SAMPLING METHODS AND SAMPLING ERRORS
1. Meaning of Sampling
Sampling is the process of selecting a representative subset (sample) from a larger population to draw conclusions about the entire population. It is used when studying the whole population is time-consuming, costly, or impractical.
2. Objectives of Sampling
To save time and cost
To obtain accurate and reliable results
To enable detailed investigation
To make research feasible and manageable
3. Sampling Methods
Sampling methods are broadly classified into:
A. Probability Sampling Methods
Each unit in the population has a known and equal chance of selection.
1. Simple Random Sampling
Every unit has an equal chance of selection
Selection through lottery method or random numbers
Merits:
Free from bias
Easy to understand
Demerits:
Not suitable for large populations
Requires complete population list
Example: Selecting 50 students randomly from a class list.
2. Stratified Sampling
Population divided into homogeneous groups (strata)
Random samples drawn from each stratum
Merits:
More representative
High accuracy
Demerits:
Time-consuming
Requires prior knowledge of population
Example: Sampling students separately from science, commerce, and arts streams.
3. Systematic Sampling
Selecting every kth unit after a random start
Merits:
Simple and quick
Even coverage of population
Demerits:
Can be biased if population has a pattern
Example: Selecting every 10th employee from a list.
4. Cluster Sampling
Population divided into clusters
Entire clusters are randomly selected
Merits:
Economical for large areas
Saves time
Demerits:
Less accurate than stratified sampling
Example: Selecting certain schools and surveying all students in them.
B. Non-Probability Sampling Methods
Selection is not based on chance, and probability of selection is unknown.
1. Convenience Sampling
Sample chosen based on ease of access
Merits:
Cheapest and quickest
Demerits:
Highly biased
Low reliability
Example: Surveying people at a shopping mall.
2. Judgment (Purposive) Sampling
Researcher selects units based on judgment
Merits:
Useful for expert studies
Demerits:
Subjective and biased
Example: Selecting experienced teachers for curriculum study.
3. Quota Sampling
Population divided into groups
Fixed quotas assigned for each group
Merits:
Ensures representation
Demerits:
Selection within quota is biased
Example: Interviewing fixed numbers of men and women.
4. Snowball Sampling
Existing subjects recruit future subjects
Merits:
Useful for hidden populations
Demerits:
Lack of representativeness
Example: Studies on drug users or rare diseases.
4. Sampling Errors
Sampling error refers to the difference between sample results and actual population values.
Types of Sampling Errors
1. Random Sampling Error
Occurs due to chance variations
Decreases with larger sample size
Example: Sample average income differs slightly from population average.
2. Systematic Error (Bias)
Occurs due to faulty sampling design
Consistent and predictable
Causes:
Biased selection
Incomplete sampling frame
Non-response
3. Non-Sampling Errors
Errors not related to sample selection.
a) Response Error
Incorrect answers from respondents
b) Measurement Error
Faulty instruments or unclear questions
c) Processing Error
Errors in coding, tabulation, or analysis
5. Minimizing Sampling Errors
Use probability sampling
Increase sample size
Ensure proper sampling frame
Train investigators
Use pilot surveys
6. Importance of Sampling
Makes large studies possible
Reduces cost and time
Ensures accuracy if properly designed
Useful in social, economic, and business research
7. Conclusion
Sampling is a vital research technique that enables reliable conclusions about a population using limited data. Proper selection of sampling methods and control of sampling errors are essential for the validity and accuracy of research findings.
1. Sampling means
a) Studying the entire population
b) Selecting a part of population
c) Census method
d) Data tabulation
✅ Answer: b) Selecting a part of population
2. A sampling method where each unit has equal chance of selection is called
a) Judgment sampling
b) Convenience sampling
c) Random sampling
d) Quota sampling
✅ Answer: c) Random sampling
3. Which of the following is a probability sampling method?
a) Convenience sampling
b) Quota sampling
c) Snowball sampling
d) Stratified sampling
✅ Answer: d) Stratified sampling
4. In systematic sampling, units are selected
a) Randomly without order
b) By researcher judgment
c) At regular intervals
d) From each stratum
✅ Answer: c) At regular intervals
5. Dividing population into homogeneous groups is called
a) Cluster sampling
b) Stratified sampling
c) Systematic sampling
d) Quota sampling
✅ Answer: b) Stratified sampling
6. Which sampling method is best for geographically scattered populations?
a) Simple random sampling
b) Stratified sampling
c) Cluster sampling
d) Judgment sampling
✅ Answer: c) Cluster sampling
7. Convenience sampling is a type of
a) Probability sampling
b) Random sampling
c) Non-probability sampling
d) Systematic sampling
✅ Answer: c) Non-probability sampling
8. Snowball sampling is mainly used for
a) Large populations
b) Hidden or rare populations
c) Educated respondents
d) Small samples only
✅ Answer: b) Hidden or rare populations
9. Sampling error refers to
a) Error in data collection
b) Difference between population and sample results
c) Data entry mistake
d) Measurement mistake
✅ Answer: b) Difference between population and sample results
10. Which error occurs due to chance and reduces with larger sample size?
a) Systematic error
b) Random sampling error
c) Non-sampling error
d) Measurement error
✅ Answer: b) Random sampling error
11. Bias in selection leads to
a) Random error
b) Systematic error
c) Processing error
d) Response error
✅ Answer: b) Systematic error
12. Errors arising from wrong answers given by respondents are called
a) Sampling error
b) Measurement error
c) Response error
d) Processing error
✅ Answer: c) Response error
13. Which of the following is NOT a sampling method?
a) Stratified sampling
b) Random sampling
c) Census method
d) Cluster sampling
✅ Answer: c) Census method
14. Selecting every 5th unit from a list is an example of
a) Random sampling
b) Stratified sampling
c) Systematic sampling
d) Quota sampling
✅ Answer: c) Systematic sampling
15. The main advantage of sampling is
a) Complete accuracy
b) Saves time and cost
c) No errors
d) Studies whole population
✅ Answer: b) Saves time and cost
16. Non-response leads to
a) Sampling error
b) Systematic bias
c) Random error
d) No error
✅ Answer: b) Systematic bias
17. Which method uses researcher’s judgment?
a) Random sampling
b) Systematic sampling
c) Judgment sampling
d) Stratified sampling
✅ Answer: c) Judgment sampling
18. Quota sampling ensures
a) Equal chance to all
b) No bias
c) Fixed representation of groups
d) Random selection
✅ Answer: c) Fixed representation of groups
19. Which error occurs during coding and tabulation?
a) Sampling error
b) Response error
c) Processing error
d) Measurement error
✅ Answer: c) Processing error
20. Increasing sample size will
a) Increase random error
b) Increase bias
c) Reduce random sampling error
d) Eliminate all errors
✅ Answer: c) Reduce random sampling error
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