Scope of Statistics and Data Collection
What the domain covers
Statistics is the science of collecting, organising, presenting, analysing and interpreting data. The exam tests the scope of statistics and the distinction between primary and secondary data, plus the main collection methods.
Core rules
- Descriptive statistics summarises data; inferential statistics generalises from a sample to a population.
- Primary data are collected first-hand by the investigator; secondary data were collected by someone else for another purpose.
- Primary collection methods include direct personal interview, indirect oral interview, information from correspondents, mailed questionnaire and schedule filled by an enumerator.
- A questionnaire is completed by the respondent; a schedule is completed by the investigator or enumerator.
- Variables may be discrete or continuous, and qualitative (nominal or ordinal) or quantitative.
- A census covers every unit; a sample covers part of the population.
Worked example
An investigator visits 40 households in a village and records monthly income in a structured schedule. This is primary data because it is collected first-hand for the present study. Income is a continuous quantitative variable, although recorded in whole rupees. The study is cross-sectional because the 40 households are observed at one point in time, not repeatedly over years. The schedule, not a questionnaire, is used because the investigator fills it in.
Traps
- Calling published data primary just because it is in a report; use the collection history.
- Mixing up questionnaire and schedule.
- Saying a sample is always better than a census; it depends on cost, accuracy and feasibility.
- Forgetting that statistics cannot handle purely qualitative facts without coding.