Probability and Non-probability Sampling Methods

Probability Sampling Methods: Here, each and every item in the population has an equal probability (chance) being included in sample. There are five methods of probability sampling methods, as detailed hereunder:

Simple Random Sampling: This method is also known as lottery sampling where each and every item of the population has an equal chance of being included in the sample. This method is commonly used in case of a homogenous group by using random numbers. 

Systematic Sampling: Here, the entire population is arranged alphabetically, serially ordered etc., and samples appearing at a fixed interval are selected, like every 25th number from the list may be included in the sample.

Randomness is incorporated in picking the first number randomly to start with.

Stratified Sampling: When the population does not comprise of a homogenous group, this method is used by dividing the population into homogenous sub population, ensuring to avoid any overlapping.

Thereafter, random items are selected from each starta, proportionally or equally.

Cluster Sampling: In this method, the population is divided into heterogeneous groups, called clusters. Thereafter, few samples are selected using simple random sampling method.

Multi-stage Sampling: In cases, where big surveys are to be conducted which extend to a large geographic area with heterogeneous population, this method is used.

This is a multi stage method where in the first stage, few random selections are made.

For example, if we have to select a few families from across the country, first, some states are selected randomly and then, from each selected state, few districts are selected on random basis.

In the third stage, towns are selected from the selected districts and in the final stage, some families are selected from the chosen tower.

Non-Probability Sampling Methods: Under these methods, deliberate selections are made of particular items constituting a sample.

If the investigator considers that some units are not representative, these may not get equal chance of being included in the sample. There are the following methods under this category: 

• Convenience Sampling: Under this method, samples are chosen from the population based on the ease of access.

For collecting data about cinemagoers in a city, a few cinema halls may be selected and by going there, interviews of few persons may be taken for collecting relevant data.

• Judgement Sampling: Here, judgement of the investigator is used to select sample items for making a representative sample.

This method is used in case of qualitative research surveys which intend to develop hypothesis.

Quota Sampling: Under this non-probability method of sampling, the whole population is divided into homogenous groups and the interviewers are allotted quotas to be filled from each group.

The actual sample is left to the judgement of the interviewer. The quote allocated is generally proportionate to the size of the group in the population.

Any of the above-mentioned methods may be used whichever is suitable for the purpose of the research. But in case of random sampling, errors on account of personal judgement may be eliminated. 

In all cases, the sampling method used should be decided by considering the nature and scope of the enquiry, time available, cost, staff available etc.

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