Question

In your opinion, which of the below three study design is best to assess the relationship between smoking and CRC? Why? (6 sentences)

No plagiarism please

Cross Sectional Study

1. Verga Enterprises is conducting a study to assess the relationship between smoking and colorectal cancer (CRC). In the survey that was conducted, 20 out of 100 non-smokers had CRC, while 40 out of 60 smokers had CRC.

(a) What makes this a Cross Sectional study design?

This particular case is an cross sectional study because of the following reasons:

1. The above study took place at a single point off time
2. Manipulating variables are not present in this case. Only variables present are smokers, non smokers and CRC, non CRC
3. This case allows us to see at numerous characteristics at once in a given population.

(b) Complete the 2x2 table.

	Smokers	Non Smoker
CRC	40	20
Non- CRC	20	80
Total	60	100

(c) What are the odds of CRC among smokers?

40/20 = 2:1 -> odds of CRC among smokers

(d) What are the odds of disease among non-smokers.

20/80 = 1:4 -> odds of CRC among non-smokers

(e) Calculate the prevalence ratio.

(40÷(40+20))/(20÷(20+80)) = 10/3

(f) Interpret the prevalence ratio.

The ratio of the proportion of the persons with CRC over the proportion of smokers
(g) what is the value of the prevalence odds ratio (POR)?

8
(h) What can be concluded about the relationship between the exposure and health outcome?

Smokers are more prevalent to the CRC while non smokers are very less prevalent to CRC

Case Control Study

2. In a study to assess the relationship between smoking and CRC, Verga Enterprises examines medical records of 1000 men who were diagnosed with CRC and 1000 men who did not have CRC. 800 men who had CRC were found to be smokers, while 300 men without disease were smokers.

(a) What makes this a Case Control study design?

Case control studies are basically comparison studies and they are the first approach to test the hypothesis, here in above given design.

1 it uses control (where disease is not present) and case (where disease is present) in equal proportion

2. Here both exposure (smoking) and outcome (colorectal carcinoma) have occurred before we could start our study

3. Our study is going in backward direction i.e. from effect to cause. These all evidences proves that our study is a case-control study.

(b) Complete the 2x2 table.

Smoking risk factor	(Cases) CRC Present	(Control) CRC Absent
Present	800	300
Absent	200	700
Total	1000	1000

(c) What is the probability of event?

Cases= 800/1000*100= 80%

(d) What is the probability of a non-event?

Control = 300/1000*100= 30%

It means people who smokes, 80% of them can develop CRC. 30% of the people can develop CRC even if they don’t smoke.

(e) What is the odds-ratio (OR)/measure of association?

800*700/300*200=  9.3

odd's ratio - it measures the strength of association between risk factor and outcome, during measurement of odd's ratio we assume that disease of our interest is a rare disease, cases must be representing with the disease and controls representing without disease odd's ratio is very important parameter of analysis in case control study.

(f) Interpret the OR.

Interpretation is that the risk of colorectal cancer is 9.3 times higher in smokers in contrast to non smokers

In a study to assess the relationship between smoking and CRC in men, Verga Enterprises conducts a study wherein 100 smokers and 100 non-smokers were followed for 10 years (all the men were 60 at the start of the study). Ten men who were smokers developed CRC whereas 5 non-smokers developed disease.

(a) What makes this a Case Control study design?
(b) Complete the 2x2 table.
(c) What is the risk of developing CRC among smokers?
(d) What is the risk of developing CRC among non-smokers?
(e) What is an appropriate measure of association or risk ratio?
(f) What is the risk difference?
(g) What can be concluded about the association between the smoking and CRC?

1. A case control study is one in which one group which is exposed to a factor is compared with that which isn't ie there is a difference in exposure/outcome. In this case, one group is exposed to cigarette smoke and the second group isn't.

	CRC	no CRC
Smokers	10	90
Non-smokers	5	95

3. Risk of developing CRC in smokers(Odd's Ratio) = 10*95 / 90*5 = 3.8
4. Risk in non smokers = 90*5 / 10*95 = 0.47
5. Odd's Ratio is an appropriate measure of association between exposure and outcome.
6. The risk difference is 3.8/0.47 = 8.08 (relative risk)
7. A person who smokes is at 8 times more rush for developing CRC than non smokers.

4. In your opinion, which study design is best to assess the relationship between smoking and CRC? Why? (4-5 sentences)

Answer 1

A case control study will be the best of the three options given here. This is because it lays down a relation between cause and effect. Temporality is easily observed. The measure of outcome of exposure (smoking in this case) can be interpreted using the Odd's ratio. We can also calculate the relative risk ie how much more likely a person exposed to the factor is to develop the disease when compared to one who isn't.