09 Appendix D: Methodology

Appendix D: Methodology


The data reported in The Bible in America are based on a series of telephone and online interviews with nationwide random samples.



All telephone interviews were conducted by Barna Group. All households were selected for inclusion in the sample using a random-digit dial technique, which allows every telephone household in the nation to have an equal and known probability of selection. Households selected for inclusion in the survey sample received multiple callbacks to increase the probability of obtaining a representative distribution of adults. Regional quotas were used to ensure that sufficient population dispersion was achieved. There were also minimum and maximum ranges placed on the distribution of respondents within several demographic variables, which were tracked during the field process to ensure that statistical weighting would not be excessive. When a particular attribute reached one of the parameters, the sampling selection process was varied to preclude individuals who did not meet the necessary demographic criterion, with the interviewer seeking a person from the same household who fit the desired criterion. Between 20 and 40 percent of telephone interviews were conducted on cell phones.

Online interviews were conducted using an online research panel called KnowledgePanel® based on probability sampling that covers both the online and offline populations in the U.S. Panel members are randomly recruited by telephone and by self-administered mail and web surveys. Households are provided with access to the Internet and hardware, if needed. Unlike other Internet research that covers only individuals with Internet access who volunteer for research, this process uses a dual sampling frame that includes both listed and unlisted phone numbers, telephone and non-telephone households, and cell-phone-only households. The panel is not limited to current Web users or computer owners. All potential panelists are randomly selected to join the KnowledgePanel; unselected volunteers are not able to join.

Once data was collected, minimal statistical weights were applied to several demographic variables to more closely correspond to known national averages.

When researchers describe the accuracy of survey results, they usually provide the estimated amount of “sampling error.” This refers to the degree of possible inaccuracy that could be attributed to interviewing a group of people that is not completely representative of the population from which they were drawn. For general population surveys, see the table above for maximum sampling error.

There is a range of other errors that can influence survey results—e.g., biased question wording, question sequencing, inaccurate recording of responses, inaccurate data tabulation, etc.—errors whose influence on the findings cannot be statistically estimated. Barna Group makes every effort to overcome these possible errors at every stage of research.

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