Library Services Survey
Prepared by Princeton Survey Research Associates International for the Pew Research Center’s Internet & American Life Project November 2012
The Library Services Survey, conducted by the Pew Research Center’s Internet & American Life Project and funded by the Bill & Melinda Gates Foundation, obtained telephone interviews with a nationally representative sample of 2,252 people ages 16 and older living in the United States. Interviews were conducted via landline (nLL=1,127) and cell phone (nC=1,125, including 543 without a landline phone). The survey was conducted by Princeton Survey Research Associates International. The interviews were administered in English and Spanish by Princeton Data Source from October 15 to November 10, 2012. Statistical results are weighted to correct known demographic discrepancies. The margin of sampling error for results based on the complete set of weighted data is ±2.3 percentage points. Results based on the 1,945 internet users5 have a margin of sampling error of ±2.5 percentage points.
Details on the design, execution and analysis of the survey are discussed below.
DESIGN AND DATA COLLECTION PROCEDURES
A combination of landline and cellular random digit dial (RDD) samples was used to represent all adults in the United States who have access to either a landline or cellular telephone. Both samples were provided by Survey Sampling International, LLC (SSI) according to PSRAI specifications.
Numbers for the landline sample were drawn with probabilities in proportion to their share of listed telephone households from active blocks (area code + exchange + two-digit block number) that contained three or more residential directory listings. The cellular sample was not list-assisted, but was drawn through a systematic sampling from dedicated wireless 100-blocks and shared service 100-blocks with no directory-listed landline numbers.
Interviews were conducted from October 15 to November 10, 2012. As many as 7 attempts were made to contact every sampled telephone number. Sample was released for interviewing in replicates, which are representative subsamples of the larger sample. Using replicates to control the release of sample ensures that complete call procedures are followed for the entire sample. Calls were staggered over times of day and days of the week to maximize the chance of making contact with potential respondents. Interviewing was spread as evenly as possible across the days in field. Each telephone number was called at least one time during the day in an attempt to complete an interview.
For the landline sample, interviewers asked to speak with the youngest male or female ages 16 or older currently at home based on a random rotation. If no male/female was available, interviewers asked to speak with the youngest person age 16 or older of the other gender. This systematic respondent selection technique has been shown to produce samples that closely mirror the population in terms of age and gender when combined with cell interviewing.
For the cellular sample, interviews were conducted with the person who answered the phone. Interviewers verified that the person was age 16 or older and in a safe place before administering the survey. Cellular respondents were offered a post-paid cash reimbursement for their participation.
WEIGHTING AND ANALYSIS
The first stage of weighting corrected for different probabilities of selection associated with the number of adults in each household and each respondent’s telephone usage patterns.6 This weighting also adjusts for the overlapping landline and cell sample frames and the relative sizes of each frame and each sample.
This first-stage weight for the ith case can be expressed as:
Where SLL = size of the landline sample
SCP = size of the cell phone sample
ADi = Number of adults in the household
R = Estimated ratio of the land line sample frame to the cell phone sample frame
The equations can be simplified by plugging in the values for SLL = 1,127 and SCP = 1,125. Additionally, we will estimate of the ratio of the size of landline sample frame to the cell phone sample frame R = 0.60. The final stage of weighting balances sample demographics to population parameters. The sample is balanced by form to match national population parameters for sex, age, education, race, Hispanic origin, region (U.S. Census definitions), population density, and telephone usage. The Hispanic origin was split out based on nativity; U.S born and non-U.S. born. The White, non-Hispanic subgroup is also balanced on age, education and region. The basic weighting parameters came from a special analysis of the Census Bureau’s 2011 Annual Social and Economic Supplement (ASEC) that included all households in the United States. The population density parameter was derived from Census data. The cell phone usage parameter came from an analysis of the July-December 2011 National Health Interview Survey.78 Weighting was accomplished using Sample Balancing, a special iterative sample weighting program that simultaneously balances the distributions of all variables using a statistical technique called the Deming Algorithm. Weights were trimmed to prevent individual interviews from having too much influence on the final results. The use of these weights in statistical analysis ensures that the demographic characteristics of the sample closely approximate the demographic characteristics of the national population. Table 1 compares weighted and unweighted sample distributions to population parameters.
Effects of Sample Design on Statistical Inference
Post-data collection statistical adjustments require analysis procedures that reflect departures from simple random sampling. PSRAI calculates the effects of these design features so that an appropriate adjustment can be incorporated into tests of statistical significance when using these data. The so-called “design effect” or deff represents the loss in statistical efficiency that results from systematic non-response. The total sample design effect for this survey is 1.24.
PSRAI calculates the composite design effect for a sample of size n, with each case having a weight, wi as:
In a wide range of situations, the adjusted standard error of a statistic should be calculated by multiplying the usual formula by the square root of the design effect (√deff ). Thus, the formula for computing the 95% confidence interval around a percentage is:
where p is the sample estimate and n is the unweighted number of sample cases in the group being considered.
The survey’s margin of error is the largest 95% confidence interval for any estimated proportion based on the total sample — the one around 50%. For example, the margin of error for the entire sample is ±2.3 percentage points. This means that in 95 out every 100 samples drawn using the same methodology, estimated proportions based on the entire sample will be no more than 2.3 percentage points away from their true values in the population. The margin of error for estimates based on form 1 or form 2 respondents is ±3.3 percentage points. It is important to remember that sampling fluctuations are only one possible source of error in a survey estimate. Other sources, such as respondent selection bias, questionnaire wording and reporting inaccuracy, may contribute additional error of greater or lesser magnitude.
Table 2 reports the disposition of all sampled telephone numbers ever dialed from the original telephone number samples. The response rate estimates the fraction of all eligible respondents in the sample that were ultimately interviewed. At PSRAI it is calculated by taking the product of three component rates:9
Contact rate – the proportion of working numbers where a request for interview was made10
Cooperation rate – the proportion of contacted numbers where a consent for interview was at least initially obtained, versus those refused
Completion rate – the proportion of initially cooperating and eligible interviews that were completed
Thus the response rate for the landline sample was 11.4 percent. The response rate for the cellular sample was 11 percent.