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Risk of Low-Fat Intake on Melanoma - Question 1

Sundquist, K. Qvist, J. Johansson, S. Sundquist, J. “The long-term effect of physical activity on incidence of coronary heart disease: a 12-year follow-up study”

(a) What study design does this study employ, what is the exposure and what is the outcomes?

The study design employed is a cohort or longitudinal study in which the sample was derived from a national survey and they were selected in a random manner and face-to-face interviews were conducted between 1988 and 1989. The participants were followed up till the time that they were hospitalized due to CHD, or their death occurred due to any reason, migrated or the study ended in the December of 2000. The exposure variable that was decided by the researchers was leisure-time physical activity. It was categorized as no exercise, occasionally, once or twice a week and at least twice a week. The outcome variable that was measured was hospitalization due to coronary heart disease which was classified according to the international classification of disease 9 and 10. Participants who have history of hospitalization due to the same disease were excluded from the study.

(b) What justifications do the authors give for conducting this study?

In industrialized countries, the incidence of coronary heart disease is higher in people who are physically inactive. There is a higher risk of coronary heart disease in people who are inactive compared to the ones who are active. Being inactive is also responsible for the increase in body mass index, high cholesterol and hypertension. There is a gap in the literature regarding the association between coronary heart disease and low socio-economic status and women in Sweden. This is the gap that is addressed in the given research as a justification for the conductance of the study. as coronary heart disease is a chronic condition the study was conducted to assess the long-term effect of physical inactivity and the development of the disease. To only the effect of these two factors, the other confounding variables were adjusted, that is, age, gender, socio-economic status, smoking and body mass index.

(c) What was the proportion of those who (i) do not do any physical activity (ii) engaged in twice a week vigorous physical activity? (You are not required to report 95% confidence intervals)

The proportion has many definitions depending on the use. In epidemiology, rate ratio and proportion are the measures of the disease or deaths in a particular population or sample of the population. Proportion is a comparison of a particular group compared to the whole population. It can be expressed as a decimal, fraction or a percentage.

It can be calculated by the use of the following formula:

In the present context;

Number of persons has the level of self-reported physical activity as "I practically get no exercise at all"

total number of people under the study who were in the survey

To obtain a percentage this is multiplied by 100.

The total number of people under this category are 1208 and the sample under the survey is 5196

The proportion is calculated as, (12085196×100) = 23.24%

The same when calculated for people exercising for at least twice a weak gives(4645196×100)= 8.92%.

(d) What is the crude incidence rate of CHD in this sample what is the incidence rate in inactive men and inactive women? [4 marks]

Crude incidence rate is defined as the number of new cases at a specific site in a given population during the given period of time. It is usually calculated as per 100,000 population at risk.

At the start of the study, the whole population was at risk but was a disease-free population which was estimated to be 2,814,000. The total crude incidence of coronary heart disease in all the four-level of activities is 240.

For men, the crude count was 202 and the estimated population for the year of the survey was 1,429,000 and the rate obtained is (2021,429,000)×100,000 = 14.13

For women, the crude count was 73 and the estimated population for the year of the survey was 1,385,000 and the rate obtained is (731,385,000)×100,000 = 5.27.

(e) Calculate the relative risk of being non-active versus being “highly active” (i.e., vigorous physical activity at least twice a week) in men and in women and how would you interpret the relative risk in men and in women

Relative risk is defined as the ratio of the probability of an event of disease occurrence in the exposed group vs. non-exposed group.

For calculation of relative risk first, a contingency table should be made which looks as follows:

Factor Incidence of CHD Yes No

Inactivity a b

High-level of activity c d

The relative risk is calculated as incidence among exposed divided by incidence among non-exposed, that is

For men:

Factor Incidence of CHD Yes No

Inactivity 61 2289

High-level of activity 111 2289

Relative risk = (6123501112400) = 0.56

For women:

Factor Incidence of CHD Yes No

Inactivity 14 2437

High-level of activity 38 2437

Relative risk = (142451382475) = 0.38

The relative risk does not tell about the absolute risk but it just tells about the likelihood of the disease occurrence and from the relative risk it can be seen that men are more likely to be diagnosed with coronary heart disease compared to women.

(f) Looking at the sex and age adjusted HR in Table 3 (the HR stands for Hazard Ratio which is similar to rate-rato) (i) how would you describe the association between physical activity and CHD

The hazard ratio is the hazard that is seen in the exposed group compared to the one in the non-exposed group. Before the adjustment, it was seen that there was a drop in the incidence of coronary heart disease with the increase in leisure time activity. After the adjustment done for the age and gender the effect remains the same, that is, with the increase in leisure time activity there was a decrease in CHD. It can be said that physical activity is a protective factor against the development of coronary heart disease.

From the present longitudinal study and given sample size, it can be said that there is a causal association between physical activity and CHD and it does not occur due to chance. However, this association is not direct as there are other factors which can affect the formation of coronary heart disease.

