seminar-sampling methods

Information about seminar-sampling methods

Published on August 11, 2014

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PowerPoint Presentation: SAMPLING METHODS SPEAKER:- Shubhanshu Gupta TEACHER I/C: Dr. B L Verma DATE:- 05/08/2014 What exactly is a “sample”?: What exactly is a “sample”? What exactly is a “sample”?: What exactly is a “sample”? A subset of the population, selected by either “probability” or “non-probability” methods. If you have a “probability sample” you simply know the likelihood of any member of the population being included (not necessarily that it is “random.” PowerPoint Presentation: TARGET POPULATION STUDY POPULATION SAMPLE Target vs. accessible populations: Target vs. accessible populations The target population is the population a researcher would like to generalize to. Often this isn’t possible, so the accessible population is used. For example, a researcher might want to target all male elementary teachers in the India, but actually collects data from the male elementary teachers in Delhi. Need for sampling: Need for sampling Complete enumeration may not be possible. Resources: Lower cost, Lesser demand on personnel. Speed: Faster results due to lesser coverage. Reliable information: Due to small size - better trained personnel, more accurate methods, better supervision. To draw conclusions about population from sample, there are two major requirements for a sample. Firstly, the sample size should be large. 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. Disadvantages of sampling: Disadvantages of sampling Sampling entails an argument from the fraction to the whole. Validity depends on representativeness of the sample. Fails to provide precise information in case of small segments containing few individuals. 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. Definitions : Definitions Population : The target group to which the findings (of a study) would ultimately apply is called population 1 Or Population is the term statisticians use to describe a large set or collection of items that have something in common 2. Sample : is that part of the target population which is actually enquired upon or investigated 1. Or Sample is a subset of population, selected in such a way that it is representative of the larger population 2 Definitions cont..: Definitions cont.. 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 Definitions (cont.): Definitions (cont.) 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 * . . PowerPoint Presentation: SAMPLING PROCESS Identifying and defining the target population Describing the accessible population & ensuring sampling frame Specifying the sampling unit Specifying sampling selection methods PowerPoint Presentation: Determining the sample size Specifying the sampling plan Selecting a desired sample PowerPoint Presentation: Target population Sampling frame Sample Population you want to generalize results to Population you have access to for your study Study population How can you get access to study population? Study actually done on? 1. 2. 3….. Types of sampling techniques : Types of sampling techniques Probability samples Non-probability samples Probability Techniques: Probability Techniques Non-probability Sampling: Non-probability Sampling Non probability sampling does not involve  random  selection Accidental or Purposive PROBABILITY SAMPLING TECHNIQUE: PROBABILITY SAMPLING TECHNIQUE Features of the probability sampling: Features of the probability sampling It is a technique wherein the sample are gathered in a process that given all the individuals in the population equal chances of being selected. In this sampling technique, the researcher must guarantee that every individual has an equal opportunity for selection. The advantage of using a random sample is the absence of both systematic & sampling bias. The effect of this is a minimal or absent systematic bias, which is a difference between the results from the sample & those from the population . Simple random sampling: Simple random sampling This is the most pure & basic probability sampling design. In this type of sampling design, every member of population has an equal chance of being selected as subject. The entire process of sampling is done in a single step, with each subject selected independently of the other members of the population There is need of two essential prerequisites to implement the simple random technique: population must be homogeneous & researcher must have list of the elements/members of the accessible population . PowerPoint Presentation: SRS with replacement (SRSWR ): In SRSWR the units selected in the earlier draws are replaced back in the population before the subsequent draws are made. Thus a unit has a chance of being included in the sample for more than once. SRS without replacement (SRSWOR) : Most common In SRSWOR the units selected in the earlier draws aren’t replaced back in the population before the subsequent draws are made. Thus a unit has only one chance of being included in the sample. The first step of the simple random sampling technique is to identify the accessible population & prepare a list of all the elements/members of the population. The list of the subjects in population is called as sampling frame & sample drawn from sampling frame by using following methods: The lottery method The use of table of random numbers The use of computer : The first step of the simple random sampling technique is to identify the accessible population & prepare a list of all the elements/members of the population. The list of the subjects in population is called as sampling frame & sample drawn from sampling frame by using following methods: The lottery method The use of table of random numbers The use of computer The lottery method… It is most primitive & mechanical method. Each