In order to collect data from the entire population

In order to collect data from the entire population (every individual in a country, all the college students every geographic area, etc.), it would be very difficult, time consuming and expensive. So, sampling is a method which is used to analyse the data in which a fixed number of observations are taken from an entire population to perform the survey and it is studied in order to draw the assumptions about the population.

When a sampling is required to be emulated from a population, it is necessary to choose which method is to be applied and the various methods of sampling can be grouped under two heads as random sampling and non-random sampling.
Random Sampling
The various sampling methods commonly used under this method are simple random sampling, stratified sampling, systematic sampling, multi-stage sampling and cluster sampling.
Simple Random Sampling.
It is a basic or a simple sampling technique whereby we select a group of subjects from the population and each of those chosen subjects are at random. In this method selection is free from personal bias and each of those members of population has an equal chance of being included in the sample. Random sampling is sometimes also referred as representative sampling. this technique is appropriate when the population from which the sample taken is homogenous. This sampling method is similar to what Bhutan Lottery because if everyone is population then everyone who has bought lottery ticket has equal chance of winning the lottery assuming that they all have one lottery each.
Advantages of using Simple Random Sampling.
• It is considered to be a fair way of selecting a sample from the given population and there is no possibility of personal biasness because every member is given the equal chance of being selected.
• In this type of sampling, there is less chance of occurrence of failures because subjects are to be randomly selected from the population.

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Disadvantages of using Simple Random Sampling.
• In order to use the random sampling, it is very difficult because it is very difficult to interview the defined group of population individually and it is also very expensive and time consuming to prepare slips for the investigator for the survey.
• In this method, usually the data required to ensure the reliability of statistics is usually large which is very difficult to collect.

Stratified Sampling.
It is a type of probability sampling method in which the researchers categorize or divide the population into different subgroups or strata and the researchers select the final subjects proportionally from the different subgroups or strata. We may expect the measurement of interest to be varied among the different subgroup of population and this has to be accounted while selecting the sample from the population in order to obtain the sample that represents the population. This type of sampling method is generally used when the population is dissimilar where a certain similar sub-population can be isolated.

Advantages of using stratified sampling method.
• It will be possible to reduce the size of the sample the achieve the precision because we will be subgrouping the population and it would be easier to collect the data from the subjects.
• In this method, since the size of the population will be reduced, we would be able to generate the data easily and the result generated will be much more precision and cheaper compared to the simple random sampling method.
Disadvantages of using stratified sampling method.
• In this method each group must contain homogenous items as far as possible otherwise the produced result will not be reliable. However, this is a very difficult task and might also cost lot of time and money.
• The items from each of the group must be selected at random basis. In this case, which might be very difficult task to achieve in absence of skilled sampling supervisors and random selection from within the subgroup cannot be ensured.

Systematic Sampling Method
Systematic sampling is a type of sampling method which is mostly used in those type of cases where there is a complete list of population from which sampling is to be drawn are available. In this method we have to calculate sampling interval and it is calculated by dividing the size of the population by the desired sample size. In this method, population can be identified based on any sort of numbers of the desired features that will suit the purpose of the study being conducted. Some of the selection criteria may include age, gender, location, education level and also profession.

Advantage of using systematic sampling method.
• This method of sampling much convenient to adopt compare to simple random sampling and stratified sampling method because the work and time involved in this sampling is relatively less.
• The result obtained by using this method has generally been at satisfactory level and is also taken into consideration that no periodic features are connected with the sampling interval.

Disadvantage of using Systematic sampling method.
• This method becomes relatively less representative design than simple random sampling if we are dealing with those type of population who have hidden periodicities.
• If the population list is random then this technique is almost like random sampling but if it’s not then sampling is not reliable.

Multi Stage Random Sampling
It is a type of sampling which is build or constructed using series of simple random samples in stages and this sampling is often more practical than simple random sampling for studies requiring location analysis, such as door to surveys. In this method large areas are first divided into smaller regions and random samples of this divided regions are collected. In the second stage, the random samples are collected from each of the divided regions which are chosen in the first stage. Then, in the third stage, a random sample of even smaller areas are taken in within each area chosen in second stage. If the stage is relatively small and if the research feels he/she can collect the data then he /she can stop in this stage and if not he/she can still continue until appropriate small areas have been chosen.

Advantage of using Multi Stage Random Sampling
• This sampling method introduces great flexibility in sampling method which lacks in other methods.
• It needs to be carried out only for those samples which are included in the first stage. It is particularly helpful for those under developed areas where no frame is generally sufficiently detailed and result is most of the time accurate.

Disadvantage of Using Multi stage random sampling
• Large number of errors are caused because it involves process of division and sub division of various groups or clusters in different stages.
• Use of this method leads to greater variability of the assumptions than any other method of sampling.

Cluster Sampling
Cluster Sampling Is a method which is used in when natural groups are present in a population and the entire population is subdivided into groups whereby we collect the random samples from each group. This type of method is usually used in market research and also used when the researchers are not able to get information about the population as a whole. There are two types of cluster sampling they are, single-stage cluster sampling whereby all the elements each group are selected. Two-stage cluster whereby a random sampling technique is applied to those selected clusters.

Advantages of using Cluster Sampling
• It is relatively less expensive to observe clusters of units in a population rather than selecting units randomly which is scattered all over.
• The loss of accuracy per person or individual is more than compensated by the possibility of studying the larger samples for the same cost.

Disadvantages of using Cluster sampling.
• In this technique, group may be composed of the units which is like one another. This type may produce large sampling error and also reduce the representativeness of the sample.
• Large number of cases from the starting point of increasing the accuracy of sample is not selected and which will lead to overlapping effect.

Non-Random Sampling Method.
The types of non-random sampling method are as follows:

Judgement Sampling
It is a non-probability sampling technique whereby the researcher selects units which are to be sampled mostly on the basis of their knowledge and professional judgement. It is used when limited number of population possess the characteristics of interest.

Advantage of using Judgement Sampling.
• Not much of special knowledge about statistics is required.
• This method is well understood and is widely used over many years.
Disadvantage of using Judgement sampling.
• In this method, there is usually no technique to evaluate the reliability of the expert or the authority.
• Since there is no thing such as randomization while collecting the data people will not have equal chance of being selected.

Convenience Sampling.
It is a type of sampling method whereby the researchers select the samples based on ease of a proximity to the researchers. This type of sampling is totally different from the random sampling.

Advantages of using Convenience Sampling.
• This type of sampling is fast and inexpensive compared to the others.
• This type is sampling mostly works within our limits.
Disadvantages of using Convenience
• It is highly exposed to selection bias and also influences beyond the control of the researcher.
• It has high level of error of sampling.

Quota Sampling.
In this method quotas are set up according to the given criteria and within the quota, the selection items are mostly dependent on the personal judgement. The cost per person is relatively small but there are many opportunities for biases which may lead to invalid results.

Advantage of using Quota Sampling.
• It is relatively easy to administer.
• It is cost effective.
Disadvantage of using Quota Sampling.
• Selection of sample is not in random.
• There are huge chances of potential biasness.

Purposive Sampling.
This is a non-probability sample which is selected based on the features of the population and the purpose of study. This type can be used while we need to reach the target quickly whereby the proportionality of the sampling will not be the main concern.
Advantage of using Purposive sampling.
• We can reach to the targeted sample quickly.
• There are various types available to perform the sampling.

Disadvantage of using Purposive sampling.
• It is difficult to defend like defending our results.
• It is vulnerable to researcher’s biasness.


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