9.1 - Variables & Controls

The three kinds of variable

Every experiment has one variable you change, one you measure, and several you keep the same.

The three variables explained

Telling the independent and dependent apart

• Read the aim as "how does X affect Y": X is the independent variable and Y is the dependent variable, every time.
• On a graph, the independent variable goes along the bottom (the x-axis) and the dependent variable up the side (the y-axis).
• The independent variable is the one the student chose to set before starting; the dependent variable is the one they could only find out by measuring.

Controls and valid results

The word "control" is used in two different ways in an experiment - control variables and a control experiment. Don't mix them up.

Control variables keep results valid

• Control variables are the factors you keep the same throughout the experiment - for example, the pH and the volume of amylase.
• Keeping them constant is what makes the results valid: if only the temperature changes and everything else stays the same, any change in the result must be down to the temperature and nothing else.
• Always name a control variable in full: "the volume of amylase", not vague phrases like "the conditions".

A control experiment is run for comparison

• A control experiment is a separate set-up with the factor you are testing left out, run alongside the others for comparison.
• Example: to show that amylase breaks down the starch, set up a tube with boiled amylase that no longer works.
• If the starch breaks down in the ordinary tubes but not in the control, the working amylase must have caused it.

Hazards, risks, and control measures

• A hazard is anything with the potential to cause harm, such as a scalpel or a hot water bath.
• A risk is the chance that a hazard actually causes harm in your procedure, and how serious that harm would be.
• The same hazard can carry a bigger or smaller risk: cutting towards your fingers with a scalpel is riskier than cutting away from them, even though the hazard itself hasn't changed.
• A safety question usually wants three things in a row: the hazard, the risk it poses in that step, and a control measure aimed at that risk.

Common hazards and their control measures

Using less of something, or a lower setting, is a control measure in its own right - a smaller volume of iodine or a cooler water bath doesn't remove the hazard, but reduces the risk, because there's less to cause harm if something goes wrong.