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