- 1. POPULATION AND SAMPLING BY: MART NIKKI LOU M. MANTILLA Methods of Research- Population and Sampling
- 2. Methods of Research- Population and Sampling OUTLINE OF PRESENTATION: • POPULATION • POPULATION PARAMETERS a. Average b. Mean c. Median d. Mode e. Range f. Standard Deviation (SD) • ALL ABOUT SAMPLING
- 3. POPULATION The entire aggregation of items from which a samples can be drawn is known as population. In sampling, the population may refer to the units, from which the sample is drawn. A population of interest may be the universe of nation or cities. The target group to which the findings (of a study) would ultimately apply or; Is the term statisticians use to describe a large set or collection of items that have something in common. “N” represents the size of the population Methods of Research- Population and Sampling
- 4. POPULATION PARAMETERS Methods of Research- Population and Sampling Statistics such as average and standard deviations, mean, mode are some population parameters
- 5. Methods of Research- Population and Sampling Mean : (average) O The "Mean" is computed by adding all of the numbers in the data together and dividing by the number elements contained in the data set.
- 6. Methods of Research- Population and Sampling Example : O Data Set = 2, 5, 9, 3, 5, 4, 7 O Number of Elements in Data Set = 7 O Mean = ( 2 + 5 + 9 + 7 + 5 + 4 + 3 ) / 7 = 5
- 7. Methods of Research- Population and Sampling O Median : (middle) O The "Median" of a data set is dependent on whether the number of elements in the data set is odd or even. O First reorder the data set from the smallest to the largest O Mark off high and low values until you reach the middle. O If there 2 middles, add them and divide by 2.
- 8. Methods of Research- Population and Sampling O Examples : Odd Number of Elements O Data Set = 2, 5, 9, 3, 5, 4, 7 O Reordered = 2, 3, 4, 5, 5, 7, 9 ^ O Median= 5
- 9. Methods of Research- Population and Sampling O Examples : Even Number of Elements O Data Set = 2, 5, 9, 3, 5, 4 O Reordered = 2, 3, 4, 5, 5, 9 ^ ^ O Median = ( 4 + 5 ) / 2 = 4.5
- 10. Methods of Research- Population and Sampling O Mode : (most often) O The "Mode" for a data set is the element that occurs the most often. O It is not uncommon for a data set to have more than one mode. O This happens when two or more elements occur with equal frequency in the data set.
- 11. Methods of Research- Population and Sampling O Example : O Data Set = 2, 5, 9, 3, 5, 4, 7 O Mode = 5 O Example: O Data Set = 2, 5, 2, 3, 5, 4, 7 O Modes = 2 and 5
- 12. Methods of Research- Population and Sampling O Range : O The "Range" for a data set is the difference between the largest value and smallest value contained in the data set. O First reorder the data set from smallest to largest then subtract the first element from the last element.
- 13. Methods of Research- Population and Sampling Examples : O Data Set = 2, 5, 9, 3, 5, 4, 7 O Reordered = 2, 3, 4, 5, 5, 7, 9 O Range = ( 9 - 2 ) = 7
- 14. SAMPLE Methods of Research- Population and Sampling Sample: is that part of the target population which is actually enquired upon or investigated1. Or Sample: is a subset of population, selected in such a way that itis representative of the larger population2
- 15. Methods of Research- Population and Sampling Sampling: is the process of selecting a small number of elements from a larger defined target group of elements such that the information gathered from the small group will allow judgments to be made about the larger groups. conclusions based on the sample results may be attributed only to the population sampled*.
- 16. Methods of Research- Population and Sampling • Sampling unit: is the unit of selection • Unit of study or element: is the subject on which information is obtained. • Sampling frame: list of all sampling units in the target population is called a sampling frame. • Sample size: the number of units or subjects sampled for inclusion in the study is called sample size. • Sampling technique: Method of selecting sampling units from sampling frame
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- 19. NEED FOR SAMPLING Methods of Research- Population and Sampling 1. Complete enumeration may not be possible. 2. Resources: Lower cost, Lesser demand on personnel. 3. Speed: Faster results due to lesser coverage. 4. Reliable information: Due to small size - better trained personnel, more accurate methods, better supervision.
- 20. Methods of Research- Population and Sampling To draw conclusions about population from sample, there are two major requirements for a sample. a) Firstly, the sample size should be large. b) Secondly, the sample has to be selected appropriately so that it is representative of the population. Sample should have all the characteristics of the population.
- 21. DISADVANTAGE OF SAMPLING Methods of Research- Population and Sampling 1. Sampling entails an argument from the fraction to the whole. Validity depends on representativeness of the sample. 2. Fails to provide precise information in case of small segments containing few individuals. 3. Not necessary in studies where complete enumeration is needed. 4. May cause a feeling of discrimination among the subjects who are not included in the study.
