DECADE OF DRINKING: TEN-YEAR TRENDS IN DRINKING PATTERNS
AUCKLAND, NEW ZEALAND, 1990-1999
- Sally Casswell and Krishna Bhatta
- Alcohol & Public Health Research
Unit, May 2001
sampling frame was the free-dialling area of the Auckland metropolitan
region. Interviewing was
undertaken using the Alcohol & Public Health Research Unitís in-house
computer-assisted telephone interviewing (CATI) system.
This is a network of 20 computer stations and a supervisorís
station. The questions were
programmed into the computer and the interviewers read them as they appeared
on the screen. They then coded the answers directly into the computer, from
the options presented on the screen. The
supervisors were at any time able to observe any interview on their own
screen and listen in to any call without the interviewer or respondent being
aware (respondents were told this might happen before they began).
The ability to monitor all interviewing at the one central location
ensured a high degree of quality control.
Quality was also emphasised in the extensive training given to
interviewers. To ensure further
quality of the survey data, the information collected was checked by the
CATI supervisors to identify invalid, missing or any dubious figures.
If required, call backs were arranged to check the recorded
numbers were randomly selected. At least ten calls were made to each
household to try and obtain an interview with the qualifying respondents.
These calls were made at different times of the day and during both
week days at the weekends. The
random digit dialling system ensured that unlisted numbers, estimated at 10%
of domestic phones, were included in the sampling frame.
a household was contacted, initials of all members aged 14-65 years were
collected to allow the computer to randomly select the interviewee.
Any number of the eligible residents could be selected by the computer,
and in some cases it selected none. On
average every 2.5 persons in the eligible age range was selected. People were
told that the survey was being undertaken by the Alcohol & Public Health
Research Unit from the University of Auckland Medical School. They were also told that they could be interviewed by a Maori
interviewer if they wished. Confidentiality
was assured and only first names were collected.
New Zealandís Household Economic Surveys show that telephone penetration
in New Zealand is comparatively high. Over
the period 1990 to 1996 it ranged from 93.5% to 96%.
Certain sectors of the population, most notably Maori and people from
the Pacific Islands, are under-represented among telephone owners, and they
are the same groups who tend to be under-represented in face-to-face
surveys. Analyses undertaken by
the Alcohol & Public Health Research Unit (Wyllie et al., 1994) have
shown that this under-representation has little effect on results at a
OF THE DATA
effort to determine whether the data from the annual Auckland alcohol surveys is
a valid representation of patterns of change, comparisons were made with other
data relating to alcohol consumption and purchasing. Although direct comparisons cannot be made, they provide
of Alcohol Consumption Measures
choice of questions for measuring alcohol consumption in the Auckland
surveys was made after careful consideration of the advantages and
disadvantages of various approaches (Wyllie et al., 1994).
The questions used are based on typical drinking behaviour at the
different locations and it is possible that they are not sufficiently
sensitive to detect small changes in quantities being consumed. For example, a person could have more than one drinking
pattern at home and change the quantities they consume in one of them, but
if this is not their most typical pattern at home the change will not be
detected by the survey questions. Likewise,
as a result of compulsory breath testing, a person may sometimes not drink
at locations away from home, but their typical quantity at those locations
may remain unchanged.
other measures in the surveys show patterns that are consistent with the
consumption measures, which suggests that their sensitivity is not a major
issue (see Internal Validity below for further discussion of this issue).
Sampling and Measurement Errors
in data can result from changes in research methods, questionnaire design,
interviewing procedures, or variations in the samples.
Considerable efforts have been made to minimise any change due to these
Interviewer quality control
noted above, there is considerable emphasis placed on interviewer quality
As well as the monitoring of work, this included each new interviewer
receiving over 20 hours of training and experienced interviewers having time to
familiarise themselves with the questionnaire prior to each survey.
design and administration
have been no changes to the questionnaire which would have affected the
There was a change in the computer software in 1995 and a change in the
way in which random numbers were generated, but neither of these changes was
expected to have affected the data obtained.
mentioned in the Research Methods section , there was found to be variation in
the consumption data by time of interview.
This has been controlled for in the analyses.
in response rates could affect the data if those persons with higher response
rates are different from those with lower response rates.
As shown in table D1, the response rate was lowest in the first year and
was also lower than usual in 1993, 1997 and 1998.
