Sampling Techniques PDF

Sample and sampling Methods Assoc. Prof. Dr. Hatice PARS Objectives of this session: Students able to ◼ (1) Understand meaning of sampling process and its importance ◼ (2) Know the definations of different terms used in the sampling process ◼ (3) Learn how to apply different random sam...

Sample and sampling Methods Assoc. Prof. Dr. Hatice PARS Objectives of this session: Students able to ◼ (1) Understand meaning of sampling process and its importance ◼ (2) Know the definations of different terms used in the sampling process ◼ (3) Learn how to apply different random sampling & non random sampling techniques  Sampling in Epidemiology ◼ Why Sample?  Unable to study all members of a population  Reduce bias  Save time and money  Measurements may be better in sample than in entire population  Feasibility Sampling Sampling is the process or technique of selecting a sample of appropriate characteristics and adequate size.  Representativeness (validity) A sample should accurately reflect distribution of relevant variable in population ◼ Person e.g. age, sex ◼ Place e.g. urban vs. rural ◼ Time e.g. seasonality Representativeness essential to generalise Ensure representativeness before starting, Confirm once completed  Sampling and representativeness Sampling Population Sample Target Population Target Population ➔ Sampling Population ➔ Sample Definitions ◼ Population – group of things (people) having one or more common characteristics ◼ Sample – representative subgroup of the larger population  Used to estimate something about a population (generalize)  Must be similar to population on characteristic being investigated Population: a set which includes all measurements of interest to the researcher (The collection of all responses, measurements, or counts that are of interest) Sample: A subset of the population  Def. – Cont. Sampling Frame This is the complete list of sampling units in the target population to be subjected to the sampling procedure. Completeness and accuracy of this list is essential for the success of the study. Sampling Units These are the individual units / entities that make up the frame just as elements are entities that make up the population.  Def. – Cont. Sampling Error This arises out of random sampling and is the discrepancies between sample values and the population value. Sampling Variation ◼ Due to infinite variations among individuals and their surrounding conditions. ◼ Produce differences among samples from the population and is due to chance. ◼ Example: In a clinical trail of 200 patients we find that the efficacy of a particular drug is 75% If we repeat the study using the same drug in another group of similar 200 patients we will not get the same efficacy of 75%. It could be 78% or 71%. “Different results from different trails though all of them conducted under the same conditions”  How to sample ? In general, 2 requirements 1. Sampling frame must be available, otherwise develop a sampling frame. 2. Choose an appropriate sampling method to draw a sample from the sampling frame. The Sampling Design Process Define the Population Determine the Sampling Frame Select Sampling Technique(s) Determine the Sample Size Execute the Sampling Process Sampling Methods Probability Sampling Non-Probability Sampling ◼ Simple random sampling ◼ Deliberate (quota) ◼ Stratified random sampling sampling ◼ Convenience sampling ◼ Systematic random ◼ Purposive sampling sampling ◼ Snowball sampling ◼ Cluster (area) random ◼ Consecutive sampling sampling ◼ Multistage random sampling Simple Random Sampling ◼ Equal probability ◼ Techniques  Lotterymethod  Table of random numbers ◼ Advantage  Most representative group ◼ Disadvantage  Difficult to identify every member of a population Table of random numbers 684257954125632140 582032154785962024 362333254789120325 985263017424503686  Random Number table 1 2 3 4 5 49486 93775 88744 80091 92732 94860 36746 04571 13150 65383 10169 95685 47585 53247 60900 12018 45351 15671 23026 55344 45611 71585 61487 87434 07498 89137 30984 18842 69619 53872 94541 12057 30771 19598 96069 89920 28843 87599 30181 26839 32472 32796 15255 39636 90819 How to select a simple random sample 1. Define the population 2. Determine the desired sample size 3. List all members of the population or the potential subjects ◼ For example:  4th grade boys who have demonstrated problem behaviors  Lets select 10 boys from the list So our selected subjects are with numbers 10, 22, 24, 15, 6, 1, 25, 11, 13, & 16. ◼ Simple random sampling  Estimate hemoglobin levels in patients with sickle cell anemia 1. Determine sample size 2. Obtain a list of all patients with sickle cell anemia in a hospital or clinic 3. Patient is the sampling unit 4. Use a table of random numbers to select units from the sampling frame 5. Measure hemoglobin in all patients 6. Estimate the levels (normal & abnormal) of hemoglobin Systematic random Sampling ◼ Technique  Use “system” to select sample (e.g., every 5th item in alphabetized list, every 10th name in phone book) ◼ Advantage  Quick, efficient, saves time and energy ◼ Disadvantage  Not entirely bias free; each item does not have equal chance to be selected  System for selecting subjects may introduce systematic error  Cannot generalize beyond population actually sampled Example ◼ If a systematic sample of 500 students were to be carried out in a university with an enrolled population of 10,000, the sampling interval would be: ◼ I = N/n = 10,000/500 =20 ◼ All students would be assigned sequential numbers. The starting point would be chosen by selecting a random number between 1 and 20. If this number was 9, then the 9th student on the list of students would be selected along with every following 20th student. The sample of students would be those corresponding to student numbers 9, 29, 49, 69,........ 