Cambridge International A Level Biology Revision Guide

Chapter P2: Planning, analysis and evaluation
The practical skills that you will develop during your
A level course are tested by a written paper. In this
examination, you will write answers on a question paper,
but the questions actually test skills that you will have
developed as you carried out practical work during your
AS and A level course. The skills that are tested build on
those that are tested at AS level. They fall into two main
groups:
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planning
analysis, conclusions and evaluation.
Each time you carry out an experiment or analyse data
collected through experiment, you will use a variety of
these skills. In this chapter, we will look at the different
components of the skills in detail, and consider what you
must be able to do in order to demonstrate these skills
most effectively.
Some of the skills are the same as those that were tested
at AS level. However, the questions are usually a little more
demanding at A level.
Planning an investigation
As you progress through your A level biology course, you
should develop the ability to plan your own experiments,
rather than simply following instructions on a worksheet
provided by someone else. In a practical examination,
you are likely to be asked to plan an experiment that
investigates the effect of one factor (the independent
variable) on another (the dependent variable).
The question will usually give you a scenario to start
you off. Sometimes, this will be familiar – you will be able
to remember a similar experiment that you have done
yourself. Sometimes it will be completely new, and you will
have to use your experience in other experiments to think
about the best way to design the particular one you have
been asked to plan.
It is very important to read the instructions in the
question extremely carefully. For example, the question
might tell you what apparatus you should use in your
experimental design. If you do not use that apparatus in
your plan – even if you do not think it the best apparatus
for that particular investigation – then it is unlikely that
you are answering the question fully.
Constructing a hypothesis
Once you have been given a scenario, the question may ask
you to construct a hypothesis. A hypothesis is a prediction
of how you think the two variables are related to one
another. It must be stated in a way that you can test by
experiment, through the collection of quantitative results.
(Quantitative results are ones that involve numerical data
rather than descriptions.)
For example, you might be asked to plan an experiment
to investigate the relationship between temperature
and the rate of respiration of yeast. So what will your
hypothesis be? Here are two possibilities.
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The rate of respiration of yeast increases with
temperature.
As temperature increases, the rate of respiration of
yeast will increase up to a maximum temperature,
above which it will fall.
Either of those hypotheses is fine. Both of them can be
tested by changing the temperature and measuring the
rate of respiration.
Note, however, that you cannot ‘prove’ that a
hypothesis is correct just by doing one experiment. Your
results may support your hypothesis, but they cannot
prove it. You would need to do many more experiments
before you can be sure that your hypothesis really is
correct in all situations.
But you can disprove a hypothesis more easily. If
you found that the rate of respiration did not increase
as temperature increases, then this suggests that the
hypothesis is incorrect. Nevertheless, it would be a good
idea to do the experiment two or three times more, to
make sure that the results can be repeated.
Sometimes, you could be asked to sketch a graph of
your predicted results, if your hypothesis is supported. A
sketch graph relating to the first hypothesis is shown in
Figure P2.1a, and one relating to the second hypothesis is
shown in Figure P2.1b.
Using the right apparatus
Imagine the question has asked you to plan your
experiment using the apparatus shown in Figure P2.2.
As the yeast respires, it produces carbon dioxide gas.
This collects above the liquid in the syringe, increasing
the pressure and causing the meniscus to move down the
capillary tubing.
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