- 22. TYPES OF SAMPLES Methods of Research- Population and Sampling
- 23. SIMPLE RANDOM SAMPLES Methods of Research- Population and Sampling Equal probability of selection of units for inclusion in the study Requires a list of all sampling units (sampling frame) Each individual is chosen randomly. Methods: a. Lottery method (possible for finite population) b. Random number tables c. Software that generate random numbers
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- 27. Methods of Research- Population and Sampling Simple random method • With replacement • Without replacement Advantage • Very scientific method • Equal chance of all subjects for selection Disadvantage • Requires sampling frame
- 28. SYSTEMATIC RANDOM SAMPLING Methods of Research- Population and Sampling Systematic sampling is a commonly employed technique, when complete and up to date list of sampling units is available. A systematic random sample is obtained by • Selecting the first unit on a random basis • Then others are included on the basis of sampling interval I = N/n.
- 29. Methods of Research- Population and Sampling For example, if there are 100 patients (N) in a hospital and to select a sample of 20 patients (n) by systematic random sampling procedure, • Step 1: write the names of 100 patients in alphabetical order or their roll numbers one below the other. • Step 2: sampling fraction: divide N by n to get the sampling fraction (k).In the example k=100/20 = 5. • Step 3: randomly select any number between 1 to k i.e. between 1 to 5. Suppose the number we select is 4. • Step 4: patient number 4 is selected in the sample. • Step 5: Thereafter every 4+k th patient is selected in the sample until we reach the last one.
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- 31. Methods of Research- Population and Sampling Advantage: • easy to draw, simplicity. • assurance that the population will be evenly sampled. Disadvantage: • Requires sampling frame.
- 32. STRATIFIED RANDOM SAMPLING Methods of Research- Population and Sampling • Preferred method when the population is heterogeneous with respect to characteristic under study. • Population is divided into groups or strata on the basis of certain characteristics. • A simple random sample is selected from each strata. • Ensures representation of different strata/ groups in the study population. • Can be done by selecting individuals from different strata in certain fixed predetermined proportions.
- 33. Methods of Research- Population and Sampling • For example, if we draw a simple random sample from a population, a sample of 100 may contain • 10 to 15 from high socioeconomic group • 20 to 25 from middle socioeconomic group • 70 to 75 from low socioeconomic group • To get adequately large representation for all the three socio economic structures, we can stratify on socioeconomic class and select simple random samples from each of the three strata.
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- 35. Methods of Research- Population and Sampling Advantage: • All groups, however small are equally represented. • When we want to highlight a specific subgroup within the population. Ensures presence of the subgroup. • Observe existing relationships between two or more subgroups. • Can representatively sample even the smallest and most inaccessible subgroups in the population. To sample the rare extremes of the given population. • Higher statistical precision compared to simple random sampling. (d/t lesser variability). So less time and money. Disadvantage: • Requires a sampling frame for each stratum separately. • Requires accurate information on proportions of each stratum
- 36. MULTISTAGE RANDOM SAMPLING Methods of Research- Population and Sampling Multistage sampling refers to sampling plans where the sampling is carried out in stages using smaller and smaller sampling units at each stage. As the name suggest, multistage sampling is carried out in steps. This Method is regularly use in conducting national surveys on large scale. It is an economical and time saving method of selecting a sample out of widely spread population. In this method first the population will be divided on the state basis, then districts, then cities, then locality, wards, individuals who are sampled at different stages until a final sample unit.
- 37. MULTIPHASE SAMPLING Methods of Research- Population and Sampling Is slightly different from multi-stage sampling. With multi- phase sampling, the sampling unit at each phase is the same, but some f them are interviewed in detail or asked more questions than others ask. In other words, all the members of the sample provided basic information and some of them provided more and detailed information.
- 38. CLUSTER SAMPLING Methods of Research- Population and Sampling The population is divided into subgroups (clusters) like families. A simple random sample is taken of the subgroups and then all members of the cluster selected are surveyed. Cluster sampling is used when the population is heterogeneous. Clusters are formed by grouping units on the basis of their geographical locations. Cluster sampling is a very useful method for the field epidemiological research and for health administrators.
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- 40. Methods of Research- Population and Sampling Types: • One stage – when all units in the selected cluster are selected. • Two stage – only some units from a selected cluster are taken using simple random or systematic random sampling. Advantages • Simple as complete list of sampling units within population not required • Low cost • Can estimate characteristics of both cluster and population • Less travel/resources required Disadvantages • Potential problem is that cluster members are more likely to be alike, than those in another cluster (homogenous). • Each stage in cluster sampling introduces sampling error— the more stages there are, the more error there tends to be
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