Anomalous Results, Conclusions & Evaluation (Edexcel IGCSE Biology): Revision Note
Exam code: 4BI1
What is an anomalous result?
An anomalous result is a value that appears to be outside the general pattern of the results
It is usually an extreme value that is either very high or very low compared with the others
Describe an anomalous result by saying the way in which it does not fit the pattern
Saying that it is "wrong", "a mistake" or "an error" is not enough to describe an anomalous result
When you have anomalous results you should:
leave them out when you calculate a mean
ignore them when you draw a line of best fit
If you ignore a point on a graph, show this clearly, e.g. by drawing a circle around it
Repeating a reading does not stop anomalous results happening, but it helps you to spot anomalies
The question may not tell you that a value is anomalous
Examiner reports show that students lose marks by including an anomalous value when they calculate a mean
Worked Example
A student investigated the digestion of gelatine by an enzyme. The student repeated the experiment four times. The table shows the results.
Repeat | Volume of gelatine digested in cm3 |
|---|---|
1 | 0.48 |
2 | 0.53 |
3 | 1.63 |
4 | 0.47 |
Calculate the mean volume of gelatine digested in cm3.
Give your answer to two decimal places.
[3]
Two-mark answer:
(0.48 + 0.53 + 1.63 + 0.47) ÷ 4 [1 mark]
= 0.78 cm3 [1 mark]
Full-mark answer:
Repeat 3 (1.63 cm3) is anomalous, so leave it out
mean = (0.48 + 0.53 + 0.47) ÷ 3 [1 mark]
mean = 0.4933… [1 mark]
mean = 0.49 cm3 [1 mark]
Examiner Tips and Tricks
The two-mark answer calculates the mean correctly and gives it to two decimal places, but it includes repeat 3, which is anomalous. The question does not tell you there is an anomalous result, so you have to spot it.
The full-mark answer identifies repeat 3 as anomalous, leaves it out, calculates the mean of the other three repeats and gives the answer to two decimal places.
To find an anomalous result in a table, look at how the values change from one row to the next. An anomalous result will break the pattern; it may not just be the largest or smallest number.
How to write a conclusion
A conclusion says what your results show, and should relate to the aim of the investigation
Before you write it, reread the aim in the question
In your conclusion you should:
link the independent variable to the dependent variable
For example, "as the temperature increased, the rate of water loss also increased"
support your conclusion with evidence from your results by stating values from the table or graph
Only conclude what your results actually show
Be careful not to state a causal link when one hasn't been proven; sometimes a conclusion may just state that two variables are linked but cannot state that a change in one variable caused a change in another (i.e. correlation, not causation)
Do not apply the results of a conclusion too broadly; results gained from studying woodlice may not be applicable to humans, for example
Examiner Tips and Tricks
Describing your results again is not a conclusion. A conclusion answers the question the investigation was asking and then uses your results to show why.
You may be asked to evaluate a conclusion that someone else has made
To do this:
describe the data that support the conclusion
describe any data that oppose it, or that it does not cover
comment on the quality of the investigation, e.g. the sample size, whether it was repeated and whether other variables were controlled
finish with a judgement on whether the conclusion is valid
Sources of error & improvements
When you evaluate a method, use the terms accurate, reliable and valid precisely:
accurate results are close to the true value
reliable results are similar when the experiment is repeated
valid results come from an investigation that tests what it sets out to, with other variables controlled
Repeating an experiment and calculating a mean improves reliability, but it does not make each measurement more accurate
To improve accuracy, improve the method or the measurement, e.g. take readings at smaller time intervals
Parallax error happens when you read a scale from an angle, so be sure to read scales at eye level, e.g. when reading a volume of liquid in a measuring cylinder or reading a thermometer
For every source of error you identify, suggest a specific, matching improvement that would reduce it
Only suggest improving something that the given method does not already control
Examiner Tips and Tricks
Never write "human error", "be more careful", or "use better equipment". Always name the specific piece of apparatus or step.
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