The act of conducting a study begins with the collection of some
measurement or number commonly called a random variable.
Random variables are any measurable characteristic or trait, in
other words a measurement!
Example
We could measure the diameter of a tree or the
spread rate of a forest fire. The object from which we collected the data is called an
experimental unit. So for the last two examples the experimental
unit would be a tree and the forest fire.
Population
A population is the set of all values of a single measurement
(random variable) collected under certain conditions called the
environmental conditions.
example 1 |
EXAMPLE 2 |
The population is equal to the set of all diameters of all
trees on the Boise National Forest in 2006. |
The population is equal to the fire spread rate for all
forest fires on the Boise National Forest during 2006. |
|
|
In both cases our environmental conditions limit our population
to the Boise National Forest in 2006.
Sample
A sample is a subset of the population. For example, we may only
measure the diameter of 20,000 trees or collect the spread rate for
10 of forest fires on the Boise National Forest in 2006.
Although we are usually interested in the population we can often
not obtain measurements on the entire population. So a sample
provides us with some information about the population.
Remember that both populations and samples are sets of
numbers,
they are never sets of physical units or experimental units.
Example
Population is equal to the set of all trees on the Boise
national Forest in 2006. This statement is wrong; it should read the
population is equal to the set of diameters for all trees on the
Boise National Forest in 2006. |