9929, 9949, 9969 and 9989. Systematic sampling Stratified Random Sampling ◼ Technique  Divide population into various strata  Randomly sample within each strata  Sample from each strata should be proportional ◼ Advantage  Better in achieving representativeness on control variable ◼ Disadvantage  Difficult to pick appropriate strata  Difficult to Identify every member in population Stratified Random selection for drug trail in hypertension Mild Moderate Severe  Sampling in Epidemiology ◼ Stratified random sample  Assess dietary intake in adolescents 1. Define three age groups: 11-13, 14-16, 17-19 2. Stratify age groups by sex 3. Obtain list of children in this age range from schools 4. Randomly select children from each of the 6 strata until sample size is obtained 5. Measure dietary intake Cluster (Area) random sampling ◼ Randomly select groups (cluster) – all members of groups are subjects ◼ Appropriate when  you can’t obtain a list of the members of the population  have little knowledge of population characteristics  Population is scattered over large geographic area  Cluster sampling Section 1 Section 2 Section 3 Section 5 Section 4 Cluster (Area) Sampling ◼ Advantage  More practical, less costly ◼ Conclusions should be stated in terms of cluster (sample unit – school) ◼ Sample size is number of clusters Multistage random sampling ◼ Stage 1  randomly sample clusters (schools) ◼ Stage 2  randomly sample individuals from the schools selected Sampling Methods Probability Sampling Non-Probability Sampling ◼ Simple random sampling ◼ Deliberate (quota) ◼ Stratified random sampling sampling ◼ Convenience sampling ◼ Systematic random ◼ Purposive sampling sampling ◼ Snowball sampling ◼ Cluster (area) random ◼ Consecutive sampling sampling ◼ Multistage random sampling Deliberate (Quota) Sampling ◼ Similar to stratified random sampling ◼ Technique  Quotas set using some characteristic of the population thought to be relevant  Subjects selected non-randomly to meet quotas (usu. convenience sampling) ◼ Disadvantage  selection bias  Cannot set quotas for all characteristics important to study Convenience Sampling ◼ “Take them where you find them” - nonrandom ◼ Intact classes, volunteers, survey respondents (low return), a typical group, a typical person ◼ Disadvantage: Selection bias Purposive Sampling ◼ Purposive sampling (criterion-based sampling)  Establishcriteria necessary for being included in study and find sample to meet criteria ◼ Solution: Screening  Use random sampling to obtain a representative sample of larger population and then those subjects that are not members of the desired population are screened or filtered out  EX: want to study smokers but can’t identify all smokers Snowball Sampling In snowball sampling, an initial group of respondents is selected.  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. Consecutive sampling Outcome of 1000 consecutive patients presenting to the emergency room with chest pain Natural history of all 125 patients with HIV-associated TB during 5 year period Explicit efforts must be made to identify and recruit ALL persons with the condition of interest Choosing probability vs. non-probability sampling method Probability Evaluation Criteria Non-probability sampling sampling Conclusive Nature of research Exploratory Larger sampling Relative magnitude Larger non-sampling errors sampling vs. error non-sampling error High Population variability Low [Heterogeneous] [Homogeneous] Favorable Statistical Considerations Unfavorable High Sophistication Needed Low Relatively Longer Time Relatively shorter High Budget Needed Low Suniti Solomon et al Prevalence and risk factors of HIV 1 and HIV 2 infection in Urban and rural areas in TN. Int. J. of STD & AIDS 1998;9:98- 103 Objective: Find prevalence and risk factors. Setting: Centres in metropolitancity & municipality. Subjects: Individuals in Tamil nadu. Sampling Porcedure: “ Health camps were organised in 5 urban and 5 rural centres to cover entire state graphically” “ Every third person screened, in the active reproductive age group, were recruited as a subject. At each camp the inclusion of subjects continued until 200 persons were recruited” Mary Sexton et al. Sex differences in the use of asthma drugs: Cross-sectional study. BMJ 1998; 317: 1434-7 Objective : To assess the use of asthma drugs. Design : Cross-sectional study. Setting: Six general practices in East Anglia. Subjects : Adults aged 20-54 with Asthma Sampling method “identify cases with asthma received drugs one year before – through database from each participating practices. The sample was stratified into three categories of severity corresponding the prescribed drugs Bronchodilator alone (mild) 38% Steroids (moderate) 57% Nebulizer treatment (severe) 5% Use SRS to select subject in each practice based on proportion of use of each type of drug within the practice S. Anuradha Genital ulcer disease and acquisition of HIV infection. Indian J Med Microbiol 1992; 10(4):265-269 Objective : To find out the association of HIV infection with genital ulcer disease. Setting : Dept. of STD, GGH, Chennai. Subjects : Individials attending the STD dept. Sampling procedure ‘ Blood samples from first 20 patients were taken for analysis once a week for 40 weeks’. Statistical analysis : Data were analysed by using SPSS/PC + ver 4.0. The strength of association between the variables and HIV serological status was estimated using odds ratio (OR) and their 95% Confidence Intervals(CI) Reidy A et al. Prevalence of series eye disease and visual impairement in a north London population: Population based, cross sectional study. BMJ 1998; 316:1643- Objective: To estimate eye disorders and of visual impairement Design: Cross-sectional survey. Setting : General Practices in metropolitan in England. Subjects: aged 65 or older & registered Sampling 17 general practice group Procedure Random sampling 7 were selected People age 65 or older were registered with the general practices. Total 750-850 in each Gen Pract Use SRS to select eligible people in each practice One third in each practices were selected to form survey sample In Conclusion, For any research, based on its study design and objectives an appropriate random sampling technique should be used, so as to generalize the findings.