Controlling Variables in Biology (DP IB Biology): Revision Note

Controlling variables in Biology

• This is the practical application of the 'Controlled Variables' you identified in your design

• The main goal of controlling variables is to minimise systematic errors—errors that consistently shift your results in one direction

• By keeping all other factors constant, you can be:

  • more confident that any effect you measure is due to the change in your independent variable

  • ensure that the results are valid

  • ensure that only the independent variable affects the results

Methods of control

Appreciate when and how to calibrate measuring apparatus

• To ensure your data is accurate, the instruments you use must be checked and calibrated

• Calibration is the process of checking an instrument's readings against a known, reliable standard and adjusting it if necessary

• You should consider calibration for key instruments in your design:

  • pH meters

    • Before taking readings, a pH meter must be calibrated using at least two standard buffer solutions (e.g. at pH 4.00 and 7.00)

    • This adjusts the probe's output to give correct pH values

  • Sensors and data loggers

    • A CO₂ sensor or O₂ sensor used to measure rates of photosynthesis or respiration should be calibrated against the known concentration of gases in the air

  • Microscope eyepiece graticule

    • Must be calibrated with a stage micrometer to determine the length of each graticule unit at a specific magnification

  • Digital thermometers and sensors

    • Their accuracy can be checked by placing them in boiling, distilled water (which should read 100.0°C at standard pressure) and a beaker of crushed ice (0.0°C)

      

      

  • Digital balances

    • Must always be zeroed (tared) before use to ensure you are only measuring the mass of your chemical.

      

Appreciate when and how to maintain constant environmental conditions

• Living organisms are sensitive to their environment, so controlling conditions is crucial

• Maintaining temperature

  • For experiments involving organisms or enzymes, temperature is often the most important variable to control

    • The best way to do this is to use a thermostatically controlled water bath or an incubator

    • Place all your samples in the controlled environment for a set amount of time before starting the experiment to allow them to acclimatise

• Maintaining other conditions

  • Depending on the experiment, you may also need to control factors like light intensity (using a lamp at a fixed distance) or humidity

• Sometimes, a variable cannot be perfectly controlled (e.g. ambient room temperature might fluctuate slightly)

  • In these cases, the best practice is to monitor and record it

  • You can then discuss its potential impact in your evaluation

Choose representative random samples and minimise sampling errors

• Biological materials (e.g. potatoes, leaves, insects) are naturally variable

  • It is impossible to have two individuals that are perfectly identical

• To account for this, you must use representative samples and replication

• Random sampling: In ecological studies, this involves using techniques like quadrat sampling to collect data that is representative of the entire area and avoids bias

• Minimising sampling error in the lab:

  • Try to select samples that are as similar as possible (e.g. use potatoes from the same variety, leaves from the same plant, or organisms of the same age and sex)

  • Use a large number of replicates (e.g. 5 or more potato cylinders at each concentration) to reduce the impact of any single anomalous sample

Set up a control run where appropriate

• A control or control group is a setup that is treated in exactly the same way as your experimental setups, but without the independent variable being applied

• The purpose of a control is to prove that it is your independent variable, and nothing else, that is causing the observed effect

  • Example 1:

    • In an experiment testing the effect of different antibiotic concentrations on bacterial growth, the control would be a petri dish with bacteria but no antibiotic (a negative control)

    • This shows the bacteria are capable of growing normally

  • Example 2:

    • In an experiment testing the effectiveness of amylase, a control tube could be set up with starch solution but no enzyme

    • This proves that the starch does not break down on its own