Chalk−1

Biology · Introductory biology · Worked example

Count the experimental units in a design

A biologist tests whether warmer water slows zebrafish growth. She keeps one tank at 28 °⁢C and one at 32 °⁢C, with 15 young fish in each. After eight weeks the fish average 2.9 cm long in the 28 °⁢C tank and 2.6 cm in the 32 °⁢C tank. How many replicates does each group have, and can the difference be credited to temperature?

Find what was assigned

Temperature was set for each tank, not for each fish. The fish in one tank share its water, heater, filter and light. The tank is the experimental unit, since it is the thing that received a temperature independently.

Count the replicates

Each temperature has one tank, so n=1 per group. The 15 fish in a tank are 15 measurements of one unit, much like measuring one plant 15 times. They are not 15 replicates.

Look at the difference

The tanks differ by 0.3 cm on average. Anything else that differs between the two tanks, such as a weaker filter or an infection, changes every fish in it at once. With one tank per temperature, a tank effect and a temperature effect cannot be told apart.

2.9−2.6=0.3

Redesign

Use eight tanks of one size and setup. Assign four to 28 °⁢C and four to 32 °⁢C at random, with 15 fish in each. Average the fish in each tank, then compare the four tank means of one group with the four of the other, so n=4 per group.

Know what the fish still add

Measuring 15 fish per tank is not wasted, because it makes each tank’s mean more precise. It cannot replace more tanks, since only more tanks show how much tanks differ from each other.

Result

Each group has one replicate, a single tank, so n=1, not 15. The 0.3 cm difference cannot be credited to temperature, because anything else that differs between the two tanks affects all of its fish. Replicate tanks, assigned to the temperatures at random, fix the design.

Your turn

1. One like the example

A teacher tests a new reading app. Class A, 28 students, uses it for a term, and class B, 27 students, does not. Class A’s mean score rises 6 points more than class B’s. What is the experimental unit, and what is n per group?

Show the answer and explanation

The class is the experimental unit, so n=1 per group.

The app was given to a whole class, and the students in a class share a teacher, a room and a timetable. Any difference between the two classes is tangled up with the app. Several classes, assigned to the app at random, would be needed.

2. A variation

A drug is added to the drinking water of 4 cages of mice, and 4 other cages get plain water, with 5 mice in each cage. What is n per group, and how should the data be compared?

Show the answer and explanation

n=4 cages per group. Average the mice in each cage and compare the 4 cage means of one group with the 4 of the other.

The drug went into each cage’s water, so the cage is the experimental unit. Mice in one cage share water, food and germs, so they are not independent. Treating them as 20 replicates per group would overstate the evidence.

3. Somewhere new

A student treats one flask of cultured cells with a hormone and leaves another flask untreated. She counts cells in 3 wells taken from each flask and finds more in the treated wells. How many independent replicates are there, and how could she get more?

Show the answer and explanation

One per group. She should repeat the whole experiment on independent cultures, for example on three separate days, treating a fresh flask each time.

The 3 wells come from one flask, so they share everything that happened to that flask. Only a separately prepared culture, treated on its own, is a new experimental unit.

Keep exploring

The Experiment planner opens with one tank per temperature and 15 fish measured in each. It warns that repeated measurements of one biological unit do not replace independent biological replicates.

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After reading

Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 12 key terms Open the tank design in the Experiment planner Open worked example on a board Experimental design in Biology Reference

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