Question

Tasmanian researchers conducted a case –control study to investigate the effect of dietary fat intake on skin cancer. They hypothesised that people whose dietary fat intake is low will be more susceptible to skin cancer. The study compared 500 cases of melanoma with 500 controls who were randomly selected from the state’s electoral roll. The researchers categorised the dietary fat intake into three categories High, Moderate and Low. They found that among skin cancer patients 150 were classified at the low and 80 at the high dietary fat intake whereas among control 130 were at the low and 100 were at the high.

a) Build a table to summarise the data above which will help you estimate the association between dietary fat intake and Skin cancer, pay attention to headings of columns and rows

b) Calculate the appropriate measure of relative risk (RR) of having melanoma between those consuming low fat intake versus high fat intake; calculate the appropriated measure of RR of having melanoma between those consuming medium to high fat intake and explain in words the meaning of what you found

c) Calculate the percent attributable risk due to exposure to low-dietary fat intake on Melanoma and explain in one or two sentences the meaning of your findings

d) Calculated the population attributable risk of low-fat intake on melanoma and explain in words the meaning of such finding

e) What do you think about the conclusions of the PAR regarding exposure to low fat diet to reduce melanoma? (no more than 3-4 sentences)

Answer 1

a. Table Summary :

Dietary Fat Intake	Melanoma	Control	Total
Low	150	130	280
Medium	270	270	540
High	80	100	180

b.

Dietary Fat Intake	Melanoma	Control	Total	Cumulative Incidence
Low	150	130	280	150 / 280 = 0.53
Medium	270	270	540	270/540 =0.59
High	80	100	180	80/180 = 0.44

Risk Ratio: Ratio of cumulative incidence of an event in a group and the cumulative incidence of the same event in another group.

Interpretation of Risk Ratio :

Ratio =1: No difference or negligible difference in the risk

Ratio >1: Increased risk of the outcome specified in the exposed group

Ratio <1: Decreased risk of the outcome specified in the exposed group

Formula: % Exposed group in Event 1 / % Exposed group in Event 2

b. i. Risk Ratio (RR) of having melanoma between those consuming medium to high fat intake

= 0.53 / 0.44

=1.20

Interpretation: Those who have low fat intake have 1.20 times the risk of melanoma than those having High-fat intakes.

b. ii. Risk Ratio (RR) of having melanoma between those consuming low fat intake versus high fat intake

= 0.59 / 0.44

=1.34

Interpretation: Those who have Medium fat intake have 1.34times the risk of melanoma than those having High-fat intakes.

c.Percent attributable risk due to exposure to low-dietary fat intake on Melanoma

Percent Attributable Risk: Difference between the % incidence rate of the exposed group and non-exposed group

The Incidence rate has to be computed for the same number of Patients.

Low Fat Intake	Melanoma	Control	Total	Incidence	Adjusted Incidence per 720
Yes	150	130	280	150/280	(150 * 2.57 / 280 * 2.57) = 0.53
No	270 + 80 = 350	270+100=370	720	350/720	( 350/720) * 100 = 0.48

Note : Adjusted Incidence is the Ration of Higher Sample Size / Lower Sample size

= 270/280

=2.57

Formula: ((Incidence Rate of exposed group - Incidence Rate of non-exposed group ) / (Incidence Rate of exposed group )) *100

= ( (0.53-0.48)/0.53 ) * 100

=9.43 %

Explanation: There is a 9.43 % higher chance of developing Melanoma from Low fat intake per 720 people.

d.  Population attributable risk (PAR) of low-fat intake on melanoma :

PAR = It is the proportion of incidence in the population due to exposure to the event.

Low Fat Intake	Melanoma	Control	Total	Incidence	Adjusted Incidence per 1000
Yes	150	130	280
No	270 + 80 = 350	270+100=370	720	350/720	(350*1.389)/(370*1.389) = 0.48
Total	500	500	1000	500/1000	500/1000= 0.50

Note : Adjusted Incidence is the Ration of Higher Sample Size / Lower Sample size

PAR = Incidence of general Population - Incidence of non-exposed Population

PAR = 0.5-0.48

=0.02

PAR % = ( (Incidence of general Population - Incidence of non-exposed Population ) / Incidence of general Population ) * 100

= ((0.5-0.48)/0.5) * 100

=4%

Interpretation: The overall risk of developing Melanoma for the total population due to Low Fat intake is 20 per 1000 patients.

d. We could conclude that 4% cases of Melanoma can be attributed due to Low fat intake. There by implementing a reduction in the exposure of Low fat intake to reduce Melanoma.