Measuring Disease Frequency

Frequency: the number of times that something occurs.
Frequency distributions provide:
- The different values of a variable.
  - Ex: the different intervals of pulse/min.
- The number of observations within each value.
  - The number of cases (observations) within each pulse/min interval.

Measures for morbidity (disease) and mortality (death) are ratios, proportions, and rates.
All are based on a single formula: $(x / y) \times 10^n$
- $10^n$ is based on your population size of interest.
- i.e., $10^0 = 1$ , $10^1 = 10$ , $10^2 = 100$ , $10^3 = 1000$

The most common rate to measure disease frequency and is a measure of the risk or probability of becoming a case.
Only refers to NEW cases.
Must include unit of time in final expression.
Formula:
$Incidence = \frac{new \ cases \ / \ given \ time}{population \ at \ risk \ / \ same \ given \ time} \times (10^n)$
Example: There are 200 new cases of an illness in a population of 20,000 in a year. What is the incidence? Use the multiplier of $10^3 = 1000$
$(\frac{200}{20,000}) \times 1000 = 10$ new cases of illness per 1000 population per year.

Formula:
$Prevalence \ rate = \frac{new + pre-existing \ cases \ / \ given \ time}{population \ at \ risk \ / \ same \ given \ time} \times (10^n)$
Prevalence rate is better at describing chronic (long term) disease than incidence as it includes all cases.
Example: A nursing home has 10 new cases of Klebsiella pneumonia occur in January. For the month of January, there were a total of 15 cases of Klebsiella pneumonia. The facility had 200 residents during January. Use the multiplier of $10^2$ to get a percentage.
- Incidence: $(\frac{10}{200}) \times 100 = 5\%$
- Prevalence rate: $(\frac{15}{200}) \times 100 = 7.5\%$

Measures the proportion of the population with a disease that only leaves the prevalent pool by recovery or death.
The greater the duration of illness will increase prevalence.

Measures the rapidity of occurrence or the flow of new cases into the diseased population.
Greater incidence increases prevalence.

Measure of frequency of morbidity, or speed of spread, in an at-risk population.
It is used in hypothetical predictions and during actual outbreaks of disease.
In the outbreak setting, the term attack rate is often used as a synonym for risk.
A type of incidence rate.
Formula:
$Attack \ rate = \frac{new \ cases \ / \ given \ time}{pop. \ at \ risk \ / \ at \ the \ beginning \ of \ time \ period} \times (10^2)$

Calculate attack rate for those who ate Food X compared to those who did not eat Food X.
Attack rate = $\frac{sick}{sick + well} \times 100$ during a time period

Persons in an age group with the highest attack rate are those in the population at greatest risk of the disease.

Formula:
$Crude \ Mortality = \frac{deaths \ from \ all \ causes \ / \ time}{pop. \ originating \ the \ deaths \ / \ same \ time} \times (10^n)$
In 2009, crude mortality for the world was 8.37 deaths / 1,000 / year.

Formula:
$CFR = \frac{Deaths \ among \ the \ cases \ / \ given \ time}{All \ cases \ of \ the \ disease \ / \ same \ time} \times (10^n)$
CFR is a measure of the severity or virulence of the disease.
- i.e., CFR for viruses if $10^n = 10^2$ or a %
  - Hepatitis A virus ~ 0.5%
  - West Nile virus ranges from 3% to 15%
  - Rabies virus approaches 100%