Valid measurements

Planning to collect measurements that are valid

Controlling factors

The conclusions that can be drawn from the experiment are valid if the method makes sure that the effects observed and measured are due to the cause claimed.

It is important that other factors are accounted for. The factors that need to be controlled should be identified and the plan should include how they will be controlled.


Two students investigate how light intensity affects the rate of photosynthesis of pondweed. They test the first two concentrations on day one. They test the other concentrations the following week when the weather is much hotter.

Why is any conclusion from this experiment not valid?

A lamp is moved away from a beaker containing pondweed, by 10 cm intervals up to 50 cm along a ruler, in an experiment to investigate light intensity on the rate of photosynthesis.A light intensity experiment

The conclusion cannot be valid because the surrounding temperature was different on the two days. An increase in the rate of photosynthesis could be caused by this increase in temperature, not just by a change in light intensity.


What factors should the students have controlled?

The length of the pondweed, the volume of water and its temperature.

Deciding range

The range of measurements made must be enough to draw a valid conclusion. If the measurements are too close together, it is difficult to be confident in any trend shown.


The students record the number of bubbles produced each minute for five minutes.

Their first set of results are:

Distance from light (cm)Mean number of bubbles of gas per minute

How could the students improve the experiment to collect a broader range of results?

The students could increase their distance from the light in 5 cm intervals, not every 10 cm. They could have used a different length or species of pondweed.

Planning to collect measurements that are precise and accurate


Measurements are precise if the results are very close in value. The choice of measuring apparatus can affect the precision of data collected.

For example, during an investigation into zonation of algae on a seashore, the distances along a transect should be measured using a tape measure. If they were paced out without using a tape measure to save time, the quadrats are less likely to be in the correct position. This placement would not be precise.


The accuracy of a numerical result is how close it is to the true value.

For example, during an investigation into the effect of temperature on the decay of milk, the pH change could be measured with a pH probe. If Universal Indicator solution was to be used, human interpretation of the colour when comparing against a chart might mean the reading would differ. This would not be precise.

Results are precise if they are close in value but they must also be close to the true value to be accurate.

Identify hazards and suggest ways to minimise the risk

The substances used in a biology experiment may be hazardous. They could be harmful to the eyes or skin, or cause harm if breathed in.

The risk of harm from these hazards may be minimised by using the lowest concentration of any solutions such as iodine or Benedict's solution, wearing eye protection and wearing gloves.

Procedures can also be hazardous. For example, care must be taken not to touch very hot equipment. Loose clothing or hair should be kept away from flames.

All experiments should be risk assessed. Always follow teacher instructions and safety advice.