24 Non probable sampling techniques

Pa . Raajeswari

Sampling is not anything, which is followed only in statistics. It is used in everyday life. When rice is purchased in provision store, a small quantity initially purchased and tested. Sometimes, the small quantity is cooked and if it is found good, then the bulk is purchased. There is always difference of opinion in using sampling as the basis for data collection and conclusion. Those who aruge against sampling state that whatever care is taken in selection of sample, cenpercent representation of the population cannot be achieved.

A sample design is a defined plan for obtaining a sample from a given population. It refers to the technique or the procedure the researches would adopt in selecting items for the sample. Sampling design may as well lay down the number of items to be included in the sample [i.e.] the size of the sample. Sample design is determined before data are collected researcher must select/prepare a sample design which should be reliable and appropriate for his research study.

OBJECTIVES OF SAMPLING

The main objectives of the sampling are as follows:

To make an inference about an unknown parameter from a measurable sample statistic.
To test the hypothesis relating to population.
To avoid the vast study about the entire population.
To obtain quick result.

SAMPLING AND ITS ELEMENTS IN THE PROCESS OF SAMPLING

Sampling is the process of drawing a sample from a larger population or universe. The following three elements are in the process of sampling

Selecting the sample
Collecting the information
Making an inference about the population

REASONS FOR USING SAMPLING TECHNIQUES

The sampling technique is used for the following reasons:

Economy: The sampling technique is less expensive than the census technique.
Less time consuming: The data can be collected and summarised more quickly by studying a sample than by studying the entire universe. Hence, the sampling technique is less time-consuming than the census technique.
Reliability: If the choice of sample units is made with due care and the matter under survey is not heterogeneous, the conclusion of the sample survey will have almost the same reliability as those of census survey
Detailed study: Since the number of sample units is fairly small these can be studied intensively and elaborately.
Sampling is a scientific technique.
Greater suitability in most situations is obtained.

CHARACTERISTICS OF A GOOD SAMPLE

To obtain worthwhile results by using sampling techniques, the sampling should possess the following characteristics.

Representativeness: A sample should fully represent the whole data. Hence, the researcher should be careful in selecting those units which have the same set of qualities and features of the population.
Independence: All units of the sample should be included independently of one another.
Adequacy: The number of units selected as sample should be sufficient to derive conclusions applicable to the whole data.
Homogeneity: The units included in sample must have likeness with other units, otherwise sample will be unscientific.

STEPS TO BE TAKEN TO MAKE THE SAMPLE USEFUL AND RELIABLE The following steps are to be taken in the process of selection of a sample:

Defining the population or universe to be surveyed: The population to be surveyed has to be defined in terms of element, nature, units, extent and time.
Specifying the sampling frame: It is necessary to specify the list of sampling units from which a sample is taken. The researcher has to give the means of representing the elements of the population, such as, telephone directory, map or any other list used for this purpose.
Specifying sampling unit: The sampling unit is to be selected, which may contain one or several population elements.
Specifying method of sampling: The sampling method used for selecting the sampling units are to be described.
Determining the size of sample: The number of elements or units of the population to be sampled is chosen.
Specifying the sampling plan: The operational procedure for selection of the sampling units has to be given.
Selection of the sample: The selection of the sample is carried out in the field.

ADVANTAGES OF SAMPLING

Sampling reduces the time and cost of research studies.
It saves the number of staffs required for field work, processing and analysing the data.
It gives much scope for a detailed study.
By proper selection of the sample accuracy of results can be ensured.
It is more convenient than census.
It provides much quicker results than census.
Sampling is more useful, when the number of units is very large and scattered.

DISADVANTAGES OF SAMPLING

Sampling technique is less accurate than the census technique.
It is not scientific to extent the conclusions derived from one set of sample to other sets which are unlike or are changeable.
Faulty method of sampling may lead to biased selection and hence false generalisation.
The sampling technique can be successful only if a competent and able scientist makes the selection. Otherwise the selection is liable to be wrong.
If the universe is small then it is not possible to make selection of the sample.
If the population is heterogeneous, then this technique cannot be used.

TYPES OF SAMPLING

There are two types in methods of sampling. They are:

Random sampling& probability sampling
Non-Random or Non-Probable sampling

NON PROBABLE SAMPLING TECHNIQUE

Non-random or non-probability sampling refers to the sampling process in which, the samples are selected for a specific purpose with a pre-determined basis of selection. This type of sampling is also required at times when random selection may not be possible. Therefore, the reliability of conclusions based on this type of sampling is less. Whenever a researcher uses this type of sampling, he would make a specific mention of it in the thesis, so that the conclusions would be evaluated accordingly.

Non – probability sampling do not follow the theory of probability in the choice of elements from the sampling population.

Non-probability sampling designs are used when the number of elements in a population is either unknown or cannot be individually identified.

In such situations the selection of elements is dependent upon other considerations.

There are four non-random designs each based on a different consideration, which are commonly used in qualitative and quantitative research.

It is also known as deliberate purposive or non-random sampling or judgement sampling. In this type of sample items are selected deliberately by researcher. There is no assurance that every element has some specifiable chance of being included.

It is not based on the theory of probability. Under this method of sampling, researcher cannot assure that every element has an equal chance of being chosen.

