Sample

Studying every person is usually too slow, costly or impractical, so they collect information from a sample and use it to draw conclusions about a larger target population.

Increasing the number of representative samples usually reduces the effect of random variation.

The number of pathogens describes how many disease-causing particles or organisms are present in a sample or exposure. The number of infected cases counts infected hosts. A single case may involve many pathogens, so these numbers must not be treated as interchangeable.

Each patient must be counted once in the correct class, and the sum of the frequencies should equal the sample size.