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BTS and MARAD developed an 8-page, 36-item questionnaire1 to collect information from a sample of merchant mariners. The first mailing of the questionnaire occurred on May 17, 2002. A second mailing was conducted on June 17, 2002 for nonrespondents whose questionnaires had not been returned as undeliverable by the postal service. Beginning July 22, 2002, a telephone follow-up was conducted for all nonrespondents (including mariners whose surveys had been returned by the post office). A third mailing was sent to nonrespondents who agreed to complete the questionnaire (see Table 1 for actual contact rates). Data collection was closed on September 6, 2002, ending a 16-week data collection period.
The U.S. Coast Guard (USCG) is the lead federal agency for regulating, licensing, and documenting professional merchant mariners. The USCGs Merchant Mariner Licensing and Documentation (MMLD) system provides information on the qualifications and reported seafaring employment for both actively sailing and inactive mariners.
At the time of the survey, the MMLD contained information on over 200,000 individuals. From that file, a target population was identified of all mariners with documents appropriate for deep-sea employment who held an Unlimited License or who were unlicensed and who had completed a document transaction within the last six years.
The six-year time frame was selected because all mariners are required to renew licenses and documents on a five-year cycle. Extending the time frame to 6 years allowed inclusion of mariners whose documents were less than 12 months out of date on the assumption that some proportion of these mariners would renew before the 12-month window for renewal had elapsed.
The final target population contained a total of 78,921 mariners18,394 Unlimited License (UL) Mariners and 60,527 Unlicensed Mariners. A sample of mariners was randomly selected from each of the two groups. Sample sizes were large enough to ensure sufficient numbers from each group to maintain 95% confidence intervals for subgroups (e.g., sailing status or specialty). Calculations included projected response rates for each group based on the response rates from the 2001 survey. The final sample included 1,796 UL Mariners and 3,479 Unlicensed Mariners.
Reliability of the Estimates
The findings summarized in this report are estimates derived from a sample survey. There are two major components of error in a sample surveysampling and nonsampling error.
Sampling Error. Sampling error occurs because findings are based on a sample, rather than on the entire population. The total respondent pool for the mariner survey is 2,255 for an estimated sampling error of about 2% at the 95% confidence level. This means that if a series of comparably sized samples were drawn from the current mariner population, 95% of the time results from those samples would fall within a range of 2% of the results reported for the current sample. Sampling errors are larger for sample subgroups, such as Unlicensed Mariners (3%), because the respondent pool is smaller.
Nonsampling Error. Estimates are subject to various errors during the survey process, such as data collection, response coding, transcription, and data-editing errors. These errors would also occur if a complete census was conducted under the same conditions as the sample survey. Explicit measures of the effects of these errors are not available. However, stringent quality control procedures were followed during data entry, and the questionnaire was reviewed and pretested in an effort to minimize nonsampling errors associated with data entry and questionnaire design.
Nonresponse error is a function of both the nonresponse rate and the differences, if any, between respondents and nonrespondents. Approximately 2,600 nonrespondents had telephone numbers in the database. About 900 of those mariners or their representatives were reached by phone. Of that group, 237 agreed to complete the survey, 327 refused to participate, and 391 were either not available (on a ship) or were physically unable to participate. Of the 327 mariners who refused to participate, the majority disconnected from the interviewer without answering the questions about their future plans. However, data collected from nonrespondents during the 2001 survey showed little differences between respondents and nonrespondents on the critical issues addressed by the survey (see 2001 Mariner Survey: Comparison of Phone and Mail Survey Results).
Final response rates for the survey were calculated using the American Association for Public Opinion Research guidelines (known as RR3) and are reported in Table 1.
Unlimited Licensed (UL) Mariners (63%) were much more likely than Unlicensed Mariners (33%) to complete a survey questionnaire. In addition, there is a substantial difference in the percent of mariners located during the survey. While 75% of UL Mariners were located, only 53% of Unlicensed Mariners were located which may, to some degree, account for the differences in response rate.