member of the population is assigned a unique number. Each number is placed in a bowl or hat & mixed thoroughly. The blind-folded researcher then picks numbered tags from the hat. All the individuals bearing the numbers picked by the researcher are the subjects for the study. : The lottery method… It is most primitive & mechanical method. Each member of the population is assigned a unique number. Each number is placed in a bowl or hat & mixed thoroughly. The blind-folded researcher then picks numbered tags from the hat. All the individuals bearing the numbers picked by the researcher are the subjects for the study . Lottery method: Lottery method Lottery method PowerPoint Presentation: Different random number table: Tipetts (1927) Random Number Table Fisher & Yates (1938) Kendall & Babington Smith`s (1939) Rand Corporation (1955) table of random numbers. C.R- Rao , Mitra & Mathai (1966) table of random numbers Random number table: Random number table 76 58 30 83 64 47 56 91 29 34 10 80 21 38 84 00 95 01 31 76 07 28 37 07 61 The use of table of random numbers… This is most commonly & accurately used method in simple random sampling. Random table present several numbers in rows & columns. Researcher initially prepare a numbered list of the members of the population, & then with a blindfold chooses a number from the random table. The same procedure is continued until the desired number of the subject is achieved. If repeatedly similar numbers are encountered, they are ignored & next numbers are considered until desired numbers of the subject are achieved. : The use of table of random numbers… This is most commonly & accurately used method in simple random sampling. Random table present several numbers in rows & columns. Researcher initially prepare a numbered list of the members of the population, & then with a blindfold chooses a number from the random table. The same procedure is continued until the desired number of the subject is achieved. If repeatedly similar numbers are encountered, they are ignored & next numbers are considered until desired numbers of the subject are achieved . The use of computer… Nowadays random tables may be generated from the computer , & subjects may be selected as described in the use of random table. For populations with a small number of members, it is advisable to use the first method, but if the population has many members, a computer-aided random selection is preferred. Excel: enter the function =RND() on any blank cell : The use of computer… Nowadays random tables may be generated from the computer , & subjects may be selected as described in the use of random table. For populations with a small number of members, it is advisable to use the first method, but if the population has many members, a computer-aided random selection is preferred. Excel: enter the function =RND() on any blank cell Merits and Demerits : Merits and Demerits Merits Ease of assembling the sample Fair way of selecting a sample Require minimum knowledge about the population in advance It unbiased probability method Free from sampling errors Demerits It requirement of a complete & up-to-date list of all the members of the population. Does not make use of knowledge about a population which researchers may already have. Lots of procedure need to be done before sampling Expensive & time-consuming Systematic Random Sampling: Systematic Random Sampling It can be likened to an arithmetic progression, wherein the difference between any two consecutive numbers is the same. It involves the selection of every Kth case from list of group, such as every 10 th person on a patient list or every 100 th person from a phone directory. Systematic sampling is sometimes used to sample every Kth person entering a bookstore, or passing down the street or leaving a hospital & so forth Systematic sampling can be applied so that an essentially random sample is drawn. If we had a list of subjects or sampling frame, the following procedure could be adopted. The desired sample size is established at some number (n) & the size of population must know or estimated (N). Number of subjects in target population (N) K = N/n or K= Size of sample For example, a researcher wants to choose about 100 subjects from a total target population of 500 people. Therefore, 500/100=5. Therefore, every 5th person will be selected. : If we had a list of subjects or sampling frame, the following procedure could be adopted. The desired sample size is established at some number (n) & the size of population must know or estimated (N). Number of subjects in target population (N) K = N/n or K= Size of sample For example, a researcher wants to choose about 100 subjects from a total target population of 500 people. Therefore, 500/100=5. Therefore, every 5 th person will be selected. Systematic random sampling(contd.) : Systematic random sampling(contd.) 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. Systematic random sampling(contd.) : Systematic random sampling(contd.) Merits and Demerits: Merits and Demerits Merits Convenient & simple to carry out. Distribution of sample is spread evenly over the entire given population. Less cumbersome, time-consuming, & cheaper Demerits If first subject is not randomly selected, then it becomes a nonrandom sampling technique Sometimes this may result in biased sample. If sampling frame has nonrandomly, this sampling technique may not be appropriate to select a representative sample. Systematic random sampling(contd.) : Systematic random sampling(contd.) Example: District level household & facility survey for Reproductive and child health – 1998-99, 2002-03, 2010-11. Random blinded rechecking of slides under RNTCP. Slides are drawn from the register by systematic random sampling. Stratified Random Sampling: Stratified Random Sampling This method is used for heterogeneous population. It