Intro to Epi Final Formulas

Relative Risk

Risk in exposed/Risk in unexposed

Cumulative incidence in exposed/Cumulative incidence in unexposed

[A/(A+B)] / [C/(C+D)]

Odds Ratio

Odds of exposure in cases / Odds of exposure in controls

AD/BC

Incidence Rate

# of new cases of disease in a time period/person-time at risk (population at risk)

Period Prevalence Rate

#of cases present in a population at a specified time/population at risk x time

Cumulative Incidence/Risk

Number of cases/Population at the start of the time period

Point Prevalence

Incidence x Duration

Number of people with a disease/Total population at risk of having or getting the disease

Attack Rate

Number of new cases of a specified disease / Population at start of epidemic period

Secondary attack rate

Number of new cases of a specified disease / Number of people exposed to people that were already sick (size of contact population at risk)

Crude Mortality Rate

Number of deaths in a given time period/people in population x time in population x 10^n

Cause-Specific Mortality Rate

Number of deaths from a specific disease/people in population x time in population x 10^n

Case-Fatality Rate

Number of people dying from a disease/Number of people with that disease x 100

Proportionate Mortality Rate

Number of people dying from a disease/Number of deaths in the population x 100

What is epidemiology?

The study of the distribution of a disease or a physiological condition in human populations and of the factors that influence this distribution. 

What is the purpose of epidemiology?

Identify the cause of a specific disease or a group of diseases

Provide the basis for developing and evaluating preventive procedures and public health practices.

ARR (Absolute Risk Reduction)

Decrease in risk due to new treatment compared to standard treatment

I (Standard) - I (New Treatment)

Attributable Risk (AR)

Amount of disease due to an exposure in the exposed population

I (Exposed) - I (Non-exposed)

Attributable Risk Percent

Percent of incidence of disease due to an exposure in the exposed population

[I (Exposed) - I (Non-Exposed)] / I (Exposed)

Population Attributable Risk (PAR)

Amount of disease due to an exposure in the total population

I (Total population) - I (Non-exposed)

Population Attributable Risk Percent

Amount of disease due to an exposure in the total population

[ I (Total Population) - I (Non exposed) ] / I (Total population)

Sensitivity

The ability of the test to identify correctly those who have the disease

a / a+c

Specificity

The ability of the test to identify correctly those who do not have the disease

d / b+d

Positive Predictive Value (PPV)

Probability that a patient who tests positive has the disease

a / a+b

Negative Predictive Value

Probability that a patient who tests negative does not have the disease

d / c+d

Relationship between PPV and point prevalance

PPV improves when Point Prevalence of a disease increases