Non-probability or Non-random samplings are of four types. They are

Convenience sampling
Judgement sampling
Quota sampling
Snow-ball sampling

i.CONVENIENCE SAMPLING:

Some times known as grab or opportunity sampling or accidental or haphazard sampling
A type of nonprobability sampling which involves the sample being drawn from that part of the population which is close to hand. That is, readily available and convenient.
The researcher using such a sample cannot scientifically make generalizations about the total population from this sample because it would not be representative enough.
This method is also called as Chunk method. A chunk refers to the fraction of the population to be investigated. This chunk is not selected by probability but selected by judgement or by convenience.
This method of sampling involves selecting the sample elements using some convenient method without going through the rigour of sampling method. The researcher may make use of any convenient base to select the required number of samples.

FOR EXAMPLE: Sample obtained from readily available list such as car booking, telephone directories, etc. is a convenient sample.

The results obtained in this method can hardly be representative of the population.

They are generally biased and unsatisfactory. However, it is often used for pilot studies.

SUITABILITY: This method is suitable for simple purposes like testing ideas or rough impressions about a subject of interest.

ADVANTAGES:

It is the cheapest and the simplest method.
It does not require a list of population.
It does not require any statistical expertise.

DISADVANTAGES:

It is highly biased.
It is the least reliable sample method.
The findings cannot be generalized.

ii. JUDGEMENT OR PURPOSIVE SAMPLING:

Judgemental sampling is a form of convenience sampling in which the population elements are selected based on the judgement of the researcher or those conform to some criterion of interest.

The primary consideration in purposive sampling is the judgement of the researcher as to who can provide the best information to achieve the objectives of the study.

The researcher only goes to those people who in his/her opinion are likely to have the required information and be willing to share it.

This type of sampling is extremely useful when you want to construct a historical reality, describe a phenomenon or develop something about which only a little is known.

It is that method of sampling, in which the samples are drawn on the basis of personal judgement of a person. Generally the researcher uses his judgement in the choice of the samples which he thinks most suitable for his study. While choosing the samples, only the average items are considered and extreme items are omitted.

EXAMPLE: Suppose 100 boys are to be selected from a college from 1000 boys. If nothing is known about the students in this college, then the investigator may visit the college and choose the first 100 boys he meets. Or he may select 100 boys all belonging to

III year. Or he may select 25 boys from Commerce course, 25 from Science courses, 25 boys from Arts courses and 25 from Fine arts courses. Hence, when only the sample size is known, the investigator uses his discretion and selects the sample.

SUITABILITY: This method is appropriate when what is important is the typicality and specific relevance of the sampling units to the study and not their overall representatives of the population.

ADVANTAGES:

It is less costly and more convenient.
It guarantees inclusion of relevant elements in the sample.

DISADVANTAGES:

This method does not ensure the representativeness of the sample
This method is less efficient than random sampling for generalisation.

iii. QUOTA SAMPLING

The main consideration directing quota sampling is the researcher’s ease of access to the sample population.

The sample is selected from a location convenient to the researcher, and whenever a person with this visible relevant characteristic is seen that person is asked to participate in the study.

The process continues until the researchers have been able to contact the required number of respondents.

There are advantages and disadvantages with this design.

It is one of the commonly used methods of sampling in market surveys and opion polls. Though it is a non-random sampling, it combines the technique of probability sampling and purposive selection. This method is convenient and economical.

In this method, the quote has to be determined in advance and intimated to the investigator. The quote for each segment of the population may be fixed at random or with a specific basis. Normally, such a sampling method does not ensure representativeness of the population.

SUITABILITY: This method is suitable for marketing surveys, opinion poss and readership survey.

ADVANTAGES:

It is considerably less costly than probability sampling.
It consumes less time.
There is no need for a list of population.
Field work can easily be organised.

DISADVANTAGES:

It may not yield a representative sample.
It is impossible to estimate the sampling error.
The finding is not generalizable to any significant extent.
It is difficult to sample more than three variable dimensions.

iv. SNOWBALL SAMPLING:

Snowball sampling is a special type of non-probability sampling technique through which a respondent list is built-up by using an initial set of its members as informants.

Snowball sampling is the process of selecting a sample using networks.

To start with a few individuals in a group or organisation are selected and the required information is collected from them.

They are then asked to identify other people in the group or organisation, and the people selected by them become apart of the sample.

Information is collected from them, and then these people are identified other members of the group and in turn those identified become the basis of further data collection.

The process is continued until the required number or a saturation point has been reached in terms of the information being sought.

EXAMPLE 1:

Snow ball sampling is a highly specialised method of sampling. It involves starting a process with one individual or group and using their contacts to develop the sample.

EXAMPLE 2: If a researcher wants to study the problems faced by the Sri Lankan refugees or Refugees from Bangladesh or Burma, he may identify an initial group of refugees through some Government sources like collectorate, embassy and from a list of camp officer. Then he can ask each one of them to supply names of other refugees known to them and continue this procedure until he gets an exhaustive list from which he can draw a sample.

EXAMPLE 3:

SUITABILITY: This method is suitable for a study for which no sample frames are readily available.

ADVANTAGES:

It is useful for smaller populations for which no frames are readily available
It is very useful in studying social groups of various kinds.

DISADVANTAGES:

It is difficult to apply this method when the population is large.
It does not ensure the inclusion of all elements in the list.
This method does not allow the use of probability statistical methods. Elements included are dependent on the subjective choice of the originally selected respondents

CONCLUSION

Whenever a scientific study is planned it may not always be feasible to study the entire population. In such situation we need to apply some sampling technique to select samples and it’s better to select a non-probability sampling technique which has its own advantages and disadvantages. This type of sampling techniques gives no assurance that every element has some specific chance of being included. It is clear that for the non-probability samples there is no way of calculating the margin of error and the level of confidence. Non-probability sampling procedures are much less desirable and easy to carry out.

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