is a probability sampling technique wherein the researcher divides the entire population into different homogeneous subgroups or strata, & then randomly selects the final subjects proportionally from the different strata. The strata are divided according selected traits of the population such as age, gender, religion, socio-economic status, diagnosis, education, geographical region, type of institution, type of care, type of registered nurses, nursing area specialization, site of care, etc. Stratified Sampling : Stratified Sampling In proportionate stratified sampling , the size of the sample drawn from each stratum is proportionate to the relative size of that stratum in the total population. In disproportionate stratified sampling , the size of the sample from each stratum is not proportionate to the relative size of that stratum and to the standard deviation of the distribution of the characteristic of interest among all the elements in that stratum. Stratified random sampling(contd.) : Stratified random sampling(contd.) 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. PowerPoint Presentation: POPULATION LOW SOCIOECONOMIC MIDDLE SOCIOECONOMIC HIGH SOCIOECONOMIC PowerPoint Presentation: STRATIFIED SAMPLING GROUP 1 GROUP2 GROUP 3 GROUP 4 Merits and Demerits: Merits and Demerits Merits It representation of all group in a population For observing relation between subgroup Observe smallest & most inaccessible subgroups in population Higher statistical precision Save lot of time, money, & effort Demerits It require accurate information on the proportion of population in each stratum. Large population must available from which select sample Possibility of faulty classification Cluster sampling: Cluster 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. Cluster sampling: Cluster sampling Cluster sampling: Cluster sampling Cluster 4 Cluster 5 Cluster 3 Cluster 2 Cluster 1 Cluster sampling (contd.) : Cluster sampling (contd.) A special form of cluster sampling called the “ 30 X 7 cluster sampling ”, has been recommended by the WHO for field studies in assessing vaccination coverage . In this a list of all villages (clusters) for a given geographical area is made. 30 clusters are selected using Probability Proportional to Size (PPS). From each of the selected clusters, 7 subjects are randomly chosen. Thus a total sample of 30 x 7 = 210 subjects is chosen. The advantage of cluster sampling is that sampling frame is not required Probability proportional to size (PPS) : Probability proportional to size (PPS) Steps: List of all clusters (villages and sectors/wards) is made. Population of each cluster is written against them. Cumulative population is then written in serial order. Sampling interval is calculated = Total cumulative population/30 Choose a random number between 1 and the SI . This is the Random Start (RS) . The first cluster to be sampled contains this cumulative population . The clusters selected are those for which the cumulative population contains one of the serial numbers . CLUSTER SAMPLING: CLUSTER SAMPLING It is sometimes called area sampling because this is usually applied when the population is large. In this technique, groups or clusters instead of individuals are randomly chosen . PowerPoint Presentation: For each selected cluster, either all the elements are included in the sample (one-stage) or a sample of elements is drawn probabilistically (two-stage). Merits and Demerits : Merits and Demerits Merits It cheap, quick, & easy for a large population. Large population can be studied, & require only list of the members. Investigators to use existing division such as district, village/town, etc. Same sample can be used again for study Demerits This technique is the least representative of the population. Possibility of high sampling error This technique is not at all useful . MULTI-STAGE SAMPLING : MULTI-STAGE SAMPLING Refers to a sampling techniques which is carried out in various stages. Population is regarded as made of a number of primary units each of which further composed of a number of secondary units. Consists of sampling first stage units by some suitable method of sampling. From among the selected first stage units, a sub- sample of secondary stage units is drawn by some suitable method of sampling which may be same as or different from the method used in selecting first stage unit. Multistage random sampling: Multistage random sampling In this method, the whole population is divided in first stage sampling units from which a random sample is selected. The selected first stage is then subdivided into second stage units from which another sample is selected. Third and fourth stage sampling is done in the same manner if necessary. Example: NFHS data is collected by multistage sampling. Rural areas – 2 stage sampling – Villages from list by PPS, Households from village Urban areas – Wards (PPS) – CEB (PPS) – 30 households from each CEB PowerPoint Presentation: Advantages: II stage units are necessary only for selected I stage units Flexible & allows different selection procedure Easier to administer A large number of units can be sampled for a given cost. . Sequential Sampling: Sequential Sampling This method of sample selection is slightly different from other methods. Here the sample size is not fixed. The investigator initially selects small sample & tries out to make inferences; if not able to draw results, he or she then adds more subjects until clear-cut inferences can be drawn. Merits and Demerits: Merits and Demerits Facilitates to conduct a study on best-possible smallest representative sample. Helping in ultimately finding the inferences of the study. With this sampling technique it is not possible to study a phenomenon which needs to be studied at one point of time. Requires repeated entries into the field to collect the