Week 4- Sampling 2.pdf

Sampling Techniques Dr.Fawaz P’shery BDS;MDS;DDPH;MMed Division of Community Dental Practice &Research Introduction Sampling is very often used in our daily life. For example, while purchasing food grains or fruits from a shop we usually examine a handful from the bag to a...

Sampling Techniques Dr.Fawaz P’shery BDS;MDS;DDPH;MMed Division of Community Dental Practice &Research Introduction Sampling is very often used in our daily life. For example, while purchasing food grains or fruits from a shop we usually examine a handful from the bag to assess the quality of the commodity. Sampling techniques- Dr.Fawaz Pullishery  A doctor examines a few drops of blood as sample and draws conclusion about the blood constitution of the whole body. Thus most of our investigations are based on samples. Sampling techniques- Dr.Fawaz Pullishery Terminologies Population The entire group under study as defined by research objectives. Sometimes called the “universe.” ▪ Sample A subset of the population that should represent the entire group. Sampling techniques- Dr.Fawaz Pullishery  Sampling units The elementary units/ group/ cluster that form basis of sampling process. The basic level of investigation… school children, consumers, store managers etc. The research objective should define the sample unit. Sampling frame A master list of the population (total or partial) from which the sample will be drawn. Sampling techniques- Dr.Fawaz Pullishery  Sampling size The number of sampling units to be selected. Researcher should calculate a minimum sample size Sampling method (sampling design) Definite plan for obtaining a sample from the sampling frame. Sampling bias (errors) Errors those arise on account of sampling. Sampling techniques- Dr.Fawaz Pullishery Population Sample Sampling techniques- Dr.Fawaz Pullishery WHY SAMPLING IS NECESSARY ? Cost Time Physical impossibility to cover the whole population Sampling techniques- Dr.Fawaz Pullishery  SAMPLING Procedure by which some members of a given population are selected as representatives of the entire population. Sampling techniques- Dr.Fawaz Pullishery Sampling: central concepts The aim of sampling is to produce a miniature copy of the population. Each member of the population has an equal likelihood of being selected into the sample. Hence we can make inferences about the larger population based on the sample. Sampling techniques- Dr.Fawaz Pullishery Characteristics of a good sample design Must be truly representative. A representative sample has all the important characteristics of the study population from which it is drawn. Must be which results in a small sampling error. Viable in context of funds available for research study. Must be such that systematic bias can be controlled in a better way. Should be such that the results of the sample study can be applied, in general, for the universe with a reasonable level of confidence. Sampling techniques- Dr.Fawaz Pullishery  TYPES OF SAMPLING NON PROBABILITY PROBABILITY SAMPLING SAMPLING Sampling techniques- Dr.Fawaz Pullishery Classification of sampling methods Probability samples: Ones in which members of the population have a known chance (probability) of being selected. Non-probability samples: Instances in which the chances (probability) of selecting members from the population are unknown. Sampling techniques- Dr.Fawaz Pullishery  Sampling methods Probability Sampling Non-probability sampling Simple random sampling Convenience sampling Stratified random sampling Quota Sampling Systematic sampling Judgment sampling Cluster random sampling Volunteer sampling Multi stage random sampling Snow ball sampling/ network Sampling techniques- Dr.Fawaz Pullishery PROBABILITY SAMPLING Sampling techniques- Dr.Fawaz Pullishery Each sampling unit has same probability of being selected Law of chance alone decide which of the individual unit in the “parent” population will be selected “Strict random sampling” Sampling techniques- Dr.Fawaz Pullishery  SIMPLE RANDOM SAMPLING Each and every unit of the population has equal chance of being selected. The probability of being selected is “known and equal” for all members of the population. Blind Draw Method (e.g. names “placed in a hat” and then drawn randomly) Random Numbers Method (all items in the sampling frame given numbers, numbers then drawn using table or computer program). Lottery Method Computer Assisted Software Sampling techniques- Dr.Fawaz Pullishery Simple Random Sampling 2, 6, 7, 12, 18 SMALL HOMOGENOUS POPULATION Sampling techniques- Dr.Fawaz Pullishery  Advantages: Known and equal chance of selection. Easy method when there is an electronic database. This sampling method is a very basic method of collecting the data. There is no technical knowledge required and need basic listening and recording skills. Disadvantages: (Overcome with electronic database) Complete accounting of population needed. The larger population means a larger sample frame. It is difficult to manage the large population Cumbersome to provide unique designations to every population member. Very inefficient when applied to skewed population distribution (over- and under-sampling problems) – this is not “overcome with the use of an electronic database) Sampling techniques- Dr.Fawaz Pullishery  SYSTEMATIC RANDOM SAMPLING Selecting one unit at random & additional units at evenly spaced intervals. The sample is chosen by selecting a random starting point and then picking every “nth” element in succession from the sampling frame. Systematic sampling is an extended implementation of probability sampling in which each member of the group is selected at regular periods to form a sample. This method is at times more efficient than simple random sampling. Sampling techniques- Dr.Fawaz Pullishery  A systematic sample is to be selected from 1200 students of a school. The sample size selected is 100. The sampling interval is, therefore, 1200/ 100= 12. The number of the first student to be included in the sample is chosen randomly, for e.g., by blindly picking 1 out of 12 pieces of paper, numbered 1- 12. If number 6 is picked, then every twelfth student will be included in the sample, starting with student number 6, until 100 students are selected. Then numbers selected would be 6, 18, 30, 42. Sampling techniques- Dr.Fawaz Pullishery  Advantages: Known and equal chance