Methodology Statement

Survey Methodology

April 2013 Winthrop Poll 

The April 2013 Winthrop Poll interviewed 1069 adults living in South Carolina.  After weights (for sex, age, and race) have been applied, results which use all respondents have a margin of error of approximately +/- 3% at the 95% confidence level. Results that use less than the full sample will naturally have a higher margin of error. For results using only Registered voters, n=832, margin of error of approximately +/- 3.5% at the 95% confidence level.
   

The survey was in the field from 4/6-4/14, 2013. Phone calls were made during weekday evenings, all day Saturday, and Sunday afternoon and evening.  Weekday daytime calls are generally not made to avoid oversampling those who are more likely to be at home during the day (e.g. retirees, stay-at-home-moms, etc.).  Conducting weekend calls is important to avoid systematically excluding certain populations (such as those who may work 2nd or 3rd shift during the week).

The survey used (1) Random Digit Dialing (RDD) and (2) wireless phone number sampling. Both RDD and wireless samples are crucial to ensure no adult in the geographical area of interest is systematically excluded from the sample. Both the RDD sample and the wireless sample were purchased from Survey Sampling International (SSI). A further explanation of RDD methodology, with descriptions taken from SSI’s website, may be found below.

Phone numbers selected for the survey were re-dialed five or more times in an attempt to reach a respondent.  Once a household was reached, we also employed procedures to randomize within households for RDD sample.  

Additionally, we: 

1. Screen the wireless sample for wireless-only status since individuals who have a cell phone and a land line already have an established probability of appearing in the RDD. 
2. Weight responses based on sex, age, and race according to the known population of residents of SC age 18 and older. 
3. Weight Responses for the subset of registered voters based on sex, age, and race according to the known population of those registered to vote in South Carolina

Computerized autodialers were not used in order to ensure the survey of wireless phones complied with the Telephone Consumers Protection Act and all FCC rules regarding contacting wireless telephones. 

The Winthrop Poll is paid for by Winthrop University with additional support from The West Forum on Politics and Policy at Winthrop University.  
 

Samples are generated using a database of "working blocks." A block (also known as a 100-bank or a bank) is a set of 100 contiguous numbers identified by the first two digits of the last four digits of a telephone number. For example, in the telephone number 203-567-7200, "72" is the block. A block is termed to be working if some specified number of listed telephone numbers are found in that block. 

Samples of random numbers distributed across all eligible blocks in proportion to their density of listed telephone households are selected. All blocks within a county are organized in ascending order by area code, exchange, and block number. Once the quota has been allocated to all counties in the frame, a sampling interval is calculated by summing the number of listed residential numbers in eligible blocks within the county and dividing that sum by the number of sampling points assigned to the county. From a random start between zero and the sampling interval, blocks are systematically selected in proportion to their density of listed households.  Once a block has been selected, a two-digit number is systematically selected in the range 00-99 and is appended to the exchange and block to form a 10-digit telephone number.