epidemiology with part 1(complete) 2.ppt
- 1. Cross-sectional studies Cross-sectional study is also called prevalence study. Cross-sectional study is the simplest form of observational study. It is based on single examination of cross-section of population at one point of time. If the sampling methodology is accurate, results can be projected to the entire population. They are more useful for chronic illnesses, e.g. hypertension. Cross-sectional studies save on time and resources, but provide very little information about natural history of disease and incidence of illness.
- 2. Case- control studies It start from effect and then proceed to cause Both exposure and outcome have occurred before start of the study The study proceeds backwards from effect to cause
- 3. Select subjects based on their disease status. A group of individuals that are disease positive (the "case" group) is compared with a group of disease negative individuals (the "control" group). The control group should ideally come from the same population that gave rise to the cases.
- 4. Basic steps in a case-control study 1. Selection of cases and controls 2. Matching 3. Measurement of exposure 4. Analysis and interpretation.
- 5. A 2×2 table is constructed, displaying exposed cases (A), exposed controls (B), unexposed cases (C) and unexposed controls (D). To measure association is the odds ratio (OR), which is the ratio of the odds of exposure in the cases (A/C) to the odds of exposure in the controls (B/D), i.e. OR = (AD/BC). CASE CONTROLS EXPOSED A B UNEXPOSED C D
- 6. Case with lung cancer Control without lung cancer Smokers (less than 5 ) 33 (a) 55 (b) Non-Smokers (less than 5 ) 2 (c) 27 (d) The first step is to find out 1. Exposure rates among cases =a/(ac) = 33/35 =94.2% 2. Exposure rate among the controls =b/(bd) = 55/82 =67%
- 7. If the exposure rate among the cases is more than the controls. We must see if the exposure rate among the cases is significantly more than the controls. This is done by using the chi-square test It is significant if p is less than 0.05.
- 8. Odds ratio It is a measure of strength of association between the risk factor and outcome. The derivation of the odds ratio is based on three assumptions: ● The disease being investigated is relatively rare ● The cases must be representative of those with the disease ● The controls must be representative of those without the disease.
- 9. Odds ratio a.d/b.c 33X27/55X2 = 8.1 People who smoke less than 5 cigarettes per day showed a risk of having lung cancer 8.1 times higher as compared to non-smokers.
- 10. OR is > 1- "those with the disease are more likely to have been exposed," OR close to 1 then the exposure and disease are not likely associated. OR <1-exposure is a protective factor in the causation of the disease.
- 11. Case control studies are usually faster and more cost effective Sensitive to bias (selection bias). The main challenge is to identify the appropriate control group; The distribution of exposure among the control group should be representative of the distribution in the population that gave rise to the cases.
- 12. Cohort Study It look at cause and proceed to effect study before the disease is manifest and proceed to study over a period of time for the disease to occur. Cohort means a group of people sharing a common experience. Cohort studies are often prospective studies, they can be retrospective also, or a combination of both prospective and retrospective components can be brought in.
- 13. Steps in a cohort study: 1. Selection of study subjects 2. Obtaining data on exposur 3. Selection of comparison groups 4. Follow-up 5. Analysis.
- 14. CHD Develop CHD does not develop total Smoker 84 (a) 2916 (b) 3000 (a+b) Nno-smoker 87 (C) 4913 (d) 5000 (c+d) Total 171 (a+c) 7829 (b+d) 8000 The incidence rates of CHD among smokers i.e. a/(a+b) =84/3000 =28 per 1000 The incidence rates of CHD among non-smokers i.e. =c/(c+d) =87/5000 =17.4 per 1000
- 15. Then, we must determine if the incidence rate among the smokers is significantly more than among the non-smokers by using the chi-square test.
- 16. Relative risk (RR) It is ratio of incidence of the disease among the exposed and incidence among the non-exposed. RR (incidence of disease among exposed)/ (Incidence of disease among non-exposed) =a/(ab)/c/(cd) =28/17.4 =1.6 If RR is more than 1, then there is a positive association If RR is equal to 1, then there is no association Smokers develop CHD 1.6 times more than nonsmokers.
- 17. Attributable risk (AR) This is defined as amount or proportion of disease incidence that can be attributed to a specific exposure. It indicates to what extent the disease under study can be attributed to the exposure: (incidence of disease among exposed) - (incidence of disease among non exposed)/ (Incidence of disease among exposed) =28-17.4/28 = 10.6/28 = 0.379 = 37.9% 37. 9% of CHD among the smokers was due to smoking.
- 18. Differences between case-control and cohort studies Case-control Cohort studies Proceeds from effect to Proceeds from cause to ● cause effect Starts with the disease Starts with people ● exposed to risk factor Rate of exposure among Tests frequency of disease ● exposed and those not among those exposed and exposed is studied those not exposed First approach to testing Reserved for testing ● hypothesis precisely define hypothesis
- 19. Involves small number of Involves large number of ● subjects subjects Less time and resources More time and cost ● intensive Suitable for rare diseases Difficult to conduct for ● rare diseases Yields odds ratio Yields incidence rates, ● RR, AR and population atributable risk Cannot yield information Information about more ● about diseases one other than disease is possible than selected for
- 20. Randomized Control Studies Essential elements are: Drawing up a strict protocol, selecting reference and experimental populations, randomization, intervention, follow-up assessment of outcome.
- 21. Randomization is a statistical procedure where participants are allocated into groups called study and control groups to receive or not to receive an experimental therapeutic or preventive procedure, intervention. Randomization is an attempt to avoid bias and allow comparability.
- 22. Study designs include Concurrent parallel Crossover type of study designs. In the former, study and control groups will be studied parallel whereas in the latter all the participants will have the benefit of treatment after a particular period because the control group becomes study group.
- 23. Types of randomized control studies are: Clinical trials, e.g. drug trials Preventive trials, e.g. trials of vaccines Risk factor trials, e.g. trials of risk factors of cardiovascular disease, e.g. tobacco use, physical activity, diet, etc. Cessation experiments, e.g. smoking cessation experiments for studying lung cancer.
- 24. What is bias? Bias is systematic error that comes in. Bias on the part of participants if they know they belong to study group—participant bias bias because of observer if he knows that he is dealing with study group—observer bias bias because of investigator investigator bias, if he knows he is dealing with study group. In order to prevent this, a technique called blinding is adopted.
- 25. Concept of blinding Single blind trial means participant will not know whether he belongs to study group or control group. In double blind studies, both the participant and the observer will not be aware. In triple blind study, the participant, observer as well as the investigator will not be aware