Evaluating Experimental Methods (OCR AS Biology)

• 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 consider
  • Method limitations
  • Accuracy
  • Precision
  • Reliability
  • Validity
• Method limitation
  • A method limitation is any experimental design flaw or fault in the method that affects the accuracy of the results
  • It is crucial that any limitations within an experiment are identified and removed/corrected
    • For example, in an experiment using a potometer to measure the rate of water uptake in plants, there could be an air bubble inside the plant xylem
    • The bubble in the apparatus is a limitation as it prevents the accurate measurement of water uptake
    • It can be corrected by ensuring that all plant stems are cut underwater to prevent the entry of air
• Accuracy
  • Accuracy is how close a reading/measurement is to its true value
  • Accuracy can be reduced by the presence of errors in an experiment
    • 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
    • Unexpected environmental changes or incorrect use of equipment can produce random errors that are different every time the experiment is carried out, e.g. a breeze blowing during a potometer experiment may not blow at the same speed throughout the experiment
• 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

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

Instructions

• 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