Methodology of AJC poll

Abt SRBI conducted the survey of registered voters in Georgia on behalf of The Atlanta Journal-Constitution. The survey included telephone interviews with a representative sample of 1,012 registered voters living in Georgia. Telephone interviews were conducted by land line (n=612) and cellphone (n=400, including 293 without a land line phone or “cell-only”). Interviewing was conducted from May 5 to May 8.

Sampling

The sample design was a random digit-dialed sample of cellphone numbers and land-line numbers with a Georgia telephone exchange. This sample design is referred to as a “dual-frame” because it includes cellphones and landlines. Both samples were provided by Survey Sampling Inc.

The land-line frame is constructed by compiling all Georgia telephone exchanges that are classified as providing regular telephone service. The frame is referred to as “list-assisted” because a complete file of directory-listed residential numbers is used to remove 100-banks from the frame if they contain zero residential listings. The remaining 100-banks are “working” and used to enumerate all the telephone numbers within the bank from which a sample is drawn. All land-line numbers (directory-listed and unlisted) in the working banks are eligible to be randomly dialed. Telephone numbers known to belong to businesses are removed.

The cellular telephone frame begins with 1,000-blocks constructed from exchanges that provide cellular telephone service. The frame of 1,000-blocks is then expanded to the 100-block level to identify and remove “mixed-use” 100-blocks, or those that include land-line numbers. The result is a sampling of cellular 100-blocks that is mutually exclusive of the list-assisted RDD sampling frame described above. All cellphone numbers in the working banks are eligible to be randomly dialed.

For the land-line sample, interviewers were asked to speak with the youngest adult male or female currently at home based on a random rotation. If no male/female was available, interviewers asked to speak with the youngest adult of the other gender. For the cell sample, interviews were conducted with the person who answered the phone. Interviewers verified that the person was an adult and in a safe place before administering the survey.

All cooperating respondents from both samples were asked about their voter registration status. Registered voters continued with the full interview. People who said they were not registered to vote were only asked demographic questions for weighting purposes.

Weighting

The final weights produced for this survey compensated for the dual-frame sample design and aligned the sample to match the population parameters of the adult noninstitutionalized population in Georgia. To construct the weights, we use the sample of 1,012 registered voters and 290 nonregistered voters. The first stage of weighting adjusts for the overlapping land line and cell sample frames by calculating the probability that dual users are selected in either the land-line or the cellphone sample. Cellphone-only respondents were only eligible for selection in the cell sample and land line-only respondents were only eligible for selection in the land-line sample.

The second stage of weighting balanced sample demographics to estimated population parameters for the state of Georgia. The sample was balanced to match population parameters for sex, age, education level, race/Hispanic ethnicity, region (north, Atlanta Metro, Atlanta exburbs, Southeast, Southwest), and telephone usage (cell-only, dual-user, land line-only). The demographic and region population parameters were computed from the 2012 American Community Survey. The telephone usage population estimates were constructed from the model-based estimates for Georgia released by the National Center for Health Statistics for the year 2012 . Since the cellphone-only adult population has increased every year since 2012, these substate-level estimates were updated to reflect national trends according to the 2013 NCHS report.

The second-stage weighting was conducted using an operation known as raking ratio estimation, or “raking”. Raking is used to reduce the risk of biases due to nonresponse and noncoverage in sample surveys. The raking procedure uses an iterative technique that simultaneously calibrates the sample to population distributions defined by sociodemographic parameters. After the raked weights were generated, we examined the distribution of values.

Margin of error

The margin of error for an estimate is a measure of uncertainty that reflects the fact that the estimate is derived from a sample drawn from the population. If one were to draw a second sample in the exact same manner, the estimate would be different from the first simply due to the fact that the sample contains different members of the population. A third sample would be different from the first two, and so on. The margin of error measures how different estimates could be based on drawing different samples from the same population.

The error margin for the sample of 1,012 registered voters is +/-4 percent. This includes a “design effect” of 1.67. The design effect is the amount of variability introduced by the sample design, such as the dual-frame sample and weighting.

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