Domestic Travel Survey data reliability
The Domestic Travel Survey drew on a sample of approximately 15,000 New Zealand residents per year to represent the behaviour of all domestic travellers in New Zealand.
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Survey results subject to measurement errors
The results for the Domestic Travel Survey were subject to measurement errors, including both sampling and non-sampling errors. These errors should be considered when analysing the results from the survey.
Sampling errors
Sampling errors arise from estimating a population characteristic by looking at only a sample of the population rather than the entire population. It refers to the difference between the estimate derived from a sample survey and the 'true' value that would result if everyone in the population was surveyed.
The Domestic Travel Survey sample included approximately 15,000 New Zealand residents out of the approximately 4 million New Zealand residents each year.
The sampling error:
- is different for each estimate in the survey
- generally decreases as the sample size increases
- depends on the size of the population
- depends on the variability of the characteristics of interest in the population.
For example, total expenditure for the year ended December 2008 was NZ$8.1 billion. This estimate has a sampling error of plus or minus 2.6% (at the 95% confidence level). This means that there is a 95% chance that the true expenditure estimate lies between NZ$7.8 and NZ$8.3 billion.
For the Domestic Travel Survey, sampling errors of selected estimates have been modelled as a function of the estimate itself.
Domestic Travel Survey generalised sampling errors — at the 95% confidence level
|
Size of estimate |
Day trips |
Overnight trips |
All trips |
Nights |
Expenditure |
|
1,000 |
>100% |
>100% |
>100% |
>100% |
>100% |
|
2,000 |
>100% |
>100% |
>100% |
>100% |
>100% |
|
3,000 |
>100% |
>100% |
>100% |
>100% |
>100% |
|
4,000 |
>100% |
>100% |
>100% |
>100% |
>100% |
|
10,000 |
>100% |
>100% |
>100% |
>100% |
>100% |
|
20,000 |
>100% |
>50% |
>100% |
>100% |
>100% |
|
30,000 |
>100% |
>50% |
>100% |
>100% |
>100% |
|
40,000 |
>100% |
>50% |
>100% |
>100% |
>100% |
|
50,000 |
>100% |
>50% |
>100% |
>100% |
>100% |
|
100,000 |
>50% |
40.4% |
>50% |
>50% |
>100% |
|
200,000 |
>50% |
27.8% |
>50% |
>50% |
>100% |
|
300,000 |
46.8% |
22.4% |
43.1% |
46.1% |
>100% |
|
400,000 |
40.4% |
19.2% |
36.9% |
40.3% |
>100% |
|
500,000 |
36.1% |
17.0% |
32.7% |
36.3% |
>100% |
|
1,000,000 |
25.3% |
11.7% |
22.5% |
26.2% |
>100% |
|
2,000,000 |
17.8% |
8.1% |
15.5% |
18.9% |
>100% |
|
3,000,000 |
14.4% |
6.5% |
12.5% |
15.7% |
>100% |
|
4,000,000 |
12.5% |
5.6% |
10.7% |
13.7% |
>100% |
|
5,000,000 |
11.1% |
4.9% |
9.5% |
12.3% |
>100% |
|
10,000,000 |
7.8% |
3.4% |
6.5% |
8.9% |
>50% |
|
15,000,000 |
6.3% |
2.7% |
5.2% |
7.4% |
>50% |
|
20,000,000 |
5.5% |
|
4.5% |
6.4% |
>50% |
|
30,000,000 |
|
|
3.6% |
5.3% |
>50% |
|
40,000,000 |
|
|
3.1% |
4.7% |
44.2% |
|
50,000,000 |
|
|
|
|
39.5% |
|
100,000,000 |
|
|
|
|
27.9% |
|
200,000,000 |
|
|
|
|
19.7% |
|
400,000,000 |
|
|
|
|
14.0% |
|
600,000,000 |
|
|
|
|
11.4% |
|
800,000,000 |
|
|
|
|
9.9% |
|
1,000,000,000 |
|
|
|
|
8.8% |
|
5,000,000,000 |
|
|
|
|
3.9% |
|
7,000,000,000 |
|
|
|
|
3.3% |
Non-sampling errors
Aside from the sampling errors associated with the process of selecting a sample, a survey is also subject to a wide variety of other errors. These errors are commonly referred to as non-sampling errors. Non-sampling errors are defined as errors arising during the course of all survey activities other than sampling. Unlike sampling errors they are generally difficult to measure.
Some examples of non-sampling errors include:
- respondent errors
- poor questionnaire design
- non-response.
These types of errors are minimised through the use of best survey practices and monitoring.