International A Level Biology / Practical Skills in Biology II
Limitations, improvements and extensions
Edexcel International A Level Biology (YBI11), chapter 26: Practical Skills in Biology II, specification point U6.2, U6.3. Free lesson.
Read the revision notes · International A Level Biology past papers
A strong practical evaluation names a limitation that could happen in that method and explains how it affects the results. Improvements make the same investigation more reliable, while extensions ask a new question.
How to describe a limitation
A limitation must belong to the practical you are discussing. Explain both what goes wrong and how it changes the results. This makes your answer specific rather than a general comment about difficulty controlling variables. In the pondweed practical, some oxygen dissolves in the water before collection, so the volume collected is too small and the rate is underestimated. In a respirometer, oxygen can run low in the sealed tube. Seeds may respire anaerobically, taking up less oxygen, so the rate is underestimated. In a turbidity investigation, dead cells also make the culture cloudy. This…
Check that the limitation could happen
A limitation is only useful if it could affect that practical during the time it runs. Contamination by microorganisms is not automatically a good answer: it may affect seedlings grown for several days, but there is too little time for microbes to affect a short respirometer run. Name a problem that fits the method and its timescale. Explain whether it makes the result too high, too low, or harder to interpret.
How to justify an improvement
An improvement should fix a limitation in the same practical, using realistic changes to the method or apparatus. Give the reason for the change: explain what it fixes and why that makes the results better. For germinating seeds in a respirometer, oxygen may run low, causing anaerobic respiration. Less oxygen is then taken up, so the measured respiration rate is underestimated. Keep each run short and let fresh air into the apparatus between runs. This helps the seeds keep respiring aerobically, so the measurement better reflects the rate you want.
Give a reason for each improvement
Do not leave an improvement as an unsupported instruction. Saying “do more trials to increase the sample size” does not explain what problem is fixed or why the results improve. Link the change directly to the investigation. For a study of whether topshells prefer one habitat, leave them longer before recording their positions. The extra time gives any habitat preference a chance to show.
Tell an improvement from an extension
An improvement answers the same question more reliably. It changes the method, such as the apparatus, readings or control. An extension asks a new question that follows on from the result, such as testing a new independent variable, organism or condition. Keep the other parts of the original investigation the same. For the yeast and methylene blue practical at different temperatures, using a colorimeter to time the end point is an improvement. Testing different sugars at one temperature is an extension. If the question being investigated is still the same, the change is an…
Plan an extension to a hatching investigation
To extend an investigation into the effect of temperature on brine shrimp egg hatching, choose a different factor that could affect hatching. Change that factor and keep the other conditions the same as in the original investigation.