Key findings from the 2009 New Zealand ... - Ministry of Health

For example, the survey only covered the usually resident population aged 15–64 years
living in permanent private dwellings. It did not include people living in institutions (such
as prisons, hospitals, rest homes and boarding schools) and the homeless. Given that
these groups are not included in the analyses, the estimated population numbers
presented in this report will generally be slight underestimates of the numbers affected
in the entire resident population of New Zealand. Table 2 in Chapter 4 presents an
estimate of the undercount of the number of current smokers in New Zealand, as
measured by 2009 NZTUS using data from the 2006 New Zealand Census (which
includes private and non-private dwellings).
Sampling errors for survey estimates from this survey were calculated using a replicate
method, called the delete-a-group jackknife method (Kott 1998).
95% confidence intervals for prevalence estimates
Ninety-five percent confidence intervals (95% CIs) have been used in this report to
quantify the sampling errors for estimates. It should be noted that the confidence
interval is influenced by the sample size of the group. When the sample size is small,
the confidence interval becomes wider.
Ninety-five percent confidence intervals have been presented in brackets after
estimates in the text and as error bars in graphs. When the confidence intervals of two
groups do not overlap, the difference in rates between the groups is statistically
significant at the 5% level.
Sometimes, even when there are overlapping confidence intervals, the difference
between the groups being compared can still be statistically significant at the 5% level.
In instances where there was a small overlap between groups (indicating possible
significance), t-tests were conducted to determine whether the apparent difference
between groups is significant. Statistically significant differences at the 5% level of
significance are indicated in the text by ‘p-value < 0.05’.
When a linear pattern was evident in a bar graph, but no significant differences were
found between bars, the significance of the gradient was tested using logistic
regression. No variables were controlled for in the logistic regression.
95% confidence intervals for rate ratios
In rate ratio graphs, the 95% CIs are presented as vertical lines on the graph. If the
confidence interval includes 1, then there is no statistically significant difference
between the rate ratio for that particular group and the reference group. If the 95% CI
does not include 1, then the rate ratio is statistically significant (ie, there is a significant
difference between the group of interest and the reference group). For more
information, see the example rate ratio graph at the end of this chapter.
Statistical significance
In this report the term ‘significant’ (or ‘significance’) is used to refer to statistical
significance.
Tobacco Use in New Zealand 5