At a glance
CASPER uses a two-stage cluster sampling methodology. In the first stage, clusters (traditionally 30) are selected with a probability proportional to the estimated number of households in the clusters. A cluster is a non-overlapping section in a geographic area with a known number of households. In the second stage, interview teams use systematic random sampling to select seven households from each of the selected clusters.
First stage sampling
Selecting a CASPER sample
Selecting a CASPER sample requires a list of all clusters within your sampling frame. This includes the number of households within each cluster. U.S. Census blocks are most commonly used.
You can download this information from the Census website or population-based files within Geographic Information System (GIS) software.
Number each household by assigning each cluster with a cumulative sum of the number of households. Finally, select 30 clusters by using a random number generator to select 30 numbers. The numbers should be between one and the total number of households within your sampling frame. Select the entire cluster in which that random number (i.e., household) is located.
Note:
Choose all clusters without substitution. This means that clusters originally selected are those that are assessed with no changes or modifications. Any departure from this design (30×7 cluster sampling) is not considered a CASPER. In situations where the 30×7 design may not be feasible or ideal, and a change in methodology is warranted, modified CASPERs may be acceptable but must be described as modified in report(s). See Modified CASPERs for more information.
Develop maps via the Census website or GIS software so teams can easily navigate to the selected clusters. For more information and detail on selecting clusters, please see the CASPER Toolkit, Section 2.4.
Geographic Information System (GIS) CASPER toolbox
Using GIS software rather than the U.S. Census website allows the user to select portions of a:
- County
- City
- Other available geopolitical areas
This provides more flexibility in the selection of the sampling frame.
A CASPER Toolbox is available for GIS sample selection that can automatically select clusters. It is currently available in ArcGIS Desktop for those with GIS expertise. CDC also provides sampling and mapping support to requesting jurisdictions. Email CASPER@cdc.gov for more information.
Second stage sampling
Typically, a single individual will conduct the first stage of sampling (selecting 30 clusters). It is the responsibility of the interview teams to appropriately select the households within each cluster to interview.
Systematic random sampling
To select the seven households to interview, conduct systematic random sampling.
To do this, count (or estimate) the number of households within the selected cluster. Divide that number by 7 (this will be your n). Starting at a random point, travel through the cluster in a serpentine method to select every nth household for interview. Walk up one side of the street. Then, turn and walk down the other side in such a manner that every house within the selected cluster is passed.
The most scientific and representative way is to select the seven households. Then, continue to return to those households until an interview is completed.
However, it is important to balance the scientifically ideal with the real-world or disaster situation. Therefore, interview teams should attempt to revisit the selected household three times. The interview teams may replace the household if an interview wasn't successful (e.g., household refused, nobody answered after three attempts, house is vacant, or there was language barrier).
Overall, keeping the sample as complete and representative as possible requires sound judgment and quality training of interview teams.
For more information, please see the CASPER Toolkit, Section 3.4.
Things to avoid
Convenience sampling
Convenience sampling is a form of non-probability sampling that involves selection based on:
- Availability
- Opportunity
- Convenience
Examples include going to households where there are people outside or where another interviewee told you to go since they know they would answer.
Target sampling
Target sampling is a form of non-probability sampling that involves intentionally sampling a certain population or group. An example is going to the household that looks the most damaged or like it will get "best" results.
Sequential sampling
Sequential sampling is going to one house after another in order. This will likely bias your sample to one section of your cluster.
Note that there are some circumstances which may predetermine sequential sampling as necessary. This includes clusters with less than 10 households. Another circumstance is when your clusters are difficult to navigate. In such a case, sequential sampling is the only way to successfully complete the CASPER in the time allowed. In these circumstances, it is extremely important to select a random starting point prior to going into the field.
Sampling and mapping support
CDC provides sampling and mapping support to aid in the CASPER process. This support is provided to any requesting jurisdiction or agency. Email CASPER@cdc.gov for assistance.