, Answer ALL questions. Write your answers in the spaces provided.
1 The breaking strength of cables has usually been assessed by testing samples of cables
until they break (testing to destruction).
A manufacturer of carbon-fibre-reinforced polymer (CFRP) cables wants to replace
testing to destruction with a method for predicting breaking strength based on known
characteristics of a cable.
The methods to be compared are a computer simulation model and a
mathematical model.
The investigation involves selecting a sample of 18 CFRP cables. Nine of them have
their breaking strengths predicted by a computer simulation model and nine by a
mathematical model.
Each cable is then tested to destruction to determine its actual breaking strength.
The following are the percentage errors made in the predictions for each model.
A negative error means that the prediction is too low.
Previous research suggests that percentage errors such as these have skew distributions.
(a) Perform a non-parametric test to decide whether there is evidence of a difference in
the average prediction error of these models.
(9)
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3
*P69433A0320* Turn over
, Question 1 continued
Khalid, a junior researcher, instead suggested evaluating the methods by comparing
mean percentage error using a t-test.
(b) Make two comments on the suitability of this suggestion.
(2)
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Lauren, the chief engineer, suggests that, to make the comparison, it would have been
better to run both the computer and mathematical models using the same cables, and
then testing each to destruction.
(c) State, with justification, whether or not you agree with Lauren’s suggestion.
(2)
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(d) Suggest a test that Lauren could use to compare the two methods.
(2)
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(Total for Question 1 is 15 marks)
4
*P69433A0420*
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