SAMPLING Methods and its types and various techniques

Types if sampling

1. SAMPLING Methods presented by: simran

2. INTRODUCTION In clinical research, we define the population as a group of people who share a common character or a condition. Population : denotes the aggregate from which sample is to be taken Sampling is a process of selecting a number of individuals for a study in such a way that the individuals represent the larger group from which they were selected.

3. A Sample is defined as a smaller set of data that a researcher chooses or selects from a larger population by using a pre-defined selection method The process of selecting a sample population from the target population is called the “ sampling method ”

4. Sampling is based on the law of statistical regularity and the law of inertia of large numbers. The first law of statistical regularity states that the small numbers of items (sample) are picked from a large number of items (universe or population), the sample will tend to possess the same characteristics as that of a whole group of items. The second law of inertia states that the sample should be large enough to represent truly the entire population of the universe

5. the need for sampling Containing costs Speeding up the data gathering Improving effectiveness Reducing bias

6. Characteristics of a good sample Representativeness Efficiency Absence of sampling error Economically viable Feasibility Randomly selected Actual information provider

7. SAMPLING PROCESS STEPS Clearly Define Target Population Select Sampling Frame Choose Sampling Technique Determine Sample Size Collect Data & Assess Response Rate

8. Probability Sampling Non-probability sampling Simple Random Quota sampling Stratified random Multistage sampling Judgement sampling Snowball sampling Convenience sampling Consecutive sampling Systematic sampling Cluster sampling Sampling Techniques

9. Probability Sampling Methods Non-probability Sampling Methods Probability sampling is a sampling technique in which samples taken from a larger population are chosen based on probability theory Non-probability sampling method is a technique in which the researcher chooses samples based on subjective judgment, preferably random selection These are also known as RANDOM SAMPLING methods These are also called NON-RANDOM SAMPLING methods These are used for research which is conclusive These are used for research which is exploratory These involve a long time to get the data These are easy ways to collect the data quickly There is an underlying hypothesis in probability sampling before the study starts. Also, the objective of this method is to validate the defined hypothesis The hypothesis is derived later by conducting the research study in the case of non-probability sampling

10. Probability Sampling Probability sampling means that every item in the population has an equal chance of being included in the sample Probability or random sampling has the freedom from bias The benefit of using probability sampling is that it guarantees the sample that should be representative of the population More time-consuming and expensive than the non-probability sampling method

11. SIMPLE RANDOM SAMPLING The simple random sample means that every case of the population has an equal probability of inclusion in the sample Since the item selection entirely depends on the chance, this method is known as the “ method of chance selection ” As the  sample size  is large, and the item is chosen randomly, it is known as “ representative sampling ”

12. The two important aspects needed in simple random techniques are The first population must be homogeneous The researcher has a list of members in the accessible population The sampling frame can be used in the following methods – Lottery method The use of a table of random numbers The use of computer

13. Advantages Most reliable and unbiased methods Requires minimum knowledge of the study population Sampling errors can be computed easily Disadvantages Need to date complete list of all members of the population It may be uneconomical and time-consuming

14. STRATIFIED RANDOM SAMPLING This method is used for heterogeneous population In a stratified sampling method, the total population is divided into smaller groups to complete the sampling process T he small group is formed based on a few characteristics of the population After separating the population into a smaller groups, the statisticians randomly select the sample

15. Advantages It is often more convenient to recruit a stratified sample than a simple random sample Comparison is possible in two groups Ensure a representative sample in a heterogeneous population Disadvantages Require complete information about the population Large population is required Chance of faulty calculation strata

16. Systematic sampling It is obtained by selecting one unit at random and then selecting additional units at evenly spaced interval till sample of required size has been obtained. Advantages Convenient and simple to carry out Time-consuming and cheaper than simple random technique Disadvantage If the first Subject is not randomly selected, then it becomes a nonrandom sampling technique Sometimes may result in a biased sample

