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.

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.

Quantitative or numeric: Data are called quantitative or numeric if they can be written down as numbers

Quantitative measurements divide further into two types:

• Discrete variable: A variable whose values are countable. In other words, a discrete variable can assume only certain values with no intermediate values.
  
  
• 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,...
  
  
• Continuous variable: A variable that can assume any numerical value over a certain interval or intervals.
  
  
• 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.