Thinking Like a Scientist

Variables: the things that change

A variable is anything that can change in an experiment. There are three kinds you must know.

A simple way to remember: "I change, D measures." Independent goes first, dependent comes after.

A fair test

An experiment is a fair test when only the independent variable changes. If Sam changes the drop height and pushes the ball, he cannot tell which one made the bounce higher. That is why controlled variables matter so much.

Validity, reliability and accuracy

These three words come up in almost every science exam. They sound alike but mean different things.

Drawing a good graph

Graphs turn a table of numbers into a picture so you can see a pattern.

1. Put the independent variable on the x-axis (across) and the dependent variable on the y-axis (up).
2. Label both axes with the quantity and unit, for example "Drop height (m)".
3. Choose a scale that uses most of the paper and goes up in even steps.
4. Plot points with small neat crosses.
5. Draw a line of best fit. This is a smooth line or curve that passes as close as possible to most points. Do not join the dots.
6. Give the graph a title.

Which kind of graph?

• Line graph (or scatter graph): when both variables are numbers that can take any value, like height or time. These are called continuous variables.
• Column (bar) graph: when the independent variable is a name or group, like "type of ball". These are called categorical variables.

An outlier is a point that does not fit the pattern. It is usually a mistake. Circle it, ignore it for the line of best fit, and repeat that measurement if you can.

Reading the pattern. If the line is straight and passes through the origin (0, 0), the two variables are directly proportional. Double one, and the other doubles too.

The gradient of a graph

The gradient (slope) tells you how steep the line is.

$$\text{gradient}=\frac{\text{rise}}{\text{run}}=\frac{\Delta y}{\Delta x}$$

The symbol $\Delta$ (delta) just means "change in". Always pick two points far apart on the line, not data points that are off the line.

Quick summary

• Independent variable: what you change. Dependent: what you measure. Controlled: what you keep the same.
• Valid means a fair test. Reliable means repeatable. Accurate means close to the true value. Precise means close together.
• Repeating trials improves reliability, not validity.
• Independent variable on the x-axis, dependent on the y-axis, both with units.
• Use a line of best fit, never dot to dot. Circle outliers.
• Line graphs for continuous data. Column graphs for categories.
• Gradient $=\frac{\text{rise}}{\text{run}}$.

Quiz