sample. NONPROBABILITY SAMPLING TECHNIQUE: NONPROBABILITY SAMPLING TECHNIQUE Features of the nonprobability sampling: Features of the nonprobability sampling It is a technique wherein samples are gathered in a process that does not give all the individual in the population equal chances of being selected. Most researchers are bound by time, money, & workforce, & because of these limitations, it is almost impossible to randomly sample the entire population & it is often necessary to employ another sampling technique, the nonprobability sampling technique. Subject in a nonprobability sample are usually selected on the basis of their accessibility or by the purposive personal judgment of the researcher Uses of Non probability Sampling: Uses of Non probability Sampling This type of sampling can be used when demonstrating that a particular trait exists in the population. It can also be used when researcher aims to do a qualitative, pilot , or exploratory study. It can be used when randomization is not possible like when the population is almost limitless. it can be used when the research does not aim to generate results that will be used to create generalizations. It is also useful when the researcher has limited budget, time, & workforce. This technique can also be used in an initial study (pilot study) Types of the Nonprobability Sampling Purposive/judgmental sampling Convenience/grab/availability sampling Consecutive sampling Quota sampling Snow ball sampling : Types of the Nonprobability Sampling Purposive/judgmental sampling Convenience/grab/availability sampling Consecutive sampling Quota sampling Snow ball sampling Purposive/deliberate sampling: Purposive/deliberate sampling . e It is more commonly known as ‘judgmental’ or ‘authoritative sampling’. In this type of sampling, subjects are chosen to be part of the sample with a specific purpose in mind. In purposive sampling, the researcher believes that some subjects are fit for research compared to other individual. This is the reason why they are purposively chosen as subject. In this sampling technique, samples are chosen by choice not by chance, through a judgment made the researcher based on his or her knowledge about the population For example, a researcher wants to study the lived experiences of post disaster depression among people living in earthquake affected areas of Gujarat. In this case, a purposive sampling technique is used to select the subjects who were the victims of the earthquake disaster & have suffered post disaster depression living in earthquake-affected areas of Gujarat. In this study, the researcher selected only those people who fulfill the criteria as well as particular subjects that are the typical & representative part of population as per the knowledge of the researcher. : For example, a researcher wants to study the lived experiences of post disaster depression among people living in earthquake affected areas of Gujarat. In this case, a purposive sampling technique is used to select the subjects who were the victims of the earthquake disaster & have suffered post disaster depression living in earthquake-affected areas of Gujarat. In this study, the researcher selected only those people who fulfill the criteria as well as particular subjects that are the typical & representative part of population as per the knowledge of the researcher. Merits and Demerits : Merits and Demerits Merits Simple to draw sample & useful in explorative studies Save resources, require less fieldwork. Demerits Require considerable knowledge about the population under study. It is not always reliable sample, as conscious biases may exist. Two main weakness of purposive sampling are with the authority & in the sampling process. It is usually biased since no randomization was used to obtained the sample . Convenience/haphazard Sampling: Convenience/haphazard Sampling It is probably the most common of all sampling techniques because it is fast, inexpensive, easy, & the subject are readily available. It is a nonprobability sampling technique where subjects are selected because of their convenient accessibility & proximity to the researcher. The subjects are selected just because they are easiest to recruit for the study & the researcher did not consider selecting subjects that are representative of the entire population It is also known as an accidental sampling . Subjects are chosen simply because they are easy to recruit. PowerPoint Presentation: the process of including whoever happens to be available at the time …called “accidental” or “haphazard” sampling For example, if a researcher wants to conduct a study on the older people residing in Jhansi, & the researcher observes that he can meet several older people coming for morning walk in a park located near his residence in Jhansi, he can choose these people as his research subjects. These subjects are readily accessible for the researcher & may help him to save time, money, & resources.(man on the streets) : For example, if a researcher wants to conduct a study on the older people residing in Jhansi, & the researcher observes that he can meet several older people coming for morning walk in a park located near his residence in Jhansi, he can choose these people as his research subjects. These subjects are readily accessible for the researcher & may help him to save time, money, & resources.