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Preparing the Sample

Preparing the Sample. What is a sample?. The target population, defined by each site, is the population to which the results of the survey should be applicable. A sample is a carefully selected subset of the target population. Why select a sample of the target population?.

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Preparing the Sample

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  1. Preparing the Sample

  2. What is a sample? • The target population, defined by each site, is the population to which the results of the survey should be applicable. • A sample is a carefully selected subset of the target population.

  3. Why select a sample of the target population? • It is not logistically possible to survey every individual within your target population. Therefore, a sample of this population is selected for the survey. The results of this sample are then "applied" to the target population.

  4. Determining the Sample Size • STEPS recommends a minimum sample of at least 2,000 individuals for sample of 25–64-year-olds stratified by age and sex. • The sample size should be calculated specifically for your survey.

  5. Identifying the Sample Frame • A sampling frame is a list of units or elements that defines the target population. • A sampling frame should cover all of the target population. • Your sampling frame should provide the highest possible level of detail.

  6. Stages of Identifying the Sampling Frame Site • A sampling frame is most often a combination of many different lists that contain different levels of information. • Create a flowchart that details the different levels/clusters/groups that describe your site. Province District Town Village Household Individual

  7. Stages of Identifying the Sampling Frame Site • Select the lowest level in the chart that contains information on every individual in the top level. This determines the sampling design selected. • (e.g. Does the town level contain information on every individual in the site?) Province District Town Village Household Individual

  8. Choosing a Sample Design • The sample design describes how the sample is selected. • STEPS recommends and supports five different sample designs. • The type of sample design selected is determined by the sampling frame available.

  9. Sample Design 1 Site • Information is available for all the individuals in the site. • Sampling frame consists of a complete list of every individual in the site. • Ideal scenario, but extremely rare. Province District Town Village Household Individual

  10. Sample Design 2 Site • Information is available for all the households in the site. • Do not have information available on individuals within the households, but do have information on the households. Province District Town Village Household Individual

  11. Sample Design 3 Site • Information is not available for individuals or households at the site level. • You need to select part of the sample before you can get more detailed information. • Information is available at for individuals once you get to a lower level in the sampling frame (e.g. Town or Village) Province Select a subset: 1st part of sample District Town Village Household Individual

  12. Sample Design 4 Site • Information is not available for individuals or households at the site level. • You need to select part of the sample before you can get more detailed information. • Information is available for households once you get to a lower level in the sampling frame (e.g. Town or Village). Province Select a subset: 1st part of sample District Town Village Household Individual

  13. Sample Design 5 Site • Information is not available for individuals or households at the site level. • You need to select part of the sample before you can get more detailed information. • There is no information available on the households or the individuals at the lower levels in the sampling frame (e.g. Town or Village), • Perform a census of the village and select households or select every household, Province Select a subset: 1st part of sample District Town Village Household Individual

  14. Probability Proportional to Size Sampling (PPS) • Probability proportional to size sampling is used for every level of sampling except for household and individual selection. • The larger clusters/groups have a greater chance of inclusion in the sample.

  15. EA/cluster EA/cluster EA/cluster EA/cluster Household Household Household Household Household Household Village Village Village Village Village Village Basic Sampling Terminology Target Population PSU • PSU = Primary Sampling Unit • SSU = Secondary Sampling Unit • TSU = Tertiary Sampling Unit SSU TSU Participant Participant Participant Participant Participant Participant Participant Participant Participant

  16. Selecting the Sample • Once the sample design is selected, you are ready to proceed with sample selection. • There is an Excel workbook entitled STEPS Sampling that includes spreadsheets for every stage of the sample selection: STEPSsampling.xls will do the following: • Randomize your sample selection. • Provide probability proportional size. • Provide information for the weighting.

  17. STEPSsampling.xls

  18. STEPSsampling.xls

  19. Using STEPSsampling.xls • Helps automate the selection process and provides weights for the dataset. • Plug information directly into the green sections of the spreadsheet and you will draw the first level of your sample. • Draw the subsequent levels by putting in information from the selected units.

  20. Documenting the Sample • The documentation is used during • data analysis and • writing the site report. • The documentation needs to include information on • sampling frame, • sample selection method at each sampling stage, and • probability of selection at each sampling stage. • Without this information, you will not be able to weight your data!

  21. Data Collection Forms • There are two data collection forms used by STEPS: • Interview Tracking Form • Kish Household Coversheet • You always need to use the Interview Tracking Form. • The sample design selected determines if you need the Kish Household Coversheet.

  22. Common Questions • If the interviewer knocks on a selected house and no one responds, but there is someone next door who wants to take part, can the interviewer ask them the questions?

  23. Common Questions If the interviewer knocks on a selected house and no one responds, but there is someone next door who wants to take part, can the interviewer ask them the questions? • NO, if the interviewer includes the neighbour in the sample then the sample is no longer random and it can not be weighted. • If the interviewers deviate from the selected sample, the sample will no longer be representative of the target population and will not be able to be weighted during data analysis.

  24. Common Questions (cont’d) • If the interviewer knocks on the door during their second attempt at an interview and no one is home, does the interviewer need to record this information on the Interview Tracking Form?

  25. Common Questions (cont’d) If the interviewer knocks on the door during their second attempt at an interview and no one is home, does the interviewer need to record this information on the Interview Tracking Form? • YES! This information is important. It is used to calculate the household response rate. The household response rate provides valuable information during data analysis.

  26. Common Questions (cont’d) • If a potential participant refuses to participate, do you need to record this information on the Interview Tracking Form?

  27. Common Questions (cont’d) If a potential participant refuses to participate, do you need to record this information on the Interview Tracking Form? • YES! This information is used to calculate the response rate and it is also used to weight the data.

  28. Steps in Selecting a Sample • Create a chart that describes the structure of your site. • Determine what information is available for each level of the structure (sampling frame). • Select the sampling scenario that corresponds to the available sampling frame. • Select the sample. • Document the sample. • Prepare the data collection forms.

  29. Using the Generic STEPS Tools • Every STEPS tool builds onto the next STEPS tool. For example, the analysis tools use information gathered and formatted into the data entry tool and into STEPSsampling.xls. • You do not have to use the generic tools. • If you do not use the generic tools, however, you will need to use the Mapped Instrument Tool if you wish to use the generic data analysis tools.

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