17. CLUSTER SAMPLING Cluster sampling is where the whole population is divided into clusters or groups. Subsequently, a random sample is taken from these clusters, all of which are used in the final sample Advantages This is a less expensive method It is less time consuming Easier to apply large geographical area Disadvantages The chance of error exists This may be less accurate than a simple random sample

18. Define the population Cluster the population Randomly select clusters Collect data from clusters

19. Multistage sampling Multi-stage sampling is a process of moving from a broad to a narrow sample, using a step by step process The main purpose of multi-stage sampling is to select samples that are concentrated in a few geographical regions. Once again, this saves time and money

20. NON-PROBABILITY SAMPLING Random sampling is not possible in all settings as most researchers are bound by constraints such as time, money and resources Non-probability sampling refers to techniques where the sample is assembled in a process that does not give all the individuals in the population an equal chance of being selected This type of sampling can be used when it is needed to show that a particular trait is existent in a population, qualitative, pilot or exploratory study A sample of participants or cases does not need to be representative, or random, but a clear rationale is needed for the inclusion of some cases or individuals rather than others

21. QUOTA SAMPLING Quota sampling is a non-random sampling technique in which participants are chosen on the basis of predetermined characteristics so that the total sample will have the same distribution of characteristics as the wider population The population is first divided into subgroups or quotas, just as the population is divided into strata in stratified sampling Subjects are selected conveniently (not randomly) from the strata, either proportionally or disproportionally, depending on the study requirements

22. JUDGEMENT/ PURPOSIVE/ AUTHORITATIVE SAMPLING Purposive or judgmental sampling is a strategy in which particular settings persons or events are selected in order to provide important information that cannot be obtained from other choices In this type of sampling subjects are chosen to be part of the sample with a specific purpose in mind Researchers believe that some subjects are fit for research compared to other individuals

23. Uses of purposive sampling : i t is usually used when a limited number of individuals possess the trait of interest Advantages Simple to draw samples and useful in explorative studies Save resources, as it requires less fieldwork Disadvantage Require considerable knowledge about the population It is not always a reliable sample, as conscious bias may exist

24. SNOWBALL SAMPLING In this technique, the initial study participants are asked to suggest someone else who can meet the study criteria and be willing to participate in the study This is usually done when the population size is very small and the researcher is unable to locate study participants on her own those sampling units also belong to the same targeted population

25. CONVENIENCE SAMPLING Convenience sampling is selecting participants because they are often readily and easily available & t he researcher did not choose the sample that outlines the entire population It is inexpensive and an easy option compared to other sampling techniques Convenience sampling often helps to overcome many of the limitations associated with research For example, using friends or family as part of the sample is easier than targeting unknown individuals

26. Uses of convenience study In a pilot study convenience sample is usually used because it allows the researcher to obtain basic data and trends for his study without the complication of using random sample selection methods Advantages This technique is considered easiest, cheapest, and time-consuming Disadvantages It may not be representative of the entire population so bias occurs Less reliable

27. CONSECUTIVE SAMPLING The researcher picks a single observation or a group of observation or a sample, conducts research over a period, analyzes the results, and then moves on to the next sample. This method is better than other nonprobability sample techniques because it makes a better representation of the entire population. Advantages Ensure a more representative sample Convenient and less time consuming Disadvantages Researcher has no set plan for a sample schedule Results from the sampling techniques cannot be created conclusion and interpretation about the entire population

28. References : Soben Peter 6 th Edition GHAURI, P. & GRONHAUG, K. 2005. Research Methods in Business Studies, Harlow, FT/Prentice hall. BROWN, G. H. 1947. A comparison of sampling methods. Journal of Marketing, 6, 331-337 . Charan J, Biswas T. How to calculate sample size for different study designs in medical research?. Indian J Psychol Med 2013;35:121-6. TAHERDOOST, H. 2016. How to Design and Create an Effective Survey/Questionnaire; A Step by Step Guide. International Journal of Advanced Research in Management, 5(4), 37-41.

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