of any of the sampling interval being selected. Efficiency..do not need to designate every population member, just those early on on the list (unless there is a very large sampling frame). Less expensive faster than Simple random sampling. Disadvantages: Small loss in sampling precision Potential “periodicity” problems Sampling techniques- Dr.Fawaz Pullishery Systematic Random Sampling Population is large and scattered. Random Start: 2 Desired Sample Size: 5 Population Size: 20 sample size Increment: 20/5 = 4 Sampling techniques- Dr.Fawaz Pullishery  Stratified Sampling Method Stratified random sampling is a type of probability sampling using which a research organization can branch off the entire population into multiple non-overlapping, homogeneous groups (strata) and randomly choose final members from the various strata for research which reduces cost and improves efficiency. Allows us to draw a more representative sample. Members in each of these groups should be distinct so that every member of all groups get equal opportunity to be selected using simple probability. This sampling method is also called “random quota sampling”. Sampling techniques- Dr.Fawaz Pullishery  Age, socioeconomic divisions, nationality, religion, educational achievements and other such classifications fall under stratified random sampling. Then the sampling frame must be divided into groups or strata, according to these characteristics. Random or systematic samples of predetermined size will then have to be obtained from each group (stratum). Let’s consider a situation where a research team is seeking opinions about religion amongst various age groups. Instead of collecting feedback from 326,044,985 U.S citizens, random samples of around 10000 can be selected for research. These 10000 citizens can be divided into strata according to age,i.e, groups of 18-29, 30-39, 40-49, 50-59, and 60 and above. Each stratum will have distinct members and number of members. Sampling techniques- Dr.Fawaz Pullishery  The population is separated into homogeneous groups/segments/strata and a sample is taken from each. The results are then combined to get the picture of the total population. Sampling techniques- Dr.Fawaz Pullishery  Cluster Sampling Method by which the population is divided into groups (clusters), any of which can be considered a representative sample. E.g.:Area sampling Then a random sample of clusters is selected, based on a probability sampling technique such as simple random sampling. 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). Sampling techniques- Dr.Fawaz Pullishery Sampling techniques- Dr.Fawaz Pullishery  Advantages: Consumes less time and cost: Does not require a list of all members of the universe. Ease of implementation: Since cluster sampling facilitates information from various areas and groups, it can be easily implemented in practical situations in comparison to other probability sampling methods such as simple random sampling, systematic sampling, and stratified sampling or non-probability sampling methods such as convenience sampling. Disadvantage: Cluster specification error… the more homogeneous the cluster chosen, the more imprecise the sample results Sampling techniques- Dr.Fawaz Pullishery Sampling techniques- Dr.Fawaz Pullishery  MULTI STAGE SAMPLING It is the combination of one or more methods described above. Population is divided into multiple clusters and then these clusters are further divided and grouped into various sub groups (strata) based on similarity. One or more clusters can be randomly selected from each stratum. This process continues until the cluster can’t be divided anymore. For example country can be divided into states, cities, urban and rural and all the areas with similar characteristics can be merged together to form a strata. Sampling techniques- Dr.Fawaz Pullishery  Sampling procedures carried out in several stages using random sampling techniques. Applicable for large scale surveys. state 1 dist 1 block 1 state 2 dist 2 Saudi block 2 Arabia state 3 dist 3 block 3 state 4 dist 4 block 4 Sampling techniques- Dr.Fawaz Pullishery Stage 1 Stage 2 Stage 3 Non Probability Sampling Sampling techniques- Dr.Fawaz Pullishery Convenience Sampling Researcher Researcher selects “units” to be included based on ease of obtaining them or simple availability Sampling techniques- Dr.Fawaz Pullishery Convenience Sampling Method Samples drawn at the convenience of the interviewer. Often, respondents are selected because they happen to be in the right place at the right time. ◼ use of students, and members of social organizations ◼ mall intercept interviews without qualifying the respondents ◼ “people on the street” interviews Sampling techniques- Dr.Fawaz Pullishery  Judgment Sampling Method Samples that require a judgment or an “educated guess” on the part of the interviewer as to who should represent the population. Also, “judges” (informed individuals) may be asked to suggest who should be in the sample. Disadvantages: Subjectivity enters in here, and certain members of the population will have a smaller or no chance of selection compared to others. Sampling techniques- Dr.Fawaz Pullishery Volunteer Sampling I’ll do it! I’ll do it! Researcher I’ll do it! Researcher uses only people who volunteer to participate in the research Sampling techniques- Dr.Fawaz Pullishery Quota Sampling Methods Under quota sampling the interviewers are simply given quotas to be filled from the different strata, with some restrictions on how they are to be filled. In other words, the actual selection of the items for the sample is left to the interviewer’s discretion. This type of sampling is very convenient and is relatively inexpensive. But the samples so selected certainly do not possess the characteristic of random samples. Sampling techniques- Dr.Fawaz Pullishery  Network/Snowball Sampling Researcher Researcher selects a few participants, who then suggests others who may be willing to participate Sampling techniques- Dr.Fawaz Pullishery  Snowball/ Referral Sampling In snowball sampling, an initial group of respondents is selected, usually at random. After being interviewed, these respondents are asked to identify others who belong to the target population of interest. Subsequent respondents are selected based on the referrals. Sampling techniques- Dr.Fawaz Pullishery Thank You