The biological method

Biology · Introduction. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Biologists solve problems by a fixed set of steps called the biological method. It starts with observations. From them a hypothesis is made: a tentative explanation that can be tested. From the hypothesis a deduction is drawn, in the form "if the hypothesis is true, then this should happen". The deduction is tested by experiment.

A hypothesis supported by many experiments becomes a theory. A theory that keeps passing every test and is accepted as universally true becomes a scientific law.

A fair test needs a control group, which is treated exactly like the experimental group except for the one factor being tested. The discovery of the cause and spread of malaria is the textbook example.

Rules to remember

Steps in order: observation → hypothesis → deduction → experiment → theory → law.
Inductive reasoning goes from specific cases to a general statement; deductive reasoning goes from a general statement to a specific case ("if ... then").
Laveran found Plasmodium in the blood of malaria patients; this led to the hypothesis that Plasmodium causes malaria.
A.F.A. King listed observations linking malaria with mosquitoes, for example that it is commoner in low, marshy places.
Ronald Ross found Plasmodium in female Anopheles mosquitoes that had bitten patients, and showed with sparrows that mosquitoes pass it on.
Control group = the group not given the factor under test; everything else is kept the same.
Large samples and repeated experiments reduce the effect of chance; data are organised in tables and graphs.

Common mistake

Students call the "if ... then" statement a hypothesis. The hypothesis is the suggested explanation; the deduction is the testable prediction that follows from it.

Worked example

To test the hypothesis "Plasmodium is the cause of malaria", blood from 100 malaria patients and from 100 healthy people is examined under the microscope. Which group is the control, and what result supports the hypothesis?

Deduction: if Plasmodium causes malaria, then it should be present in the blood of the patients.
The patients are the experimental group; the healthy people do not have the disease, so they are the control group.
The hypothesis is supported if Plasmodium is found in the patients but not in the healthy people.

Answer: The 100 healthy people are the control group; Plasmodium in the patients' blood only supports the hypothesis.

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Chapter 1: Measurements

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