Writing a Hypothesis

What a hypothesis is

A hypothesis is a statement, based on existing knowledge or observation, that predicts how one variable affects another. It must be testable, meaning it can be investigated by an experiment that measures the responding variable while changing the manipulated variable and keeping other factors constant.

Standard format for a hypothesis

A hypothesis in SPM Biology should directly state the relationship between the manipulated variable and the responding variable, usually in the form: 'The higher/greater the [manipulated variable], the higher/lower the [responding variable].' The direction of the relationship must match the expected biological trend, based on scientific knowledge or a given pattern in the data.

Steps to write a hypothesis

1. Identify the manipulated variable and the responding variable from the question or investigation.
2. Decide the expected direction of the relationship, does the responding variable increase or decrease as the manipulated variable increases?
3. State the relationship clearly in one sentence, naming both variables.
4. Check that the hypothesis can actually be tested by measuring the responding variable in an experiment.

Worked example

Investigation: the effect of light intensity on the rate of photosynthesis in a pondweed.

Manipulated variable: light intensity. Responding variable: rate of photosynthesis.

Hypothesis: The higher the light intensity, the higher the rate of photosynthesis.

More worked examples

The same format works across the syllabus: name the manipulated variable, name the responding variable, and state the direction of the relationship between them.

How the hypothesis fits into the investigation

A hypothesis is not written on its own; it sits between the aim and the rest of the plan. The aim says what is being investigated, the hypothesis predicts the outcome, and the variables, procedure and results then test that prediction.

A well-written hypothesis names the same manipulated and responding variables that appear in the list of variables and in the results table, so the whole plan stays consistent.

Because the hypothesis fixes the direction of the expected result, it also shapes the graph you expect to draw. If the hypothesis says the responding variable rises as the manipulated variable rises, the expected graph slopes upward; if it says the responding variable falls, the graph slopes downward.

Checking that the results match the hypothesis is part of writing the conclusion.

Paper 3-style questions

Question 1, writing a hypothesis. In an investigation on the effect of carbon dioxide concentration on the rate of photosynthesis, write a suitable hypothesis.

Model answer. The higher the carbon dioxide concentration, the higher the rate of photosynthesis (up to the point where another factor becomes limiting). The statement names both the manipulated variable (carbon dioxide concentration) and the responding variable (rate of photosynthesis) and gives the direction of the relationship.

Question 2, improving a weak hypothesis. A student writes 'temperature affects the activity of catalase'. Explain why this is a weak hypothesis and rewrite it.

Model answer. It is weak because it does not state the direction of the relationship between the variables. A better version is: up to the optimum, the higher the temperature, the higher the rate of activity of catalase, which is a clear, testable prediction.

Question 3, aim to hypothesis. The aim of an investigation is 'to study the effect of salt concentration on the mass of a fish tissue'. Convert this aim into a hypothesis.

Model answer. A suitable hypothesis is: the higher the salt concentration of the surrounding solution, the greater the loss in mass of the fish tissue, because water leaves the cells by osmosis into the more concentrated solution.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions