Lecture 6

1. Sampling and Sampling Procedures Lecture 6

2. In most epidemiologic studies, we deal with a sample of the population The study population may be: An entire population A group of the population with certain characteristics - age - sex - religion - social status Hospital inpatients General considerations

3. A sample is a portion of the population, used for survey purposes when you can not examine the entire population because the resources are not adequate. A sample should be a representative group of the whole population to be studied. When a sample is representative to the whole population, generalization from the sample can be made. The sample

4. A number of techniques can make the sample a representative group of the whole population. Only the simpler techniques will be considered in this lecture, and when used with care will meet the requirements of epidemiologic studies. Complex sampling methods need the help of a statistician.

5. The precision with which the investigator make generalization from a sample to the entire population is related to the size of the sample. Sample size

6. Sample selection needs a list of all members (units) of the community from which a sample is selected. The sampling units may be : Individuals Families Blocks in a city Class in a school Whole school The sample

7. Probability sampling: which is the most recommended method because all sampling units have the same probability of being selected and included in the study. This method assures that the sample is a representative of the whole population. Inferences from the sample can be made about the whole population (generalization of results to the whole population). Sampling procedures

8. Sampling techniques include the following : Haphazard sample: Ex- in an experiment, where the experimental animal is chosen as the investigator can catch the animal from the cage. Sampling procedures

9. Selected sample: The individuals are selected according to the investigator’ opinion that they are typical of the population being studied and is appropriate for the study objective . The sample may be selected because it is handy sample. selection of groups known to be co-operative The disadvantage of this sample is that you can not be confident that this sample represents the population, so generalization cannot be made out of this sample. Sampling procedures

10. Self-selected sample: The sample consists of individuals who volunteered for an experiment. Such method can provide some information about the population and can serve as a basis for future adequate studies Generalization cannot be made out of this sample. Sampling procedures

11. A sample may be defined as random if every individual in the population has an equal chance of being included in the sample. Random selection is the basis of this sampling technique. Any method of selection based on volunteering or selection of groups known to be co-operative is excluded and cannot be used for generalization to the entire population. Simple random sample

12. It is necessary to have a complete list of sampling units of the total population: Individual persons House holds Schools cities Selecting a random sample

13. To select a simple random sample: Make a numbered list of the units in the population (assign a number to each individual) Estimate the size of the sample Select the required number of sampling units using a table of random numbers. Selecting a probability sample

14. If the size of the population is not too large: Write the numbers on small cards Place them in a bowl and mix them thoroughly The number representing the sample size is selected from the bowl. Selecting a random sample

15. Selecting a probability sample

16. Systematic sample ( more efficient): • In this sampling method, everynth person in the list is chosen and included in the sample • If the list contains 10,000 units and the researchers wants 1,000 units, he will select one person every 10 persons (nth =10) • The first person will be selected at random between numbers from 1-10, then every tenth unit will be selected. Selecting a random sample

17. This sampling method is used when a large survey is planned to cover a district, governorate or the entire country (two stage sampling). If there is no satisfactory list of the population is available ( there is no list of a city’s population), a list of groups or clusters of individuals is available, such as city districts, census neighborhoods, villages or schools. a random sample of these clusters is selected and all individuals in the cluster are included in the study (one stage sampling). Cluster sample:

18. An initial sample is taken from the units: Neighborhoods entire district Villages entire governorate Governorates entire country Then a listing of individuals within the chosen units (clusters) is made and a random sample is taken from each (two stage sampling). The disadvantage to this method is that disease may be accumulated in some clusters giving misleading results. Cluster sample:

19. Stratified random sample: If the severity of the disease differs according one or more characteristics such as sex, age, or socio-economic status, then the population to be sampled should be divided into subgroups or strata according to the characteristics that affect the disease. Then random sample is selected within each stratum and all individuals in each stratum are included in the study . Selecting a probability sample

20. Stratified random sample: To assess the prevalence of a disease like caries which is unevenly distributed among age groups, we have to obtain a sample stratified by age: The study population is subdivided into age groups such as 6-12, 13-18, 19-25 and 26 and over. A certain number of each age group is then selected randomly and all individuals in each age stratum are included in the study . Selecting a random sample

21. Thank you