http://www.ck12.org Chapter 1. An Introduction to Analyzing Statistical Data
and the scientific names for each species. When a characteristic can be neatly placed into well-defined groups, or
categories that do not depend on order, it is called acategorical variable(some statisticians use the word qualitative).
The last three columns of the previous figure provide information in which the count, or quantity of the characteristic
is most important. For example, we are interested in thetotal numberof each species of tortoise,or how many
individuals there are per square kilometer. This type of variable is callednumerical(or quantitative). Note that
repatriationis the process of raising tortoises and releasing them into the wild when grown to avoid local predators
that prey on hatchlings. The figure below explains the remaining variables in the previous figure and labels them as
categorical or numerical.
TABLE1.2: Explanation of Remaining Variables.
Variable Explanation Type
Climate Type Many of the islands and volcanic
habitats have three distinct climate
types.CategoricalShell Shape Over many years, the different
species of tortoise have developed
different shaped shells as an adapta-
tion to assist them in eating vegeta-
tion that varies in height from island
to island.CategoricalNumber of tagged individuals The number of tortoises that were
captured and marked by scientists
to study their health and assist in
estimating the total population.NumericalNumber of Individuals Repatriated There are two tortoise breeding cen-
ters on the islands. Through those
programs, many tortoises have been
raised and then reintroduced into
the wild.NumericalVariables can be further classified as eitherdiscreteorcontinuous. Adiscretenumerical variable can only have
values at specific values. For example, the number of tortoises reintroduced into the wild must be a whole number.
(How would you introduce half of a tortoise?!) But don’t get the wrong idea! It is possible for a variable to have
fractional values and still be discrete. Shoe sizes, for example, are discrete as their values occur at set increments:
7 , 712 , 8 , 812 etc... You should also know thatallcategorical variables are discrete.
On the other hand, the population density, which means the average number of tortoises per square kilometer, could
be any positive number. This is an example of acontinuous variable. Even though the numbers in the table have
been rounded, the number of square kilometers can, in theory, be any value depending on the size of the habitat.
The average (or mean) rainfall in a city is a continuous variable. Within a reasonable range of values, all amounts of
rainfall are possible. However, someone measuring that rainfall may only measure to the nearest centimeter, and it
might then be considered discrete. Practically speaking, anytime you measure a variable that can only be measured
in discrete values, you are effectively using a variable that is not truly continuous.
Population vs. Sample
We have already defined apopulationas the total group being studied. Most of the time, it is extremely difficult
or very costly to collect all the information about a population. In the Galapagos, how would you count ALL the
tortoises of one species? It would be very difficult and perhaps even destructive to search every square meter of the