Content
Sample Design
Sample Construction
Screening to Determine Household Eligibility
Selection of Respondent Within Household
Young Adult Oversample
Initial Contact
Spanish Language Interviews
Refusal Conversion
Field Outcomes
Sample Weighting
Precision of Sample Estimates
Estimating Statistical Significance
Statistical Comparisons Between Samples
References
Technical Report Documentation
APPENDIX A1: English Language Questionnaire Version 1
APPENDIX A2: English Language Questionnaire Version 2
APPENDIX B1: Spanish Language Questionnaire Version 1
APPENDIX B2: SPANISH Language Questionnaire Version 2

Field Outcomes

Survey data collection by the Federal government requires prior approval by the Office of Management and Budget (OMB). Before submitting the formal request for data collection to OMB, NHTSA published a Notice in the Federal Register soliciting comments on the information collection. The Notice appeared in the Federal Register, 63:50, pages 12858-12859, March 16, 1998. The closing date for comments was May 15, 1998. No comments were received in response to the Notice. NHTSA then submitted the request for data collection to OMB June 29, 1998. OMB approved the information collection September 9, 1998, assigning it the OMB number 2127-0596 with and expiration date of December 31, 1999.

The field interviewing for the study commenced on November 5, 1998, following training of the field interviewers, and was completed on January 12, 1999. This is approximately the same time period in which the 1996 Occupant Protection Survey (November 4, 1996 to January 5, 1997) and the 1994 Occupant Protection Survey (October 5, 1994-December 11, 1994) were conducted. Status of cases as of the end of the field period are reported using the categories defined below.

For survey Version 1 - Seat Belt Usage Issues, a total of 30,620 randomly selected telephone numbers were sampled within a geographically stratified national sampling frame for both sample components (the cross-section of youth and adults age 16 and older and the oversample of persons age 16-39):

• 59% of the numbers were not active residential phone numbers, including 32% Pre-screened NIS/DIS; 10% NIS/DIS/Change#/Wrong# after the number was dialed; 7% SSI Pre-screened Business Numbers; and 3% computer or fax tones;

• 7% of the numbers were no answers (despite repeated attempts) and 2% were answering machines; and

• 2% were households in which the designated respondent was not interviewable (not available; away for an extended period, incapacitated, or deaf) and an additional 1% were non-interviewable due to language barriers (non-Spanish).

At the close of the field period, only 521 cases (2%) were in callback status.

The participation rate represents one of the most critical measures of potential sample bias because it indicates the degree of self-selection by potential respondents into or out of the survey. The participation rate is calculated as the number of completed interviews (including respondents who screen out as ineligible) divided by the combined total number of completed interviews, terminated interviews, and refusals to interview. (The inclusion of screen outs in the numerator and denominator is mathematically equivalent to discounting the refusals by the estimated rate of non-eligibility among refusals.) The participation rate for Version 1 is based on the following elements:

• 4,094 completed interviews;

• 2,436 cases in which someone in the household completed the household screen, but no one in the household was found to be eligible for the full interview (2362 were age-related screen-outs among the age 16-39 oversample); and

• 1,627 refusals to be interviewed (including 1220 second refusals) and 43 terminated interviews.

Based on the standard calculations of participation rate, as defined by the Council of Applied Statistical Research Organizations (CASRO), the participation rate for Version 1 was 79.6%. This formula treats the numerator as all respondents who participate by completing required survey questions, while the denominator includes those who complete required questions, those who begin but terminate before completing all required questions, and those who refuse entirely.

The Final Summary Disposition of the Version 1 sample is given in Table 3. The table includes breakouts for each survey component (national youth and adult cross-section and the age 16-39 oversample). The average interview length for Version 1 was 22.7 minutes.

For survey Version 2 - Child Safety Seat Issues, a total of 29,592 randomly selected telephone numbers were sampled within a geographically stratified national sampling frame for both sample components (the cross-section of youth and adults age 16 and older and the oversample of persons age 16-39):

• 59% of the numbers were not active residential phone numbers, including 31% Pre-screened NIS/DIS; 10% NIS/DIS/Change#/Wrong# after the number was dialed; 8% SSI Pre-screened Business Numbers; and 3% computer or fax tones;

• 6% of the numbers were no answers (despite repeated attempts) and 2% were answering machines; and

• 3% were households in which the designated respondent was not interviewable (not available; away for an extended period, incapacitated, or deaf) and an additional 1% were non-interviewable due to language barriers (non-Spanish).

At the close of the field period, there were 235 cases (1%) in callback status.

The participation rate for Version 2 is based on the following elements:

• 4,121 completed interviews;

• 2,693 cases in which someone in the household completed the household screen, but no one in the household was found to be eligible for the full interview (2,614 were age-related screen-outs among the age 16-39 oversample); and

• 1,538 refusals to be interviewed (including 1024 second refusals) and 40 terminated interviews.

Based on the standard calculations of participation rate, the participation rate for Version 2 was 81.2 percent.

The Final Summary Disposition of the Version 2 sample is given in Table 4, on the next page. The table includes breakouts for each survey component (national youth and adult cross-section and the age 16-39 oversample). The average interview length for Version 2 was 16.3 minutes.