Cambridge International A Level Biology Revision Guide

Chapter P1: Practical skills for AS
The reliability of a measurement is the degree of trust
that you can have in it. If your measurements are reliable,
then you would expect to get the same ones if you repeated
them on other occasions.
Reliability is affected by both the accuracy and
precision of your measuring instruments, and also by the
kind of measurements that you are making. For example,
in the rennin experiment, you have to make a decision
about exactly when the end-point is reached. This is really
difficult: there is no precise moment at which you can say
the clots definitely form, so your measurement of the time
at which this happens will be very unreliable.
One of the best ways of dealing with poor reliability is
to repeat the readings several times. For example, with the
rennin experiment, you could set up three tubes of each
concentration of the enzyme and measure the time to the
end-point for each of them. You would then have three
results for this particular enzyme concentration, which
you could use to calculate a mean.
Estimating uncertainty in
measurement
When you use a measuring instrument with a scale, such
as a thermometer or syringe, you will often find that your
reading doesn’t lie exactly on one of the lines on the scale.
For example, if a thermometer has a scale marked off in
intervals of 1 °C, you could probably read it to the nearest
0.5 °C. We can assume that each reading you make could
be inaccurate by 0.5 °C. This is the possible error in your
measurement. So, if your temperature reading was 31.5 °C,
we would show this as 31.5 °C ± 0.5 °C. In general, the
potential error in a measurement is half of the value of
the smallest division on the scale you are reading from.
Figure P1.11 explains why this is so.
In some situations, you might make two
measurements, and your result is the difference between
them. For example, you might want to measure a change
in temperature. You measure the temperature at the
start as 28.5 °C (error = ±0.5 °C) and at the end as 39.0 °C
(error once again = ±0.5 °C). You then calculate the rise
in temperature as 39.0 – 28.5 °C = 10.5 °C. But you have
to assume that both of the measurements could be out by
0.5 °C in the same direction. Your total error is therefore
1 °C, and you should write the answer as 10.5 °C ±1 °C.
You might be asked to express the size of the error as
a percentage. To do this, you divide the error in the
measurement by the measurement itself, and multiply
by 100.
Here, the temperature rise was measured as 10.5 °C,
and the error was ±1 °C. Therefore:
percentage error = (1 ÷ 10.5) × 100
= 9.5%
Errors in measurement are not the only important sources
of error in biology experiments, and you can read more
about this in the section on identifying significant sources
of error on page 261.
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The meniscus is at, or very
nearly at, half way
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between graduation lines,
so you can read the
volume as 5.55 cm 3 . But it
could possibly be closer to
5.50 or 5.60.
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cm 3
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Figure P1.11 Estimating uncertainty in measurement when
using a scale.
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5
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Your reading error can therefore be up to half the smallest
division on the 2 scale. The error here is therefore 2 ± 0.05 cm 3 .
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Here the meniscus is just
above the 5.5 line, but you
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cannot read how much
above. In this case you
read to the nearest ruling,
which is 5.50 cm 3 .
Recording quantitative results
Most of the experiments that you will do, either during
your course or in the practical examination, will involve
the collection and display of quantitative (numerical)
results. You may be given a results table to complete, but
often you will have to design and draw your own
results table.
Table P1.1 shows a results table that you could use
for your results from the experiment investigating the
effect of enzyme concentration on the rate of activity of
rennin. Three repeat readings were made for each enzyme
concentration, and a mean has been calculated.
There are several important points to note about
this results table, which you should always bear in mind
whenever you construct and complete one.
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