1/26/00 Random digit dialing Delineate the geographic boundaries of the sampling area Identify all of the exchanges used in the geographic area Identify the distribution of prefixes with the sampling area –Example: There may be 8 exchanges, but you may find that 3 of them are used for nearly two-thirds of residential lines.
1/26/00 Random digit dialing You may stratify based on the distribution of prefixes –Ex. Take more samples of the 3 exchanges that account for the most residential lines Try to identify vacuous suffixes –These are suffixes not yet assigned or assigned in large groups to a business –Usually consider suffixes in 100s ex ,
1/26/00 Random digit dialing May randomly select the four-digit suffixes –ex. use a random-numbers table Alternatively, you may use a plus-one approach –When you reach residence, use the number as a seed, and add fixed digits (one or two) to get the next sample
1/26/00 Random digit dialing What are the advantages and disadvantages of the random method? What are the advantages and disadvantages of the plus-one method?
1/26/00 Random digit dialing Provides a nonzero chance of reaching any household within a sampling area that has a telephone line regardless of whether the number is listed Is the probability of reaching every household equal? –No. Households with more than one phone line will have a greater probability than households with one phone line. –Adjust for unequal probability by weighting
1/26/00 Random Digit Dialing Advantages: Inexpensive and easy to do Disadvantages: 1. Large number of unfruitful calls 2. Will exclude individuals without phones 3. May be difficult to ascertain geographic area
1/26/00 Cluster sampling Each member of the study population is assigned to a group or cluster, then clusters are selected at random and all members of a selected cluster are included in the sample Clusters are often naturally-occurring groupings such as schools, households, or city blocks (Henry, 1990)
1/26/00 Multistage sampling strategy Underlying concept is similar to cluster sampling Clusters are selected as in the cluster sample, then sample members are selected from the cluster members by simple and random sampling Clustering may be done at more than one stage (Henry, 1990)
1/26/00 School example Assume 20,000 students; 40 schools Desired sample size=2,000 students (i=10)
1/26/00 Possible Approaches Select all schools, list all students, and select 1/10 of students (SRS) Select 1/2 of schools, then select 1/5 of students (multistage) Select 1/5 of schools, then select 1/2 of students (multistage) Select 1/10 of schools, then collect data on all students in those schools (cluster)
1/26/00 Area probability sampling Geographic basis Divide a total geographic area into exhaustive, mutually exclusive subareas Sample subareas List all housing units within selected subareas Sample from list or collect data from entire list (Fowler, 1993)
1/26/00 Example City has 400 blocks with a total of 20,000 housing units Sample size is 2,000 housing units (i=10)
1/26/00 Approaches Sample 80 blocks, list housing units, then sample 1/2 of the housing units Sample 40 blocks, list housing units, then sample all of them
1/26/00 Area probability sampling proportional to size Choose a cluster size for the last stage of sampling (for example, 10 housing units) Estimate the number of housing units on each block Order the blocks so that similar ones are contiguous Create a cumulative count of housing units
1/26/00 Determine the interval between clusters (I=i*cluster size) Choose a random start between 1 and I. Proceed through cumulative count, selecting every Ith block List units on the selected block and select cluster size (ex. 10) (Fowler, 1993)
1/26/00 What if your estimate of the number of housing units on the block was wrong? Use (cluster size/estimated units on block) for each block Result is 10/43 for block 1, 10/87 for block 2, and 10/99 for block 3.
1/26/00 Respondent selection Once you have selected a housing unit, how do you select the individual respondent? Who is the best person to provide the desired information? For self-reporting surveys, we use probability sampling to select the respondent.
1/26/00 Ascertain the number of eligible individuals in the housing unit Number them Randomly select a respondent You may need to weight the response by the number of eligible individuals in the housing unit
1/26/00 Nonprobability sampling designs Convenience: select cases based on their availability for the study Most similar/dissimilar cases: select cases that are judged to represent similar conditions or, alternatively, very different conditions Typical cases: select cases that are known beforehand to be useful and not to be extreme
1/26/00 Nonprobability sampling designs Critical cases: select cases that are key or essential for overall acceptance or assessment Snowball: group members identify additional members to be included in sample
1/26/00 Nonprobability sampling designs Quota: interviewers select sample that yields the same proportions as the population proportions on easily identified variables (Henry, 1990)
1/26/00 Terminology Universe Population Survey population Sampling frame Sampling unit Observation unit