Agricultural Census Sampling Frames and Sampling

1. Agricultural Census Sampling Frames and Sampling Section A

2. What is Sampling? Selecting a representative group of units from a population in order to make estimates about the population from the sample.

3. What is a Sampling Frame? A list of all the units from which the sample will be selected. It can take various forms, depending upon the unit of analysis.

4. Why Use Sampling? • Saves time and money in comparison to complete enumeration • Can concentrate on a particular sub-group of interest (e.g. livestock) • Where nonsampling errors are large, sampling allows for better control

5. When to Not Use Sampling • If data are needed down to the lowest administrative level in a country • Measuring small changes over time • High survey costs related to sampling, field control, etc.

6. Characteristics of a Good Sampling Plan • Known chance of selection for each unit • Measurable reliability • Feasibility • Economical and efficient

7. What Size Sample do I Need? The most practical sample size that will accurately estimate a population parameter with a specified precision using the most efficient design.

8. Systematic Sampling • Basically as accurate as simple random sampling • How to do it: • Determine your sampling rate • Use random number table to find a starting number equal or less than the sampling interval • From starting number take every nth unit

9. Sampling Frame Consideration • When a sampling frame contains units that are out of scope do not substitute: • Results in higher sampling rate • May introduce bias in the selection process

10. Sample Stratification • When characteristics of the population are known they can be used to improve sampling • Similar sampling units are grouped together into strata • A simple random sample selects at least one unit from each group or strata • Must know size of strata and have reliable frame for selection of units from each strata

11. Probability Proportional to Size • Possible when a size measure is available that is correlated to the variable of interest • Can improve accuracy by ensuring that large units which have the greatest impact on population estimates have the same probability of selection • Used when sampling units vary in size

12. Cluster Sampling Sampling units are grouped into clusters instead of a list and a sample of clusters is chosen rather than individual units. • Good for when no adequate frame exists of the units in the population • Or for saving field costs by cluster sampling on some geographic basis

13. Single-Stage Cluster Sample • A cluster of units is selected and all units in the selected cluster is sampled • Good for when there is no list of units or when sampling at the unit level is inefficient • Can also use area samples, where a geographic area is considered a cluster

14. Multi-Stage Cluster Sample • A subsample of units in the selected cluster is selected for the sample • A sample of clusters is selected • A list of all units in the selected clusters is constructed • A sample of units in the selected clusters is selected • Reduces the listing required and

15. Area Sampling Area sampling is a frequently used method of cluster sampling, used when: • Complete household lists are unavailable but reasonably good maps exist • Travel costs for covering randomly selected households is too great

16. Single-Stage Area Sampling • Obtain a detailed map and update with local authorities, noting changes on map • Number blocks serially in a serpentine manner to avoid omissions • Select a simple or systematic 1% sample of blocks • Interview all households in the selected sample blocks

17. Two-Stage Area Sampling • Take one block in every 25 as a sample block instead of 1 in a 100 used in procedure A. • Divide each sample block into 4 parts with approximately equal numbers of households • Number each block segment from 1 to 4 • Randomly select one of the four segments in each sample block and interview all households in the selected segment

18. Area Sampling with Listing When there are no maps then: • Select 1 in 25 blocks • List and number all households in the sample blocks • Select ¼ of all households in each sample block by simple random or systematic sample • Interview all selected sample households

19. Choice of Sample Design • Sampling specialists must choose an efficient design to provide the desired precision balancing costs and standard error • When cost is not important, single-stage sampling provides more accurate results • When cost and administrative convenience are important a cluster sample is often used

20. Section A Quiz • What are the characteristics of a good sampling plan? • What is area sampling and when is it used? • When should you not use sampling? • What is stratification?