Science skill: Writing a hypothesis and identifying variables

Aim

To learn how to state a testable hypothesis and correctly identify the manipulated, responding and constant variables of an experiment.

Variables

• Manipulated: The manipulated variable is the one quantity you deliberately change.
• Responding: The responding variable is the quantity you measure as it responds to the change.
• Constant: The constant variables are everything else that you keep the same so the test is fair.

Apparatus & materials

• The aim of the experiment, written out clearly
• A notebook and pencil

Procedure

1. Read the aim and identify the two quantities whose relationship is being investigated.
2. Decide which of the two you will deliberately change, this is the manipulated variable.
3. Decide which one you will measure in response, this is the responding variable.
4. List everything else that must be kept the same, these are the constant variables.
5. Write the hypothesis linking the two, stating the direction of the relationship (for example, 'as the length increases, the period increases').
6. Check that the hypothesis is testable by changing only the manipulated variable.

Tabulating results

The manipulated variable becomes the first column of your data table and the responding variable the next column; the constant variables are stated above or below the table, not given their own columns.

The graph

The manipulated variable goes on the x-axis and the responding variable on the y-axis, so the graph shows how the responding variable depends on the manipulated variable.

Analysis

Examiners award credit for correctly naming the manipulated variable, the responding variable and at least one constant variable, and for a hypothesis that is testable and clearly states the direction of the relationship.

A worked example and what it shows

This is a skill page, so instead of readings the useful practice is to work an aim through to variables and a hypothesis. Take the aim: to investigate how the extension of a spring depends on the load hung on it.

• Manipulated variable: the load, F (the one quantity you deliberately change).
• Responding variable: the extension, e (the quantity you measure in response).
• Constant variables: the same spring and the same temperature.

A good hypothesis states the direction of the relationship: as the load increases, the extension increases. What this shows is the habit examiners reward.

You read the aim, pick out the two linked quantities, decide which one you control and which one you measure, then fix everything else. Only one variable is changed at a time, so any change in the extension can be blamed on the load and nothing else.

Mapping the variables onto a graph

The variables you name decide the axes of any graph. The rule is simple: the manipulated variable goes on the x-axis and the responding variable on the y-axis, so the graph shows how the responding variable depends on the one you changed.

For the spring example you would plot extension e (y-axis) against load F (x-axis).

A hypothesis that states a direction also predicts a shape. "As the load increases, the extension increases" predicts a line that rises from left to right.

If the relationship is one of direct proportion, the line is straight and passes through the origin, and its gradient is a meaningful quantity.

Planning the variables well before you start means the table columns and the graph axes are already decided, and the readings drop straight into place when you take them.

Marks examiners look for

Common slips to avoid: do not confuse the manipulated and responding variables (one is changed, the other is measured), name only one manipulated variable per experiment, and state the constant variables specifically rather than writing "other factors".

For the Paper-3 science-process skills, the marks are awarded for:

• Naming the manipulated variable correctly.
• Naming the responding variable correctly.
• Naming at least one specific constant variable.
• A hypothesis that is testable by changing only the manipulated variable.
• A hypothesis that clearly states the direction of the relationship, such as "increases" or "decreases".

Keeping all but one variable fixed is what makes the test fair, and saying so earns the fair-test credit.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)