(g) The authors removed from the analysis any person who self-rated their health as “bad” or “anywhere between good and bad” why? (no more than 60 words)

The authors have excluded at the start of the study those people who self-rate their health as bad or anywhere between good and bad. This was done to remove any bias that may have been included in the study due to physical inactivity due to any underlying disease. The underlying disease can act as a comorbidity for the development of coronary heart disease.

(h) What possible bias could have changed the estimate for the association between physical activity and CHD – at least one, and explain why.

One of the biases that could have changed the estimate for the physical activity and coronary heart disease is social desirability bias. It is a response bias in which the study participant answers the survey questions or interviews in such a manner that the answers are reported as socially acceptable good behaviour. For example, in the present study people may have reported themselves as being active even though they are not. This might under report the physical inactivity that is actually present.

(i) Do you think there is a risk of confounding bias in this research? Any answer yes or no must be justified

There is a risk of confounding bias in the present study which can be due to the presence of genetic predilection towards the development of coronary heart disease. Confounding is the distortion that happens to the association between the exposure variable and the health outcome. This variable may have an effect on the exposure variable and as well as the outcome variable. There is the influence of genes on the BMI, weight gain and other factors which influences the development of the disease. At the same time, there is a genetic influence on coronary heart disease and cause the disease independent of other factors as well.

Risk of Low-Fat Intake on Melanoma - Question 2

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

Variables Low Moderate High

Cases 150 270 80

Control 130 270 100

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.

Variables Cases Control

Low fat dietary 150 130

High fat dietary 80 100

The relative risk is calculated as incidence among exposed divided by incidence among non-exposed, that is

Relative risk is calculated as: (15023030130) = 1.2

From the result, it can be seen that there is a higher incidence of skin cancer in people consuming low fat have a higher risk of developing the disease.

Variables Cases Control

Moderate fat diet 270 270

High-fat diet 80 100

The relative risk is calculated as incidence among exposed divided by incidence among non-exposed, that is,

Relative risk is calculated as: (27054080180) = 1.13

From the result, it can be seen that there is a higher incidence of skin cancer in people consuming moderate-fat diet have a higher risk of developing the disease compared to high-fat diet.

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

Attributable risk is defined as the difference between the incidence among the exposed and the incidence among the non-exposed.

If a contingency table is formed for the same like the one below it can be derived with the formula

Variables Cases Control

Low fat dietary 150 (a) 130 (b)

High fat dietary 80 (c) 100 (d)

Percent attributable risk is [150(150+130)⁄]−[80(80+100)⁄]×100 = [150(280)⁄]−[80(180)⁄]×100 = 7.15%

Percent attributable risk keeping the population of exposure as 50% it is calculated as 7.15%

From the result, it can be seen that the actual risk of development of skin cancer in people consuming a low-fat diet and the one consuming high-fat diet is actually less.

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

If a contingency table is formed for the same like the one below it can be derived with the formula

Variables Cases Control

Low fat dietary 150 (a) 130 (b)

High fat dietary 80 (c) 100 (d)

Population attributable risk is [150(280)⁄]−[80(180)⁄]

Population attributable risk is calculated to be 0.04. it can be interpreted as the reduction in the incidence of skin cancer if the whole population would have been consuming a low-fat diet.

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)

Consumption of low-fat diet is not a protective factor against the development of skin cancer. The more there is the consumption of a low-fat diet more is the chance that people might develop skin cancer. By the deductions obtained from population attributable risk, it is seen that if all the people were consuming low-fat diet there will be a reduction in the development of skin cancer by 0.04.

Risk of Low-Fat Intake on Melanoma - Question 3

A study aims to determine the incidence of type 2 diabetes. A cohort of 200 people age 65 years or older who were initially disease-free participated in the study. One hundred and fifty people were examined at the end of 3 years. Fifty other participants from the initial cohort could not be examined, including 11 people who had died. Does this loss of participants represent a source of bias? Justify your answer.

Not being able to examine all the study participants at the end of the survey is a type of selection bias. It is called the loss to follow-up and it attributes to the internal validity in a longitudinal study. It reduces the valuable information that might be required in making the association between the exposure variable and outcome variable. It causes the difference between the expected value of an estimator and the true value which might of the interest in the study like the present one for the population at the baseline which is the source of the population. This source is reduced due to attrition causing loss to follow-up.

Risk of Low-Fat Intake on Melanoma - Question 4

A telephone survey is being administered by several interviewers in order to collect data regarding the outcome in a randomised controlled trial. Identify the key issues the researchers should have considered in order to minimise measurement error of the outcome. Discuss the impact these issues may have on the study.

Measurement error is the difference between the measured quantity and the actual value. It is a random error which is included in the study which can reflect on the result that is obtained. As researchers, there should be identification and rectification of the key issues that are present. Like the use of formula increases the measurement error. Inter and intraexaminer validity should be good and they should be trained and calibrated. The surveys should have been taken in a controlled environment and there should pilot testing before the start of the study. There will be over estimation or underestimation of the outcome variable which will either increase or decrease the association and influence the results.

Remember, at the center of any academic work, lies clarity and evidence. Should you need further assistance, do look up to our Epidemiology Assignment Help

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