How to Use the Overseas to the U.S. Custom Report


General Information
All data presented in a custom report are statistical estimates, based on survey responses weighted with data from the Department of Homeland Security (DHS).  The estimates are subject to a certain amount of error, resulting from the sampling, data collection, and estimation processes.  Because of the complicated nature of the sample design, sampling variability has not been calculated for the estimates.  Instead, an indication of reliability is given by the number of respondents to the relevant questionnaire item (shown as the first row in each table).  The reader must exercise judgment in determining the amount of confidence to place in an estimate.


Procedure for Using Percentage Estimates to Represent Visitors
The International Trade Administration (ITA), National Travel and Tourism Office (NTTO) has weighted the Survey of International Air Travelers (SIAT) results to the DHS reported visitor arrivals by country of residence and port of entry to the U.S. Because this report presents weighted results, the percentage estimates found in the report can be expanded to estimate the total population of visitors.

To do this, start with the overall population estimate or control total for the column of interest.  To illustrate an example, start with Column #1, typically the primary column in a custom report that defines the data subset for the report.  The population estimate or control total for Column #1 is found in the last row in Table 1 in the first column.

To further the example, consider how you would calculate a volume estimate for those travelers indicating that their main purpose of trip was a convention or conference.  Use the control total for Column #1 and multiply by the cell percentage found under that column in Table 12, "Q13a. Main Purpose of Trip", Row #2 for convention or conference users.

Always round the calculated volume estimates to the nearest thousand; the survey estimates are not precise enough to accurately estimate the last three digits.  Also, remember to use the correct population control total from the corresponding column for the cell of interest.  This process can be used with any of the percentages found within tables and is facilitated by receiving the Excel format of the report.


Sample Size and Accuracy of the Estimates
In 2006 the NTTO implemented a new policy with regard to releasing visitor estimates based upon smaller sample sizes.  If the number of sampled visitors to a particular destination is under 400, NTTO will not calculate or provide an estimate of the total number of visitors to the destination even though an estimate could be developed.  Additionally, NTTO strongly suggests that all users of these statistics do not attempt to calculate visitor estimates when it appears that the sample size is less than 400.  The next paragraph contains an explanation of how to determine when a cell may have a sample that is too small to calculate an estimate.

A guideline has been developed for estimating the number of respondents in a cell.  By using the first row in a table, the number of respondents, you can estimate the unweighted, number in the cell.  Divide 400 by the number of respondents in the column in question.  For example, what is a lowest percentage that should be used for estimating visitor volumes in a custom report showing a respondent value of 5,730 for the column?  Divide 400 by 5,730 with a result of .0698 or 7.0%.  Looking down the column with 5,730 respondents at the top, any cell with a value of 7.0% or more probably1 has a sample size of 400 or greater and can be used to create an estimate for that cell.  Cells with less than 7.0% probably have sample sizes under 400 and should not be used to assess visitor characteristics.

If you have any questions regarding the logic of the process, or how to obtain the visitor estimates for other segments, please call the NTTO at (202) 482-4029.  Any staff member can answer your "how to" questions.  Mr. Richard Champley is the program manager for the SIAT.

More detailed questions relating to the survey results should be directed to Mr. Champley. For additional written information, click here.

1 Because the data are weighted, this mathematical procedure gives estimates of sample sizes, not exact sample sizes.

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