Population or target population: A population consists of all elements -individuals, items, or objects- whose characteristics are being studied. The population that is being studied is also called target population.
Sample: A portion of the population selected for study.
Survey: The collection of information from the elements of a population or a sample.
Variable: A characteristic under study that assumes differents values for different elements.
Examples of variables are the incomes of households, the number of houses built in a city per month during the past year, the makes of cars owned by people,...
A variable may be classified as quantitative or qualitative:
Qualitative or non-numeric: Data are called qualitative or non-numeric if they can not be written down as numbers.
The status of an undergraduate college student is a qualitative variable because a student can fall into any one of four categories: freshman, sophomore, junior, or senior.
Quantitative or numeric: Data are called quantitative or numeric if they can be written down as numbers
Quantitative measurements divide further into two types:
The number of cars sold in any day at a car dealership is a discrete variable because the number of cars sold must be 0,1,2,3,... and we can count it. The number of cars sold cannot be between 0 and 1, or between 1 and 2,...
The time taken to complete an examination is an example of continuous variable because it can assume any value, let us say, between 30 and 60 minutes.