Cambridge International A Level Biology Revision Guide

Chapter P2: Planning, analysis and evaluation
Describing the sequence of steps
When you are describing your planned experiment, make
sure that you describe a logical sequence of steps that
you would follow. It is always worth jotting these down
roughly first (perhaps on a spare page on the examination
paper), to make sure that the steps follow one another in
a sensible order, and that everything is fully explained.
You might like to draw labelled or annotated diagrams to
explain some of the steps – it is sometimes quicker and
easier to do it that way. Diagrams are very often the best
way to describe how you would assemble the apparatus.
Risk assessment
An extremely important part of planning any experiment
is to think about the potential hazards involved. In biology
experiments, there often are not any significant risks, and
if that is the case then you should say so. Do not invent risks
when there are none. Do, though, always mention risk, even
if it is just to say that you do not think there is any.
If you do identify any significant risks, then you should
explain how you would minimise them. For example, in the
yeast experiment, you might decide to use temperatures up
to 80 °C. This is hot enough to burn the skin, so you would
need to take precautions when handling the apparatus
in a water bath at that temperature. You should lift the
apparatus in and out using tongs, or use heatproof and
waterproof gloves.
Recording and displaying
results
You may be asked to construct a results table that you
could use to fill in your results. This is described on pages
255–256 in Chapter P1.
You may also want to describe how you would use the
data to plot a graph, explaining what you would put on
each axis and the type of graph you would draw.
Using the data to reach a conclusion
You may be asked how you would use your collected
results to reach a conclusion. Usually, the conclusion will
be whether or not the results support your hypothesis.
(Remember that they will never prove it.) This is described
on page 259 in Chapter P1.
Analysis, conclusions
and evaluation
In a written practical examination, generally at least one
question will provide you with sets of data. You could
be asked how to process the data (for example, by doing
calculations) in order to be able to use them to make a
conclusion. You could be asked to assess the reliability of
the investigation. This should be familiar from your AS
work, but there is one big step up at this stage – you need
to be able to use statistics to assess the variability of the
data, or the significance of your results.
Note that you do not have to learn any of the formulae
for the statistical tests. They will always be provided for
you in the question.
Mean, median and mode
A large lemon tree has several hundred fruits on it.
Imagine that you have measured the masses of 40 lemons
from this tree. These are the results, recorded to the
nearest gram and arranged in order of increasing mass.
57, 60, 67, 72, 72, 76, 78, 79, 81, 83, 84, 86, 87,
88, 88, 90, 92, 92, 93, 94, 95, 97, 98, 99, 100,
101, 101, 103, 105, 106, 107, 109, 111, 113,
119, 120, 125, 128, 132, 135
To calculate the mean, add up all the readings and divide
by the total number of readings.
sum of individual values 3823
mean = = = 96
total number of values 40
When you do this calculation, your calculator will read
95.575. However, you should not record this value. You
need to consider the appropriate number of decimal places
to use. Here, the masses of the lemons were recorded to the
nearest whole number, so we can record the mean either
to the nearest whole number, which is 96, or to one more
decimal place, which is 95.6.
To find the median and mode, you need to plot your
results as a frequency histogram (Figure P2.4).
The mode (modal class) is the most common class
in the set of results. For these results, the modal class is
90–94 g.
The median is the middle value of all the values
in the data set. In this case, the median class is 95–99 g.
495