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Method

          The Star-Ledger/Eagleton-Rutgers Poll uses a random-digit dial (RDD) “probability” sampling design to select households for participation in each of its quarterly surveys.

          In lay terms, telephone numbers around the state are called “blindly,” with due respect to their prevalence in the population. The first “cut” is by area code. For example, if 30 percent of the states 18+ population lives in the 201 area code, 30% of the sample is drawn from that 201 code. The next cut is by county within that parameter. For example, if 40 percent of those living in the 201 area code are known (through census figures) to live in Bergen county, 12 percent of the Star-Ledger/Eagleton-Rutgers Poll’s sample is chosen from that county. [30% in (201) x 40% in Bergen = 12% of the statewide adults 18 years of age and older.] This is done by sampling on exchanges within area codes. Following this sampling procedure of area codes and exchanges ensures a correct geographical representation of residents of the state’s 21 counties.

          The sampling of phone numbers has to this point been on the first 6 digits —the three digit “area code,” and the three digit “exchange”— guaranteeing a proportional probability sample based on county and town-within-county distribution. The next four digits making up the sample phone number are in effect randomly generated, picking up listed and non-listed number alike. Thus the RDD moniker for “random-digit dialing.” Respondents within the designated households are also sampled, generally by means of a “last-birthday” method. Interviewers will ask to speak with the person (18+) in the household who has had the most recent birthday. This ensures a random selection of eligible respondents within the households selected.

          After the sample is selected and correct respondents interviewed, the sample is “weighted” to correct for the fact that some respondents are harder to reach than others. For example, less well-educated respondents may be less comfortable talking on the telephone and may “refuse” to be interviewed in greater proportion than better-educated respondents. At the end of the interviewing process, the poll may have spoken with 40% of those who have a college degree, when census figures tell us that only 25% of New Jerseyans have attained this much education.

          To make sure the sample is representative, each of those with a college degree must be counted as .625 of a person by the computer (40% x .625 = 25%) to make sure their prevalence in the sample actually equals their prevalence in the population from which the sample was selected. Those with less than a high school education may be counted as more than 1.00 of a person by the computer. (Assume that 8 percent of the sample say they have less than a high school degree, while the true population count is 12 percent. Each person in this group must be counted as 1.5 for the sample to be truly representative.) A common Star-Leger/Eagleton-Rutgers Poll weights on age and education to adjust the sample to comport with current census-projected figures of the New Jersey adult population. This weighting ensures that findings from a sample can be generalized to the larger target population of all New Jersey adults.

          All surveys are subject to “sampling error”—that is the expected probable difference between interviewing everyone in a population, and a scientifically selected sample of that population. Sampling error is based on the size of the sample. The following chart shows the relationship between sample size and sampling error.

          By way of illustration, if the sample contains 400 respondents, the associated sampling error is plus or minus 5 percentage points. [This is true for percentages near 50% at a 95 percent confidence interval.] If 50 percent of the sample percent were found to give the Governor a positive job rating, one could be 95 percent confident that the true value among all New Jersey adults in the population would be somewhere between 45 and 55 percent (50 + 5), with 50 percent being the most likely figure.

          Sampling error does not take into account other sources of variation inherent in public opinion studies, such as non-response, question wording or context effects. The Star-Ledger/Eagleton-Rutgers Poll makes available its questionnaires on-line for those who wish to see the questionnaire wording and sequencing. Response dispositions are either available on-line or by request.

 

 
 

Rutgers-Eagleton Poll
Eagleton Institute of Politics
185 Ryders Lane
New Brunswick, NJ  08901-8557
phone: (732) 932-9384 ext. 285
fax:(732) 932-1551

 
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