Cambridge Pre-U Syllabus - Cambridge International Examinations

54
Cambridge Pre-U Draft
Skill Area Evaluation of procedures and data
Sub-skill
Identifying limitations and sources of error
Candidates should be able to:
• make criticisms of the experimental procedure
• evaluate the effectiveness of control of variables and thus the confidence with which conclusions might
be drawn
• identify the most significant sources of error in an experiment
• estimate, quantitatively, the uncertainty in quantitative measurements
• express such uncertainty in a measurement as an actual or percentage error
• show an understanding of the distinction between systematic errors and random errors
• Identify anomalous values in provided data and suggest appropriate means of dealing with such
anomalies
• Within familiar contexts, suggest possible explanations for anomalous readings
• Identify the extent to which provided readings have been adequately replicated, and describe the
adequacy of the range of data provided
• Use provided information to assess the extent to which selected variables have been effectively
controlled
• Use these evaluations and provided information to make informed judgements on the confidence with
which conclusions may be drawn.
In a table or graph of data, candidates should be able to identify values which are clearly anomalous, and
suggest strategies for dealing with such anomalies, including repeating the experiment or omitting the
affected replicate. Where investigations are set in familiar contexts that candidates are expected to have
explored during the course, candidates may be asked to suggest possible causes for such anomalies (above
and beyond ‘investigator error’), and will be rewarded for answers derived from their own experience of
problems intrinsic in the particular investigation.
Candidates should be used to looking at experiments and assessing the relative importance of errors in
measurement or in making observations so that they can judge which sources of error are most important.
Candidates should be familiar with simple means of estimating error, such as the errors intrinsic in
measuring devices or in the observer’s ability to observe, or in experiments where limitations of the method
introduce errors (e.g. heat loss when trying to assess the energy content of biological materials). They
should be able to express these errors in standard forms such as length = 73 mm ± 1 mm, or temperature
increase = 14 °C ± 4 °C.
Candidates should be able to suggest which of the sources of error described are likely to be systematic
errors such as those resulting from thermometers that consistently read 1 °C above actual temperature, or
candidates who read volumes to the wrong part of the meniscus, as well as those which are likely to be
random errors due to the variability of biological materials, or random variations in environmental conditions
such as room temperature.
For key controlled variables, candidates should be able to give a realistic estimate or appraisal of how
effectively the variable was controlled, for example, how constantly the temperature was maintained across a
number of samples, and from this, give an indication of the confidence that they would have in any
conclusions drawn.