Sampling – Types (Stats 165)

Introduction

Sampling is the process of selecting a portion or subset of a population to represent the whole. Since studying an entire population is often costly or impractical, sampling provides a quicker and more efficient way of obtaining reliable information for research or surveys.

Probability Sampling and Non-Probability Sampling

Probability sampling and non-probability sampling are the two broad types of sampling.

1. Probability sampling is also known as random sampling.
2. Non-probability sampling is also known as non-random sampling.

Systematic random sampling is also known as quasi-random sampling.

Probability Sampling

Probability sampling is the scientific technique of drawing samples from the population.

Dices

It is based on known probability, where every unit in the population has the same chance of being selected in the sample.

Forms/Types of Probability Sampling

• Simple Random Sampling
• Stratified Sampling
• Cluster Sampling

Simple Random Sampling

A sampling method in which every member of a population has an equal chance of being included in the sample is called Simple Random Sampling.

Types of Simple Random Sampling

1. Simple Random Sampling with Replacement
2. Simple Random Sampling without Replacement

Methods Used in Simple Random Sampling

i. Lottery Method:

All members of the population are numbered on slips of paper, placed in a container, and mixed thoroughly. Slips are drawn one by one according to the sample size, and those drawn constitute the random sample.

Lottery balls

ii. Random Number Method:

Each unit of the population is numbered from 1 to N, and a random sample of n is selected using a random number table or calculator.

(b) Stratified Sampling

This method is used when the population is heterogeneous.

The population is divided into strata (groups), and samples are taken from each stratum to ensure representation of all groups.

(c) Cluster Sampling

Cluster sampling 

Used when it is difficult to get the sampling frame or when the population is widely dispersed.

The population is divided into clusters, and a few clusters are randomly selected for study.

Non-Probability Sampling

Non-probability sampling is a method of sampling that does not involve random selection.

Forms/Types of Non-Probability Sampling

• Judgmental or Purposive Sampling
• Haphazard or Accidental Sampling
• Capture or Re-capture Sampling

(a) Judgmental or Purposive Sampling

Purposive sampling 

A sampling expert picks the units of the population they judge (believe or assume) to be representative of the population.

(b) Haphazard or Accidental Sampling

Samples are taken as they come along or make themselves available, without a planned selection process.

(c) Capture or Re-capture Sampling

Used to estimate the size of a population by capturing a subset of individuals, marking or tagging them, releasing them back, and later recapturing another subset.

It is commonly used in wildlife biology to estimate animal populations and is called the capture or re-capture sampling technique.

Conclusion

Sampling is an essential part of research that ensures effective data collection and analysis without studying the entire population. Whether probability or non-probability, the method chosen depends on the research purpose, the size and nature of the population, and the resources available.

References 

U24PS Stats 165 note

Stats 165 key points: By Rahmon Abass Adewale

Wikipedia contributors