__Descriptive Epidemiology:__

In this branch of epidemiology, data for disease is collected and a Hypothesis is formulated. It describes disease with reference to person, place and time. It also tells about the frequency, patterns and trends(severity) of the diseases.

__Analytical Epidemiology:__

In this branch, determinants(risk factors) of the disease are analyzed with the help of 3 studies;

**Case Control…..**In this method, hypothesis is checked to prove it correct by the means of, “Odd’s Ratio(OR)”. If OR is more than 1, hypothesis is correct. If OR is less than 1, hypothesis is wrong. If OR is equal to 1, it means that there is no relationship b/w the components of your hypothesis. E.g. a hypothesis of relating lung cancer with water. If OR is less than 1, it is called as

*protective factor.*Case control study is cheaper and easily applicable.

**Cohort studies….**Cohort study is more reliable. In this, relative risk is calculated. It is same as OR.

**Cross sectional study…..**This study is not important.

__Experimental Epidemiology:__

Lethal studies are never conducted in experimental epidemiology. In this branch, medicinal studies are conducted to check the efficacy of the drugs and to compare b/w two drugs. Drawback of this study is the ethical issues. Very first experimental study was conducted on “sulpha drugs”.

__Theoretical Epidemiology:__

In this branch, whole of the research work is presented in the form of research article or research paper. Data is always represented in the form of rates, ratios and proportions.

**Rates…..**It expresses the change in one quantity as compared to the other quantity. There are 5 components of the rate;

*Numerator(*subset value),

*denominator(*whole part),

*Time specification, Place specification*and

*constant multiplier*(There should be no decimal point after multiplying with the constant multiplier. For

*rare diseases,*constant multiplier is always larger and for

*common diseases,*constant multiplier is always smaller.

**Ratios….**It is relationship b/w two random quantities. Numerator is not a part of denominator. Ratio is expressed as 1:1.

**Proportion…**It is relationship b/w one part and the whole. It is always expressed in percentage.

Morbidity is only calculated by two ways…..

**incidence rate(IR)…..prevalence rate(PR).**$Incidence\; Rate(IR)=total\; no.\; of\; new\; cases\; of\; disease*place*time*10n/Total\; population\; at\; risk$

$Prevalence\; rate(PR)=no.of\; all\; cases\; of\; disease*time*place*10n/total\; population\; at\; risk$

**Case fatality**means to estimate how much this case is lethal. It is expressed by the case fatality rate(CFR). CFR is measured for some particular disease. It is calculated by the following formula;

$CFR=total\; deaths\; from\; particular\; disease*time*place*10n/Total\; cases\; of\; particular\; disease$

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