(man on the streets) Merits and Demerits : Merits and Demerits Merits This technique is considered easiest, cheapest, & least time consuming. This sampling technique may help in saving time, money, & resources. Demerits Sampling bias, & the sample is not representative of the entire population. It does not provide the representative sample from the population of the study. Findings generated from these sampling cannot be generalized on the population. Consecutive Sampling: Consecutive Sampling It is very similar to convenience sampling except that it seeks to include all accessible subjects as part of the sample. This nonprobability sampling technique can be considered as the best of all nonprobability samples because it include all the subjects that are available, which makes the sample a better representation of the entire population. It is also known as total enumerative sampling. In this sampling technique, the investigator pick up all the available subjects who are meeting the preset inclusion & exclusion criteria. This technique is generally used in small-sized populations. For example, if a researcher wants to study the activity pattern of post kidney-transplant patient, he can selects all the post kidney transplant patients who meet the designed inclusion & exclusion criteria, & who are admitted in post-transplant ward during a specific time period. : In this sampling technique, the investigator pick up all the available subjects who are meeting the preset inclusion & exclusion criteria. This technique is generally used in small-sized populations. For example, if a researcher wants to study the activity pattern of post kidney-transplant patient, he can selects all the post kidney transplant patients who meet the designed inclusion & exclusion criteria, & who are admitted in post-transplant ward during a specific time period. Merits and Demerits : Merits and Demerits Merits Little effort for sampling It is not expensive, not time consuming, & not workforce intensive. Ensures more representativeness of the selected sample . Demerits Researcher has not set plans about the sample size & sampling schedule. It always does not guarantee the selection of representative sample. Results from this sampling technique cannot be used to create conclusions & interpretations pertaining to the entire population. Quota Sampling : Quota Sampling It is nonprobability sampling technique wherein the researcher ensures equal or proportionate representation of subjects, depending on which trait is considered as the basis of the quota. The bases of the quota are usually age, gender, education, race, religion, & socio-economic status. For example, if the basis of the quota is college level & the research needs equal representation, with a sample size of 100, he must select 25 first-year students, another 25 second-year students, 25 third-year, & 25 fourth-year students. Types of Quota Sampling: Types of Quota Sampling Proportional quota sampling  – representing the major characteristics of the population by sampling a proportional amount of each . Non-proportional quota sampling   is a bit less restrictive. In this method, you specify the minimum number of sampled units you want in each category. Here, you're not concerned with having numbers that match the proportions in the population. Instead, you simply want to have enough to assure that you will be able to talk about even small groups in the population . Merits and Demerits : Merits and Demerits Merits Economically cheap, as there is no need to approach all the candidates. Suitable for studies where the fieldwork has to be carried out, like studies related to market & public opinion polls. Demerits It does not represent all population In the process of sampling these subgroups, other traits in the sample may be overrepresented. Not possible to estimate errors. Bias is possible, as investigator/interviewer can select persons known to him. Snowball/networking Sampling: Snowball/networking Sampling It is a nonprobability sampling technique that is used by researchers to identify potential subjects in studies where subjects are hard to locate such as commercial sex workers, drug abusers, etc. For example, a researcher wants to conduct a study on the prevalence of HIV/AIDS among commercial sex workers. In this situation, snowball sampling is the best choice for such studies to select a sample. This type of sampling technique works like chain referral. Therefore it is also known as chain referral sampling. After observing the initial subject, the researcher asks for assistance from the subject to help in identify people with a similar trait of interest The process of snowball sampling is much like asking subjects to nominate another person with the same trait. The researcher then observes the nominated subjects & continues in the same way until obtaining sufficient number of subjects. : After observing the initial subject, the researcher asks for assistance from the subject to help in identify people with a similar trait of interest The process of snowball sampling is much like asking subjects to nominate another person with the same trait. The researcher then observes the nominated subjects & continues in the same way until obtaining sufficient number of subjects. Merits and Demerits : Merits and Demerits Merits The chain referral process allows the researcher to reach populations that are difficult to sample when using other sampling methods. The process is cheap, simple, & cost-efficient. Need little planning & lesser workforce Demerits Researcher has little control over the sampling method. Representativeness of the sample is not guaranteed. Sampling bias is also a fear of researchers when using this sampling technique. Errors in Sampling: Errors in Sampling Non-Observation Errors Sampling error: naturally occurs Coverage error: people sampled do not match the population of interest Underrepresentation Non-response: won’t or can’t participate Errors of Observation: Errors of Observation Interview error- interaction between interviewer and person being surveyed Respondent error: respondents have difficult time answering the question Measurement error: inaccurate responses when person doesn’t understand question or poorly worded question Errors in data collection

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