Collection, Organisation and Presentation of Data

Primary vs Secondary Data

Primary data are collected first-hand for a specific investigation. Secondary data were collected earlier by another agency or researcher and are reused.

Population, Sample and Variable

The population is the complete group under study. A sample is a selected part of that population. A variable is a characteristic that can take different values, such as income or expenditure.

Sampling and bias

A useful sample should represent the population as closely as possible. A sample that systematically excludes important groups can produce biased results even if the calculations are perfectly correct.

Frequency Distribution

Raw observations can be organised into classes and frequencies. The frequency tells us how many observations fall into each category or class.

Diagrams and Graphs

• Bar diagram: useful for discrete categories; bars have gaps.
• Histogram: used for continuous grouped data; adjoining rectangles have no gaps.
• Pie diagram: shows components as proportions of a whole.

MCQ with explanation

Why can a poorly selected sample be misleading?
Answer: Because it may systematically over-represent or under-represent sections of the population, making the sample statistics unrepresentative of the population.

Application

A school wants to estimate monthly travel expenditure of 1,200 students. Identify the population, propose a sample and write five questionnaire items. Explain one possible source of sampling bias.