# EPIDEMIOLOGY - EVERYTHING

ODDS RATIO

Measure of association between an exposure and an outcome.

Calculated as A/B.

Used in Case Control Studies.

RELATIVE RISK

The probability of an event occurring in an exposed group vs the probability of it occurring in a non-exposed group.

Calculated as A/(A+B).

Used in Cohort studies.

INCIDENCE RATE

The proportion of new people with a disease, divided by the total number of people in the beginning of the measurements.

PREVALENCE RATE

The proportion of people with a disease, divided by the total number of people being measured.

NUMBER NEEDED TO TREAT

The number of patients who need to be treated in order to prevent one additional bad outcome.

Calculated as: NNT=1/ARR

Where, ARR=ERc-ERt

Where, ERc are the control/placebo group, and ERt are the treatment group.

SENSITIVITY

The ability of the test to detect the true positive rate - correctly identify those people with a disease.

Use the table, and go down: TP/(TP+FN)

SPECIFICITY

The ability of the test to correctly identify those without the disease

Use the table, and go up: TN/(TN+FP)

POSITIVE PREDICTED VALUE

The proportion of positive results that are true positives.

Use the table, and go to the right: TP/(TP+FP)

NEGATIVE PREDICTED VALUE

The proportion of negative results that are true negatives.

Use the table, and go to the left: TN/(TN+FN)

P-VALUE

The probability of observing a result as least as extreme as the one that was observed, given that the null hypothesis is true.

T-TEST

Used to determine whether there is a statistically significant difference in the means of two groups.

CHI-SQUARE TEST

Used to determine whether there is a statistically significant difference between the observed frequencies vs the expected frequencies.

Used for categorial variables.

PRIMARY PREVENTION

Immunisation, legally banning hazardous activities and products, education on how to be healthy

SECONDARY

Regular exams and screening tests, daily aspirins, exercise, to prevent further heart attacks/strokes, subtly modified jobs so unhealthy individuals can return to work.

TERTIARY

Chronic disease management groups, support groups, treatment itself

INTENTION TO TREAT ANALYSIS

Is an analysis of every member in a randomised control trial group, without taking into consideration adherence, non-compliance, protocol deviations, withdrawal. Therefore, the estimate of treatment effect from these studies is conservative.