However, none of these years have produced unexpected data.
comparison of the composition of the samples in each survey shows that
between 1994 and 1999 the proportion of the sample in the $40,000 and over
income category has been increasing. Since
1990, the proportion has been: 19%, 18%, 17%, 18%, 21%, 22%, 24% 25%, 27%
and 31%. To check this trend
against other data sources, data were purchased from the Household Economic
Survey (HES) undertaken by Statistics New Zealand (after
1998, HES figures went to 3 yearly, so 1999 figures were not available for
D1 shows the proportion of respondents reporting personal incomes of $40,000
and over in the Auckland surveys, compared with similar data for Auckland
and New Zealand from the nationwide HES.
Auckland alcohol survey proportions were higher because the HES
included persons aged over 65 years. The
trend in the Auckland alcohol data was broadly parallel to the trend in the
HES data. There were minor
differences in the years 1991, 1993 and 1997, but they were very small,
suggesting that the Auckland survey was sampling income groups adequately.
can be validated not only by comparisons with external criteria, but also by
examining whether there is internal consistency in the data that is
If there was decreasing alcohol consumption in Auckland over time,
one might have expected to see some upward trends in the reasons for
drinking less and downward trends for some of the reasons for drinking more.
There were in fact no such trends; rather there were trends in the
opposite direction in keeping with the self reported consumption in the
the level of problems showed increasing rates, which is not what would be
expected if consumption was actually decreasing.
1995 annual Auckland survey data was collected as part of the 1995 National
In this survey the proportion of Aucklanders reporting drinking more
was higher than for the rest of the country.
This is consistent with the Auckland survey data over those years,
which did not show the downward trend evident in the national available
E: TREND ANALYSIS PROCEDURES
major statistical analyses were performed in the tracking trend data:
analysis of covariance (ANCOVA) to compare the group means; logistic
regression analysis to compare the group percentages; and general linear
regression model (GLM) to test a linear trend. All analyses were controlling
for gender, age and interview date.
assumptions of parametric ANCOVA are that 1) each group are normally
distributed; 2) all groups have equal variances; 3) all covariate variables
have a linear relationship with the dependent variable; 4) all groups'
slopes are equal for each covariate variable.
undertaking ANCOVA, all continuous variables needed to be transformed to
make the data more nearly normal and reduce the variance. Logarithmic
transformation was used for this purpose.
Because of the number of comparisons being undertaken, Bonferroni
significance levels have been used, which adjust for the number of
major issue with data on drinking patterns is how to report it in a way that
is both easy for people without statistical skills to comprehend, but also
allows statistical significance testing and presents meaningful data.
A problem with using arithmetic means (averages) is that extreme
values have a large influence. A few extreme values in a sample can have an
influence on the data for the whole sample which would not accurately
reflect the sample as a whole. The
alternative that was used in the reporting of the 1995 national survey Drinking
in New Zealand was to use medians, the point at which half the sample is
above and half below. While
medians are useful for reporting one-off studies, they have limitations for
comparing surveys across time. For example if heavier drinkers were
increasing their consumption more than lighter drinkers, this might not be
reflected in a change in the median and yet overall consumption could show a
large increase. The current
study has utilised means that have been subjected to logarithmic
transformation; these means will be referred to as geometric means.
The logarithmic transformation reduces the influence of the more
F: CALCULATION OF ALCOHOL
was collected for drinking in each of fifteen standard locations plus any
additional drinking locations identified by the respondents.
average Quantity Consumed on a Typical
Occasion for an individual is the weighted average of the quantities consumed on a typical
occasion at each location, taking into account how often the person drank at
the location (frequency of drinking).
In this way, a location that a person only drank at once a year had
minimal influence, compared with a location that the person drank at daily.
volume of absolute alcohol consumed on a typical occasion was then
calculated from the types of drinks and volume of beverage drunk.
Beverage volumes were calculated from numbers and types of
containers. The volume of absolute alcohol in each type of beverage was
calculated using Statistics NZ conversions.
annual volume of absolute alcohol consumed in each location by an
calculated by taking sum of the volumes of absolute alcohol consumed per
occasion for each location, multiplied by the frequency of drinking at that
location. The sum of all location amounts gives the annual volume of total
absolute alcohol consumed by an individual.
frequency of drinking was the sum of the frequencies at all of the
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