Distinguish between sample and sampling.

(i) Survey data: The investigator meets the respondents and gets the required data

(ii) Experimental data: Values of the dependent variables obtained in an experiment.

(iii) Observational data: The investigator simply observes and records the information about respondent.

3. Methods of collecting primary data:

(i) Direct method:

(a) Personal Contact Method: The investigator personally interviews the respondent.

(b)Telephone Interviewing: Telephones and mobile phones are used to collect data.

(c) Computer Assisted Telephone Interviewing (CATI): Telephone interviewing is combined with immediate entry of the response into a data file.

(d) Computer Administered Telephone Survey: A computer calls the phone number, conducts the interview, places data into a file for later tabulation.

(ii) Indirect method: The information about the respondent is collected by interviewing the third party who knows the respondent well.

(iii) Questionnaire Method: A questionnaire contains a sequence of questions relevant to the study arranged in a logical order.

(iv) Local Correspondents Method: The investigator appoints local agents or correspondents in different places to collect the information.

(v) Enumeration method: The trained enumerators or interviewers take the schedules themselves, contact the informants, get replies and fill them in their own handwriting.

4. Sources of Secondary Data:

(i) The published sources:

(a) Government Publications.

(b) International Publications.

(c) Publications of Research institutes.

(d) Journals or Magazines or Newspapers.

(ii) Population: The word population or statistical population is used for aggregate of all units or objects within the purview of enquiry.

(a) Finite Population: A population is called finite if it is possible to count or label its individuals.

(b) Infinite Population: When it is not possible to count or label, such a population is called infinite or uncountable.

5. Methods to collect secondary data:

(i) Census Method: In this method, information is collected from each and every individual in the statistical population.

(ii) Sampling method:

(a) Sample is small proportion of the population taken from the population to study the characteristics of the population.

(b) Sampling unit is the basic unit to be sampled from the population which cannot be further subdivided for the purpose of sampling.

(c) To adopt a sampling procedure it is necessary to prepare a list such that there exists, one to one correspondence between sampling units and numbers. Such a list or map is called sampling frame.

6. Probability sampling:

Probability sampling or random sampling involves a random selection process to include a unit of the population in the sample.

(i) Simple random sampling:

(a) This method selects a sample in such a way that each possible sample to have an equal probability of being selected or each item in the entire population to have an equal probability of being included in the sample.

(b) Methods of random sampling is given by, simple random sampling without replacement (SRSWOR) and simple random sampling with replacement (SRSWR).

(ii) Stratified Random Sampling:

In Stratified random sampling, the heterogeneous population of size $N$ units is sub-divided into $L$ homogeneous non overlapping subpopulations called Strata, the $i^{th}$stratum having $N_i$ units such that $N_1+N_2+.....+N_L=N$.

(iii) Systematic Sampling:

(a) In systematic sampling, the population units are numbered from $1$ to$N$ in ascending order.

(b) A sampling interval, denoted by $k$, is determined as $k=\frac{N}{n}$, where $n$ denotes the required sample size.

(c) Then $n-1$ such sampling intervals each consisting of $k$ units will be formed.

(d) A number is selected at random from the first sampling interval.

(e) Let it be number $i$ where $i\le k$. This number is the random starting point for the whole selection of the sample.

(f) The unit corresponding to $i$ is the first unit in the sample.

(g) The subsequent sampling units are the units in the following positions, $i, k+i, 2k+i,.......nk+i$.

7. Types of tables of random numbers:

(i) Kendall and Smith random number table

(ii) Tippet’s random number table

(iii) Fishers and Yates random number table

8. Non probability sampling:

Non probability sampling is the sampling procedure in which samples are selected based on the subjective judgment of the researcher.

(i) Convenience Sampling: The samples are drawn at the convenience of the investigator.

(ii) Snowball Sampling: In this type, initial group of respondents are selected. Those respondents are requested to provide the names of additional respondents who belong to the target population of interest.

(iii) Judgement Sampling: It involves “hand-picking” of sampling units by the interviewer using his judgment in the selection of the sample.

(iv) Quota Sampling: The population is divided into different groups and the interviewer assign quotas to each group.

9. Sampling error:

If $\overline{x}$ is the sample mean and $\mu$ is the population mean of the characteristic then the sampling error is $\overline{x}-\mu$. The sampling error may be positive or negative or zero.

10. Non-Sampling Errors:

(i) Non-sampling error refers to any deviation between the results of a survey and the truth which are not caused by the random selecting of observations.

(ii) Non-Sampling Error sources:

(a) Non-response

(b) Errors in measurement

11. Coverage errors:

(i) Coverage error is a type of non-sampling error that occurs when there is not a one-to-one correspondence between the target population and the sampling frame from which a sample is drawn.

(ii) Under coverage errors:

(a) The selected unit in the sampling frame is not interviewed by the investigator.

(b) The selected unit is incorrectly classified as ineligible for surveys.

(c) The unit is omitted or skipped by the interviewer.

(iii) Over-coverage errors:

(a) The sampling frame covers ineligible units.

(b) The frame may contain the same unit more than once.