° with no cover
  4. Repeat Study 1 on bare sand, but spray 4 L of water per trial

How to solve it

  1. The question has two parts: slope must vary, and grass cover must be 50% in every trial.
  2. Repeating Study 1’s slopes (5°, 10°, 15° and 20°) with 50% cover on every tray does both, and everything else (2 L in 1 minute, fresh sand) stays as before.
  3. A and D never vary the slope at 50% cover, and C changes slope and cover together. The answer is B.

Why each choice is right or wrong

  • A. Incorrect. Every trial would be at 10°, so the slope never varies; it also changes the water volume, which neither study varied.
  • B. Correct. Slope varies (5° to 20°) while cover stays at 50% and the 2 L spray stays the same, so any change in sediment comes from slope. Its 10° trial should match Table 2’s 50% trial (37 g).
  • C. Incorrect. The two trials differ in slope and in grass cover, so a difference in sediment could come from either one.
  • D. Incorrect. The sand stays bare, so this plan cannot show what slope does when half of the surface is covered.

Trap A trial that changes two variables at once

A proposed trial can look reasonable and still be useless. A clay trial at 12° sounds like a soil test, but no one ran a 12° trial before, so it differs from every existing trial in soil and slope.

For each choice, find an existing row that matches it in every condition except the one being tested. If there is no such row, the choice changes two variables at once.