Level 2 Probability: Relative Risk

  1. A school dental nurse investigated student’s teeth in a primary school.

Breastfed as an infant / Normal teeth / Crooked teeth
Yes / 21 / 2
No / 16 / 7

a)What percentage of students who were breastfed as an infant now have crooked teeth?

b)What percentage of students who were not breastfed as an infant now have crooked teeth?

c)What is the relative risk of crooked teeth for a child who was a breastfed infant(compared to a non-breastfed infant)?

d)Interpret the relative risk in part c).

e)What is the relative risk of crooked teeth for a non-breastfed infant (compared to a breastfed infant)?

f)Interpret the relative risk in part f).

g)What is the percentage increase in risk for crooked teeth for a child who is not breastfed compared to a child who was breastfed?

  1. A new rub for muscle pain is being trialled. The treatment group used the rub for a week while the control group used a rub with no active ingredient.

More pain / Less pain
Treatment / 40 / 280
Control / 26 / 310

a)What proportion of people in the control group experiencedmore pain?

b)What proportion of people in the treatment group experiencedmore pain?

c)Calculate the relative risk of experiencingmore pain in the treatment group compared to the control group.

d)Interpret the relative risk in part c).

e) What is the percentage change in risk of experiencing more pain for the treatment group compared to the control group?

f) Interpret the percentage change in risk in part e).

  1. A large scale trial of a new acne medication asked participants whether they had experienced side effects while using the drug.

Side effects / No Side effects
Men / 1630 / 5550
Women / 1684 / 8232

a)What proportion of men experienced side effects?

b)What proportion of women experienced side effects?

c)Find the relative risk for men of experiencing side effects(compared to women).

d)Interpret the relative risk in part c).

e)Find the relative risk for women of experiencing side effect, using the risk for men as the baseline.

f)Interpret the relative risk in part e).

g) What is the percentage change in the risk of experiencing side effects for women compared to men?

h)Interpret the percentage change in part g).

Solutions

1a

b

c

d The children who were breastfed as infants had about 30% of the risk of having crooked teeth compared to the children who were not breastfed.

e

f The children who were not breastfed as infants had about 3.5 times the risk of having crooked teeth compared to the children who were.

g

There is a 250% higher risk of having crooked teeth for children who weren’t breastfed than for children who were.

2a

b

c

d People in the treatment group had 1.6 times the risk of more pain compared to people in the control group.

e

f There was a 60% higher risk of having more pain for people in the treatment group than for the control group. The treatment was not effective.

3a

b

c

d The men had 1.35 times the risk of side effects that the women did.

e

f The women had ¾ the risk of side effects that the men did.

g

h The women have a 26% lower risk of side effects than the men did.

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