Fair tests and variables

A scientific investigation starts with a question, such as “Does the temperature of water change how fast sugar dissolves?” To answer it fairly, you change one thing, measure one thing and keep everything else the same. That is a fair test.

Key facts

  • The independent variable is the one thing you change. The dependent variable is what you measure. Control variables are everything you keep the same.
  • A fair test changes only one variable, so any difference in the results must be caused by that change.
  • A prediction says what you think will happen and why: “The hotter the water, the faster the sugar will dissolve, because the particles move faster.”
  • Repeat each measurement and work out a mean. Repeats make results more reliable and show up anomalies (results that don’t fit).
  • A categoric variable has names or groups (the type of surface); a continuous variable is a measured number (the temperature, a length).
  • Repeatable: you get similar results when you do the test again. Reproducible: someone else gets similar results.
  • A control, such as a plant given no plant food, gives you something to compare against.

Worked examples

Example 1

Sam wants to find out whether the height a ball is dropped from changes how high it bounces. Name the independent, dependent and control variables.

  1. What does Sam change? The drop height. That is the independent variable.
  2. What does Sam measure? How high the ball bounces back. That is the dependent variable.
  3. What must stay the same? The ball, the floor and the way the height is measured. These are the control variables.

Answer: independent: the drop height; dependent: the bounce height; control: the same ball, the same floor

Example 2

Which plan is a fair test of whether plants need light? Plan A: one plant in the light and watered, one in the dark and not watered. Plan B: two of the same plants, both watered, one in the light and one in the dark.

  1. Plan A changes two things: the light and the water. If a plant grows badly, you can’t tell which one caused it.
  2. Plan B changes only the light. The plants, the water and the warmth stay the same.
  3. So Plan B is the fair test.

Answer: Plan B

Example 3

Write a prediction for this question: does the size of a parachute change how long it takes to fall?

  1. Say what will happen to the dependent variable (the time to fall) as you change the independent variable (the size).
  2. Give a scientific reason: a bigger parachute pushes against more air, so there is more air resistance.
  3. Put it together: “The bigger the parachute, the longer it will take to fall, because there is more air resistance.”

Answer: The bigger the parachute, the longer it will take to fall, because there is more air resistance.

Example 4

Is each variable categoric or continuous: the type of surface, the temperature, the colour of light, the mass? Which graph would you draw if it were the independent variable?

  1. Categoric variables have names or groups: the type of surface (carpet, wood…), the colour of light (red, blue…).
  2. Continuous variables are measured numbers that can take any value: the temperature (°C), the mass (g).
  3. Categoric independent variable: a bar chart. Continuous: a line graph.

Answer: the type of surface and the colour of light are categoric (bar chart); the temperature and the mass are continuous (line graph)