Cambridge International A Level Biology Revision Guide

Chapter P1: Practical skills for AS
Summary
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In an experiment investigating the effect of one variable
on another, the independent variable is the one that you
change and the dependent variable is the one that you
measure. All other variables should be controlled (kept
constant). The range of the independent variable is the
spread from lowest to highest value. The interval is the
distance between each value in the range. Temperature
can be kept constant or varied using a water bath. pH
can be kept constant or varied using buffer solutions.
The accuracy of a measurement is how true it is.
For example, an accurate measuring cylinder reads
exactly 50 cm 3 when it contains 50 cm 3 of liquid. The
precision of a measuring instrument is how consistent
it is in giving exactly the same reading for the same
value. The reliability of a set of measurements is the
degree of trust that you can have in them. A reliable
set of measurements are likely to be very similar if you
are able to do the same experiment again. If you are
concerned about reliability, then do at least three repeat
measurements for each value of your independent
variable, and calculate a mean.
In general, the error in any measurement is half the
value of the smallest division on the scale. For example,
on a measuring cylinder marked in 2 cm 3 divisions, the
error in any reading will be ±1 cm 3 . If you are taking two
readings and calculating the difference between them,
then the error is ±1 cm 3 for each reading, making a total
error of ±2 cm 3 .
Results tables should be constructed with the
independent variable in the first column and the
readings for the dependent variable(s) in the next
column(s). Units go in the headings, not in the body of
the table. Each value should be recorded to the same
number of decimal places. This is also the case for any
calculated values.
In a line graph, the independent variable goes on
the x-axis and the dependent variable on the y-axis.
Headings must include units. Scales must go up in even
and sensible steps. Points should be plotted as small
crosses or as encircled dots. Lines should be best-fit or
ruled between successive points. Do not extrapolate.
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Bar charts are drawn when there is a discontinuous
variable on the x-axis. Bars in a bar chart do not touch.
Frequency diagrams or histograms are drawn when
there is a continuous variable on the x-axis. Bars touch.
Conclusions should be short and to the point. They
should use the results to answer the question posed by
the investigation. They should not go beyond what is
shown by the results. Do not confuse conclusion with
discussion.
When describing data displayed on a graph, begin by
stating the general trend and then describe any points
at which the gradient of the curve changes. Quote
figures from from the x-axis and y-axis coordinates for
these points. Do not use language suggesting time (e.g.
‘faster’) if time is not shown on the x-axis or y-axis.
Show every small step whenever you are asked to do a
calculation.
Do not confuse mistakes with experimental errors.
Mistakes should not happen. Experimental errors are
often unavoidable, unless you have the opportunity to
use a better technique or better apparatus. Systematic
errors are those which have the same magnitude and
direction throughout the experiment, and are usually
caused by limitations in the measuring instruments.
Random errors are those which vary in magnitude and
direction during the experiment, and may be caused
by difficulty in controlling variables or in making
judgements. When asked to suggest improvements in
an experiment, concentrate on the main sources of error
and suggest ways of reducing them.
When making drawings from a microscope, a low-power
plan should show only the outlines of tissues and no
individual cells. Be prepared to go up to high power to
get more information about where one tissue ends and
another begins. High-power drawings should show as
much detail as possible, including details of individual
cells.
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