Planning an experiment

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Key points

  • Developing a method is an important part of planning an investigation.
  • In an experiment, evaluate the validity of the steps leading up to the conclusion for it to be valid.
  • Random and systematic errors can contribute to the uncertainty of a measurement.

What is the name of a set of instructions that is followed when doing an experiment?

A method.

Video

Watch this video on how writing a scientific method helps to get the correct amount and quality of data.

While you are watching, look out for the difference between types of errors that can happen.

Writing a method

Scientists need to be able to clearly show how they carry out experiments and what they find out. When this is done properly, other scientists can check their work to find strengths and weaknesses with the experiment.

A describes how an experiment is carried out. It should contain a set of written instructions with numbers or letters to show the order in which the steps are carried out.

A diagram can be included to show how the experiment should look when it’s set up.

Each step should describe one action during the experiment. The used should be described, using scientific terms. Everything that was done to complete the whole experiment should be listed in the method, so somebody else could follow it and get a similar outcome.

Here is an example method for heating different volumes of water using a Bunsen burner.

  1. Make sure there are no breaks or holes in the gas hose.
  2. Put the Bunsen burner on a heat-resistant mat, making sure it isn’t near the edge of the bench.
  3. Position a tripod over the Bunsen burner.
  4. Put a gauze on top of the tripod.
  5. Put a beaker containing the required amount of water on the gauze.
  6. Ensure the air hole of the Bunsen burner is closed.
  7. Hold a lit splint 1-2cm above the top of the barrel of the Bunsen burner.
  8. Turn on the gas at the gas tap, and the Bunsen burner will burn with a yellow flame.
  9. Extinguish the splint and place it on the heat-resistant mat.
  10. Turn the collar of the Bunsen burner so it’s burning with a blue flame.
  11. Using a stopwatch, time how long it takes for the water to boil.
  12. Stop the stopwatch when the water is boiling.
  13. Close the air hole so that the flame is yellow.
  14. Switch off the gas at the gas tap.

What can be used alongside a list of numbered instructions in a method to make the experiment clearer?

A diagram.

Accurate and precise data

The measurements that are taken in an experiment are called data. It is important to collect data that is and .

Accuracy is how close a measurement is to its . To make sure data is accurate, calculate a . Make sure that are not included in the calculation. Precision is how close are.

An illustration of dart-boards showing accuracy and precision variables.
Accurate measurements may or may not be precise, and similarly, precise measurements may or may not be accurate.

If repeat measurements are similar, the data is said to be . If someone else does the investigation and gets similar results, the data is said to be .

If results are repeatable and reproducible, it can help lead to conclusions. Valid conclusions are drawn from valid data.

What word is used to describe a result that is close to its true value?

Accurate.

A step-by-step guide on planning an experiment

Person doing science experiment - plotting results on a graph - test tubes to left

Plan an experiment well to get accurate and precise data. Without good quality data, conclusions won't be meaningful.

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True or false?

If an experiment is planned well, the data produced after following the method is more likely to be precise and accurate.

True.

Types of error

Errors can happen when taking a measurement, usually leading to a spread in data. There are two types of error that affect scientific measurements: and .

Random error

This can alter the spread of data and cause outliers, as some measurements may be affected and not others. This can happen when equipment isn't used properly or a reading is taken incorrectly during measurements.

An example of this is . This is caused when a measurement is not taken at eye level. The reading recorded can end up being too high or low.

A ruler on the left with three lines coming from it to three positions of eyes, showing how a reading can be read too high or too low depending on where the eye is.
Parallax error is caused by a student not reading the measurement at eye level. It can lead to the reading being too high or too low.

Systematic error

This can make measurements less accurate. It may happen if equipment is not used properly and can cause each result to differ by the same amount.

Examples of systematic error include not resetting a mass balance to zero and measuring from the end of a ruler instead of from zero.

Name the two types of error that can happen during an experiment.

Random and systematic error.

Test your knowledge

Quiz - Planning an experiment