The 2020 group quarters (GQ) sampling frame was divided into two strata: a small GQ stratum and a large GQ stratum. Small GQs are defined to have expected populations of fifteen or fewer residents, while large GQs have expected populations of more than fifteen residents.
Samples were selected in two phases within each stratum. In general, GQs were selected in the first phase and then persons/residents were selected in the second phase. Both phases differ between the two strata. Each sampled GQ was randomly assigned to one or more months in 2020 - it was in these months that their person samples were selected.
First Phase of Sample Selection for Small GQs
There were two stages of selecting small GQs for sample.
- First stage
The small GQ universe is divided into five groups that are approximately equal in size. All new small GQs are systematically assigned to one of these five groups on a yearly basis, with about the same probability (20 percent) of being assigned to any given group. Each group represents a second-stage sampling frame, from which GQs are selected once every five years. The 2020 secondstage sampling frame was used in 2013 as well, and is currently to be used in 2023, 2028, etc.
- Second stage
GQs were systematically selected from the 2020 second-stage sampling frame. Each GQ had the same second-stage probability of being selected within a given state, where the probabilities varied between states. Table 3 below shows these probabilities.
Second Phase of Sample Selection for Small GQs
Persons were selected for sample from each GQ that was selected for sample in the first phase of sample selection. If fifteen or fewer persons were residing at a GQ at the time a field representative (interviewer) visited the GQ, then all persons were selected for
sample. Otherwise, if more than fifteen persons were residing at the GQ, then the interviewer selected a systematic sample of ten persons from the GQ's roster.
Targeted Sampling Rate (Probability of Selection) for Small GQs
The targeted state-level sampling rates are the probabilities of selecting any given person in a GQ; it is around these probabilities that the sample design is based. These probabilities reflect both phases of sample selection, and they varied by state. The
probabilities for 2020 are shown in Table 3.
The sample was designed so that the second-phase sampling rate would be one-hundred percent for small GQs (i.e., select the entire expected population of fifteen or fewer persons for sample in every small sampled GQ). This means the probability of selecting
any person in a small GQ was designed to equal the probability of selecting the small GQ itself.
First phase of Sample Selection for Large GQs
All large GQs are eligible to be sampled as has been the case every year since the inception of the GQ sampling in 2006. This means there was only a single stage of sampling in this phase. This stage consists of systematically assigning "hits" to GQs
independently in each state, where each hit represents ten persons to be sampled.
In general, a GQ has either Z or Z+1 hits assigned to it. The value for Z is dependent on both the GQ's expected population size and its within-state target sampling rate, shown in Table 3. When this rate is multiplied by a GQ's expected population, the result is a GQ's expected person sample size. If a GQ's expected person sample size is less than ten, then Z = 0; if it is at least ten but less than twenty, then Z = 1; if it is at least twenty but less than thirty, then Z = 2; and so on. See below for a detailed example.
If a GQ has an expected person sample size that is less than ten, then this method effectively gives the GQ a probability of selection that is proportional to its size; this probability is the expected person sample size divided by ten. If a GQ has an expected
person sample size of ten or more, then it is in sample with certainty and is assigned one or more hits.
Second Phase of Sample Selection for Large GQs
Persons were selected within each GQ to which one or more hits were assigned in the first phase of selection. There were ten persons selected at a GQ for every hit assigned to the GQ. The persons were systematically sampled from a roster of persons residing at the GQ at the time of an interviewer's visit. The exception was if there were far fewer persons residing in a GQ than expected - in these situations, the number of persons to sample at the GQ would be reduced to reflect the GQ's actual population. In cases where
fewer than ten persons resided in a GQ at the time of a visit, the interviewer would select all of the persons for sample.
Targeted Sampling Rate (Probability of Selection) for Large GQs
As for small GQs, the targeted sampling rate is the probability of selecting any given person in a GQ. This probability reflects both phases of sample selection. The targeted sampling rate for Puerto Rico in 2020 was 2.44 percent. Note that this is the same rate as
for persons in small GQs.
For example, suppose a GQ had an expected population of 500 and the targeted sampling rate was 2.26%- this means any given person in a GQ would have an approximately 1-in44.2478 chance of being selected. This rate, combined with the GQ's expected population of 500, means that the expected number of persons selected for sample in the GQ would be approximately 11.3 (2.26% x 500); this GQ would be assigned either one or two hits (Z = 1).
Group Quarters Sample Month Assignment
All sample GQs were assigned to one or more months (interview months) - these were the months in which interviewers would visit a GQ to select a person sample and conduct interviews. All small GQs, all large GQs that were assigned only one hit, all remote Alaska GQs, all sampled military facilities, and all sampled correctional facilities (regardless of how many hits a military or correctional facility was assigned) were assigned to a single interview month. Remote Alaska GQs were assigned to either January or September; Federal prisons were assigned to September; all of the others were randomly assigned one interview month.
All large GQs that had been assigned multiple hits, but were not in any of the categories above, had each hit randomly assigned to a different interview month. If a GQ had more than twelve hits assigned to it, then multiple hits would be assigned to one or more interview months for the GQ. For example, if a GQ had fifteen hits assigned to it, then there would be three interview months in which two hits were assigned and nine interview months in which one hit was assigned. There are two restrictions to this process. One restriction is applied to college dormitories, whose hits are randomly assigned to non-summer months only, i.e., January through April and September through December. The other restriction is applied to military ships, whose hits were randomly assigned only to the last ten months of the year, i.e., March through December.
Prior to 2016, all GQs were sampled at the same time for a given year. Starting in 2016, Bureau of Prison GQs (Federal prisons) started to be sampled separately from other GQs. They are sampled using the same procedure described above, and are all assigned to the September interview month as before.
2020 Sample Sizes for Housing Unit Addresses and Group Quarters
Counts of sample addresses and GQ persons can be found in two locations on the US Census Bureau website. In American Fact Finder, base tables B98001 and B98002 provide sample size counts for the Puerto Rico and municipios. Sample size definitions and sample size counts for Puerto Rico are also available in the Sample Size and Data Quality Section of the ACS website, at
https://www.census.gov/acs/www/methodology/sample-size-and-data-quality/.
