Sampling technique and sample size..pptx

Sampling

1. University of Medical Sciences and Technology Research Methodology and Biostatics Sampling technique and Sample Size

2. Study Population and Sample

3. Why do we use sampling? Cannot get information on everyone in a population Efficiently gets information on a large population Obtains a representative sample of a population

4. Sampling is the process of selection of a number of units from a defined study population. The study or target population (N) is the one upon which the results of the study will be generalized.

5. The study or target population (N) is the one upon which the results of the study will be generalized. The Sample (n) is a number of sampling units selected (inclusion and exclusion criteria) from the target /study population to which findings are generalized providing validity of the study results

6. Study population (N) Study sample (n)

7. Sampling technique

8. Two main types of sampling methods: Non-probability sampling Probability sampling

9. Non-probability sampling

10. Non-probability Sampling Probability of selection is unknown or zero Inexpensive Results not generalizable Results often biased Not recommended in medical research It is by far the most biases sampling procedure as it is not random (not everyone in the population has an equal chance of being selected to participate in the study)

11. Common types of non-probability sampling : Convenience sampling (easiest accessible study units) Quota sampling ( researcher fixed the number of units to involved in the study) Snowball sampling/Respondent-driven sampling (convenient sampling where a study unit contribute to identify other units to be included) Voluntary sampling Non-probability Sampling

12. P robability sampling

13. Probability Sampling What are types of probability-based samples? Simple random sampling Systematic random sampling Stratified random sampling Cluster sampling Multistage sampling

14. Simple Random Sample Principle Equal chance/probability of drawing each unit Procedure List all units (persons) in a population Assign a number to each unit Randomly select units

15. Simple Random Sample Each unit has the same probability of selection (1/30)

16. Simple Random Sample Example : Calculate the prevalence of tooth decay among 1200 children attending a school. Sample size =100 List all children attending the school Each child assigned a number from 1 to 1200 Randomly select 100 numbers between 1 and 1200 Advantages Simple Disadvantages Need complete list of units Units may be scattered and poorly accessible

17. Systematic Random Sample Principle Select sample at regular intervals based on sampling fraction Procedure List all units (persons) in a population Assign a number to each unit Calculate sampling fraction (population size ÷ sample size) Select first unit at random based on sampling fraction Subsequent units are chosen at equal intervals

18. Systematic Random Sample Advantages Simple Can be implemented easily without software Disadvantages Need complete list of units

19. List all children attending the school Each child assigned a number from 1 to 1200 Sampling fraction =1200/100 = 12 Randomly select a number between 1 and 12 Example: 8 Select every 12 th child, starting with child #8 Example: 8, 20, 32, 44… Systematic Random Sample Example: Calculate the prevalence of tooth decay among 1200 children attending a school. ( sample size =100)

20. Stratified Random Sample Principle Select random samples from within homogeneous subgroups (strata) Procedure List all units (persons) in a population Divide the units into groups (called strata) Assign a number to each unit within each stratum Select a random sample from each stratum Combine the strata samples to form the full sample

21. Sampling frame divided into groups (age, sex, socioeconomic status) Stratified Random Sample - Method

22. Stratified Random Sample - Method Probability: 1/15 Probability:1/20 Units in each group have the same probability of selection, but probability differs between groups

23. Calculate the prevalence of tooth decay among 1200 children attending a school, with equal representation of males and females. Sample size = 100. Stratified Random Sample - Example List all children attending the school Divide the children into two groups 540 males and 660 females Assign each child a number Males: 1 to 540 Females 1 to 660 Randomly select 50 males and 50 females

24. Stratified Random Sample Advantages Can get separate estimates from the whole population and from individual strata (if sample is large enough ) Precision increased if less variability within strata than between strata Disadvantages Can be difficult to identify strata

25. Cluster Sample Principle Select all units within randomly selected geographic clusters Procedure Divide population into geographic groups (clusters ) Assign a number to each cluster Randomly select clusters Sample all units within selected clusters OR select a random sample of units within selected clusters

26. Advantages List of sampling units not required More efficient for face-to-face interviews when units are dispersed over a large area Disadvantages Loss of precision due to correlation within clusters This correlation needs to be taken into account in sample size calculations and analysis (“design effect”) Cluster Sample

27. Multistage sampling Multi-stage sampling procedure involving more than one sampling method (e.g. community based studies) , generally used for very large and diverse populations.

28. Multistage sampling - Example Multistage sampling method to investigate violence against pregnant women in Khartoum State State Khartoum Local authority 3 (Omdurman, Khartoum, Bahri ) Areas Simple Random sampling Blocks/Squares Simple Random Sampling Households Systematic Sampling Household members: Decide member of a household to be interviewed

29. Population and Sampling Sampling is the process of selection of a number of units with certain criteria from a defined study population to meet the needs of the research. The process of sampling involves : Identification of study population Determination of sampling population Definition of the sampling unit Choice of sampling method Estimation of the sample size

30. Practice: Working groups Each participant will review: Research topic Statement of a research problem Formulate the research question Formulate the research objective Provide study variables for collecting appropriate data related to the research question Propose and justify the study design which will address the research question Propose a sampling method to be used and why?