Lecture #15 &16SAMPLING AND SAMPLE......pptx

Lecture #15 &16SAMPLING AND SAMPLE......pptx

1. TOPIC :SAMPLE & SAMPLING AND ITS OVERVIEW

2. Sampling 2 Sampling is a process of selecting a portion of the population to represent the entire population Or A process in which selecting a group of people from which data are collected PREPARED BY SNOBIA MANZOOR

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4. Conti…… 4 Sample : A portion from whole. “ A sample is subset of population selected for observation and analysis. Or It defines the selected group of people from which data are collected for a study .” PREPARED BY SNOBIA MANZOOR

5. Population 5 A population is entire aggregation of cases in which a researcher is interested . Target population: A entire set of individual who meet the sampling criteria. Accessible population: The portion of target population to the researcher has reasonable access . PREPARED BY SNOBIA MANZOOR

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7. Sampling criteria: 7 Sampling criteria is also referred as eligibility criteria .it include the list of characteristics that are essential for eligibility .( Inclusion criteria) E.g., ability to speak Urdu , All patients are non-hypertensive Exclusion : characteristics that cause person to be excluded .E.g., Unable to speak Urdu PREPARED BY SNOBIA MANZOOR

8. Types of Sampling Probability Non-Probability 8 Simple Random sampling Stratified Random sampling Cluster Sampling Systematic Sampling Convenience Sampling Purposive Sampling Quota Sampling Network/snowball Sampling PREPARED BY SNOBIA MANZOOR

9. Probability Sampling 9 It involves random selection in choosing the elements (member) The main feature of probability sample is that every member of population will be included the sample. In this sampling selection is based on the principle of randomization .In this bias is less because subjects are chosen randomly . PREPARED BY SNOBIA MANZOOR

10. 1.Simple Random Sampling 10 It is most basic probability sampling plan in which each member has equal chance of being selected . It is unbiased representation of a group Method : Lottery method , Random Number table PREPARED BY SNOBIA MANZOOR

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12. Advantages of simple random sampling: 12 Advantages Easy to conduct Sampling error easily measured Lack of bias Disadvantage Sometime difficulty to reach all selected in sample PREPARED BY SNOBIA MANZOOR

13. 2.Stratified Random Sampling 13 It is used for heterogeneous population in which researcher divide entire population into homogenous groups called strata and then randomly select sample from each stratum. Eg , often stratified variables are age , gender , socioeconomic status , race. PREPARED BY SNOBIA MANZOOR

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15. Advantages and Disadvantages of Stratified Random Sampling : 15 Advantages : Ensure better coverage of the population More precise sample Disadvantages: More complex to analyze result, sampling error difficult to measure PREPARED BY SNOBIA MANZOOR

16. 3.Cluster Sampling 16 Sample is selected from the population, which is divided into natural groups, called clusters. Then taking these clusters as sampling units, a cluster is drawn randomly. All the members of the cluster are included in the sample. Cluster are commonly based on geographical areas /districts , so this approach is used more often in epidemiologic research. Whenever the population elements are scattered over a wide area , and a it is impossible to obtain a list of elements , cluster sampling is used. Then random sampling is used from these clusters PREPARED BY SNOBIA MANZOOR

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18. Advantages and disadvantage of cluster sampling 18 Advantages Cheap and quick for large population Less resources required Disadvantage High sampling error , least representative PREPARED BY SNOBIA MANZOOR

19. 4.Systematic Sampling 19 A sampling method in selecting sample from larger population according to a random starting point and fixed periodic interval. Systematic random sampling can be linked to an arithmetic progression wherein the difference between any two consecutive numbers is same PREPARED BY SNOBIA MANZOOR

20. Conti……… 20 Sampling interval =population size/sample size K=N/n The process involves selecting every kth individual on list using a starting point selecting randomly. The population size divided by sample size give the k . PREPARED BY SNOBIA MANZOOR

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22. Advantages of systematic random sampling : 22 Advantages Less resources required Simple It is easy to operate because it is well structured Disadvantages : All members have not equal chance of being selected Unrepresentativeness sometimes Pattern will lead to bias sampling PREPARED BY SNOBIA MANZOOR

23. NON-PROBABILITY SAMPLING 23 It is sampling technique where all the individual does not have equal chance of being selected. In this method, the samples are taken in non-random manners. PREPARED BY SNOBIA MANZOOR

24. 1.Convenience Sampling 24 It is method which is based on availability and accessibility of study participants . It is also called accidental sampling or opportunistic sampling. It is weak approach because it is hard to control bias As name implies , the sample is selected because they are convenient Example : Stopping the people at a street corner to conduct an interview in sampling by convenience. PREPARED BY SNOBIA MANZOOR

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26. Advantages and disadvantages of convenient sampling 26 Advantages : Fewer resources required Cheap and easy Least time consuming Advantages : Fewer resources required Cheap and easy Least time consuming PREPARED BY SNOBIA MANZOOR

27. 2.Purposive Sampling: 27 In this, the selection of units is carried out by a subject matter experts who picks typical or representative units . Subjects are selected purposely & are judged to be typical of the population or particularly knowledgeable about the issues understudy . PREPARED BY SNOBIA MANZOOR

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29. Advantages and Disadvantages of purposive sampling 29 Advantages cost effective Results are more accurate as compare to other non-Probability Disadvantages Lack generalization Required considerable knowledge about population PREPARED BY SNOBIA MANZOOR

30. 3.Quota Sampling: 30 A method of selecting a sample of convenience with certain control to avoid some of the more serious biases involved in taking those most conveniently available. In this quota are setup, for example, by specifying the number of interviews from urban & rural, males & females etc . PREPARED BY SNOBIA MANZOOR

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32. 4.Network /Snowball Sampling: 32 It is special type of non-probability sampling when the desired sample characteristics is rare . Snowball sampling relies on referrals from subjects to generate additional subjects. Initial group of respondent is usually random and asked to identify others who belong to same population of interest. In this method the researcher first of all finds out one respondent then the respondent leads the researcher for the next respondent and so on . PREPARED BY SNOBIA MANZOOR

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34. Factors influencing sampling : PREPARED BY SNOBIA MANZOOR 34 1.based on nature of researcher : Inexperienced investigator Lack of knowledge Lack of adequate resources Lack of interest

35. Conti …….. PREPARED BY SNOBIA MANZOOR 35  2.Based on the nature of sample: I nappropriate sample technique Inappropriate sample size Lack of sampling frame

36. Conti …….. PREPARED BY SNOBIA MANZOOR 36 Based on the circumstances : Lack of time Lack of cooperation Disasters  Lack of geographical area

37. Sampling process : PREPARED BY SNOBIA MANZOOR 37 It is systematic procedure that helps to eliminate bias and error .It consist of following steps: 1.Define the population(N) to be selected 2.Determine the sampling frame 3.Determine the sample (n) size 4.Control error 5.Select the sample

38. Sampling error / Non-Sampling Error 38 Sampling Error (Random error ) – Sample size & distribution of character of interest in population Systematic / Non-Sampling Error (Sampling Bias ) – Information bias (Systematic problem in collecting information ) –Observational or defective measurements techniques – Errors in editing and tabulating the result PREPARED BY SNOBIA MANZOOR

39. REFRENCES: prepared by Snobia Manzoor 39 Burns, N. & Groves, S.K.(2007). understanding nursing research.(4 th ed.). Philadelphia: W .B. Saunders. Polit ,D .F .& Beck, C.T.(2006).Essential of nursing research: Methods ,appraisal, & utilization.(6 th ed.).Philadelphia: Lippincott

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