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Contact lenses
Key Assessment Task Marking criteria
1 Your graph paper should have 5 points plotted for each of the 5
materials. See below.
2 There is an outlier for Silicone hydrogel 2: 150 OP units.
3a You should ignore the outlier.
b Because 150 OP units is very different from the other 4
measurements which lie in the range 100 to 109 OP units.
© University of York and Nuffield Foundation 2009
C21 IaS1 KAT Contact lenses H marking page 1
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4 and 6 Results
Material
Oxygen permeability /OP units
Mean /OP
units
Range / OP
units
Silicone
hydrogel 1
98
93
113
106
100
102
93 to 113
Silicone
hydrogel 2
105
outlier
106
100
109
105
100 to 109
Silicone
hydrogel 3
138
142
128
137
130
135
128 to 142
Hydrogel A
7
8
10
8
7
8
7 to 10
Hydrogel B
21
21
22
21
20
21
20 to 22
7 You should have given one of the following reasons:
There may be a slight difference in the samples – for example in
their thickness - which would change the measured value of the
oxygen permeability.
There might be a slight difference in the polymer composition which
would change the true value of the oxygen permeability.
The measuring equipment or the way it is used may give values
which are slightly different from the true value.
8 The data show that silicone hydrogel lenses are more oxygen
permeable than hydrogel lenses. For example the most oxygen
permeable hydrogel, Hydrogel B, has an oxygen permeability of only
21 OP units. But the least oxygen permeable silicone hydrogel,
Silicone hydrogel 1, has oxygen permeability 102 OP units.
(You could have chosen other examples to answer this question, but
these two are the best to choose, as they are the hydrogel with the
highest value and the silicone hydrogel with the lowest value.)
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9
Reason
Because the material itself might vary so different samples
have different oxygen permeabilities.

Because the thickness of the material might be different and
this would affect the oxygen permeability.

Because there is no true value for the oxygen permeability of
a sample of material.
Because they were poor scientists – they would have got
exactly the true value if they were careful enough.
Because the measuring equipment, or the way it was used,
might give a value that is different from the true value.

10a Hydrogel B
b Because the range of values in the set is smaller.
11a There is no real difference between silicone hydrogel 1 and
silicone hydrogel 2 because:
• the mean of the measurements for silicone hydrogel 1 is inside the
range of the measurements for silicone hydrogel 2
• the mean of the measurements for silicone hydrogel 2 is inside the
range of the measurements for silicone hydrogel 1.
11b There is a real difference between silicone hydrogel 1 and
silicone hydrogel 3 because:
• the mean of the measurements for silicone hydrogel 1 is below the
range of the measurements for silicone hydrogel 3
• the mean of the measurements for silicone hydrogel 3 is above the
range of the measurements for silicone hydrogel 1.
11c There is a real difference between hydrogel A and hydrogel B
because the mean of the measurements for hydrogel A is below the
range of the measurements for hydrogel B, and the mean of the
measurements for Hydrogel B is above the range of the
measurements for Hydrogel A.
NOW look again at the learning outcomes for this activity.
Which learning outcomes have you achieved?
© University of York and Nuffield Foundation 2009
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Which learning outcomes can you improve?
© University of York and Nuffield Foundation 2009
C21 IaS1 KAT Contact lenses H marking page 4