Epidemiological Methods

It is a report of a single (or a handful) case of unusual or new findings (in scientific meetings for others to learn from)

E.g. reporting the use of OCP can cause DVT;

Advantages

• We can aggregate cases from disparate (unrelated or incomparable) sources to generate Hypothesis and describe new syndromes. Example: Hepatitis, AIDS

Limitations

• We cannot test for statistical association because there is no relevant comparison group

Report of multiple/accumulated cases of similar findings; i.e. cases series reports a single individual or a group of individual with the same diagnosis

It assesses prevalent disease

Also reports on unusual or new conditions, as well

Advantages

• Useful for hypothesis generation
• Informative for very rare disease with few established risk factors

Disadvantages

• Cannot study cause and effect relationships
• Cannot assess disease frequency

Measures disease and exposure simultaneously in a well-defined population i.e. it assess the prevalence of disease and the prevalence of risk factors at the same point in time in a defined population

It is the simplest form of an observational study.

It is based on a single examination of a cross-section of population at one point in time – the results of which can be projected on the whole population provided the sampling was done correctly.

Advantages

• They cut across the general population, not simply those seeking medical care
• Good for identifying prevalence of common outcomes, such as arthritis, blood pressure and allergies
• Not costly to perform and does not require a lot of time
• Contains multiple variables at the time of ‘data snapshot’
• Many findings and outcomes can be analyzed to creates new theories/studies or in-depth research
• There is no loss of follow-up

Disadvantages

• Difficult to determine temporal relationship between exposure and outcome (because of lack of time element; i.e. data is not collected over a period of time)
• Prevalence-incidence bias (a.k.a. Neyman bias). Especially in the case of longer-lasting disease, any risk factor that results in death will be under represented among those with the disease

A retrospective study; a.k.a. case-history study

You have:

• People with a disease (= Case) e.g. Cancer of the lungs
• People with another disease (control) e.g. stomach cancer
• A suspected cause e.g. Cigarette smoking

You want to test if the suspected cause is the actual cause.

So, you impute your data like this:

Next, do your analysis. For your level,

Next, interpretation

• If OR = 1, no relationship between cigarette smoking and lung cancer
• If OR >1, smoking is a cause cancer; i.e. the more you smoke the more likelihood of developing cancer
• If OR <1. Smoking is protective; the more you smoke the less likelihood of developing cancer

Advantages of case-control

• Used to study rare diseases
• Used for disease with long-incubation period
• Easy to carry out; no manipulation of studies
• Quick to carry out
• Cheap
• Use to establish relationship btw multiple risk factors

Disadvantages

• Missing information, especially for long-period cases
• Biased, in terms of selection of cases and control
• Misclassification of cases and control
• Unable to calculate incidence rate

A prospective study

Here, you deal with healthy populations, who may be at risk of having a disease, with exposure to a particular risk factor. This is unlike the case-control study where the subjects are diseased

E.g. correlation between alcohol consumption and cirrhosis

• 2 Groups: A group of healthy patients consume alcohol; the other group of equal number do not consume alcohol. After monitoring both groups, determine the number of patients that have cirrhosis in both alcoholic group & non-alcoholic groups

Analysis: Relative Risk (RR)

Interpretation

• RR = 1; No relationship between alcohol intake and liver cirrhosis
• RR > 1; more intake of alcohol, more risk of liver cirrhosis
• RR < 1; alcohol is protective i.e. more alcohol, less cirrhosis

Advantages

• IR can be calculated
• ATTRIBUTABLE RISK can also be calculated
• RELATIVE RISK can be calculated
• Dose-response ratios can also be calculated
• Gives a more direct estimation of the risk from exposure to each factor
• Several possible outcomes related to exposure can be studied simultaneously

Disadvantages

• Very expensive
• Take long time to complete (e.g. 20-30 years)
• More difficult to do
• Involves a large no. of people
• Generally not suitable for investigating uncommon diseases or diseases with low incidence in the population.
• Problem of attrition e.g. people dropping out of the study
• Ethical problems
• Problem with follow-up