Key Points

Data are facts, especially numerical facts, collected for reference or analysis. The main purpose of data collection is to gather evidence for reaching a clear and sound solution to a problem.

Primary data is first-hand information collected by the researcher or investigator for their specific purpose. It is original data gathered directly from the source.

Secondary data is data that has already been collected and processed by some other agency or person. Using secondary data saves time and money but may not perfectly fit the current study's needs.

A survey is a method of gathering information from individuals using an instrument like a questionnaire. Its purpose is to collect data to describe characteristics of a population.

A good questionnaire should not be too long, should use simple and clear language, and avoid ambiguous or leading questions. Questions should be arranged logically, moving from general to specific.

Closed-ended questions provide options like 'Yes/No' or multiple choices, making them easy to analyze. Open-ended questions allow for detailed, individualized responses but are harder to interpret.

This method involves a face-to-face interview with respondents. It allows for clarification and has a high response rate but is expensive and can be influenced by the interviewer.

In this method, a questionnaire is sent by mail to be completed by the respondent. It is less expensive and covers a wide area, but it often has a low response rate.

An investigator asks questions over the telephone in this method. It is cheaper and quicker than personal interviews but has limited reach and does not allow for visual cues.

A pilot survey is a trial run of the questionnaire on a small group before the main survey. It helps to pre-test questions, assess clarity of instructions, and estimate the time and cost involved.

A census is a survey that includes every single element of the population under study. The Census of India, which collects demographic data, is conducted every ten years.

In statistics, a population is the entire group of individuals or items under study. A sample is a smaller, representative group selected from the population to obtain information.

Random sampling is a method where every individual unit in the population has an equal chance of being selected for the sample. This is also known as the lottery method.

In non-random sampling, the selection of sample units is based on the judgment, purpose, or convenience of the investigator. Not all units have an equal chance of being selected.

A sampling error is the difference between the result obtained from a sample and the actual value of the population. The magnitude of this error can be reduced by taking a larger sample.

Non-sampling errors are more serious as they can occur even in a census. These errors arise from issues like sampling bias, non-response, or incorrect data recording.

The Census of India and the National Sample Survey (NSS) are two important agencies at the national level. They collect, process, and publish data on major socio-economic issues.