OCR AS Biology

Revision Notes

1.1.3 Evaluating Experimental Methods

Evaluating Experimental Methods

  • Evaluating experimental methods is an important skill for a scientist
  • A good way to evaluate an experimental design is by repeating the experiment yourself (using the instructions provided) and determining if you can produce similar results
  • When analysing and criticising the design of an experiment there are several key things to look out for:
    • Limitations
    • Accuracy
    • Precision
    • Reliability
    • Validity
  • Limitations:
    • A limitation is any design flaw or fault that affects the accuracy of the experiment
    • It is crucial that any limitations within an experiment are identified and removed/corrected
    • For example, an experiment using a potometer to measure the rate of water uptake in different plant species had a small leak in the system
    • The leak in the apparatus is a limitation as it prevents the accurate measurement of water uptake
    • It can be corrected by ensuring that all fixtures are tightly screwed together or sealed using tape/wax etc
  • Accuracy:
    • Accuracy – how close a reading/measurement is to its true value
    • Faulty instruments or flaws in the experimental method produce systematic errors that are repeated consistently every time the instrument is used or the method is followed. This affects the accuracy of all readings obtained
  • Precision:
    • Precision – how similar repeat readings/measurements are to each other
    • Readings that are tightly clustered together (a small range) are described as precise
    • The precision of a measurement is reflected in the values recorded – measurements to a greater number of decimal places are said to be more precise than those to a whole number
  • Reliability
    • Experiments are repeated many times to ensure the reliability of results
  • Validity
    • The other variables in the experiment are identified and controlled in order to ensure the validity of an experiment
  • Ideally, the design of an experiment should be evaluated at the preliminary stage, this way any corrections or adjustments can be made prior to conducting the actual experiment

Accuracy and Precision, downloadable AS & A Level Physics revision notes

The difference between accuracy and precision explained using a dartboard as a metaphor.


  • Scientists always record instructions for their experiments so that they can be repeated
  • The instructions should allow an individual to successfully carry out the experiment without any additional help or input
  • It is very important to record all required details within these instructions
  • For example:
    • The apparatus used
    • The quantities of specific reactants/reagents used
    • The species of model organism used

Exam Tip

It is a very common mistake to confuse precision with accuracy – measurements can be precise but not accurate if each measurement reading has the same error.

Precision refers to the ability to take multiple readings with an instrument that are close to each other, whereas accuracy is the closeness of those measurements to the true value.


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