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Sample
and response rates
Survey
1 (baseline) 1996
More
than 40,000 women responded to the baseline surveys for the
main cohorts in 1996. Because of uncertainties about the accuracy
of the Medicare database (which was used as the sampling frame
for the stratified random samples), response rates cannot
be exactly specified. It is estimated that 41%-42%, 53-56%,
and 37-40% of the 1973-78 cohort, the 1946-51 cohort and the
1921-26 cohort, respectively, responded to the initial invitation
to participate. Confidentiality restrictions meant that the
names of the selected women were unknown to researchers. Usual
methods of encouraging participation such as by telephone
could not be used. The response rates were pleasing given
that the invitation included a request for women to participate
in the longitudinal study for up to 20 years.
In light of these response rates, it is important to assess
any response bias so that the generalizability of the study
findings can be determined. A comparison of the demographic
characteristics of respondents and nonrespondents was not
possible because privacy guidelines prevented the researchers
from having any information about women who were selected
to receive an invitation but did not respond. We were able,
however, to obtain aggregate data for nonrespondents’
use of health services (from the Australian Medicare database).
These data suggest that there are small differences in use
of health services among respondents and nonrespondents, with
nonrespondents less likely, for example, to have visited a
medical specialist in the last 2 years (1946-51 cohort, 49%
versus 54%; 1921-26 cohort, 65% versus 72%). There was not
a significant difference in health service use between respondents
and nonrespondents from the 1973-78 cohort.
A proportion of this difference may be explained by the fact
that some women who were selected may no longer be living
in Australia or may have died, as the Medicare database is
not routinely linked to emigration records or the National
Death Index in Australia.
Although we were not able to ascertain reasons for non-response
(because we were not allowed to know any details about the
selected women), we were able, through comparison with the
1996 census data, to confirm that the participants in each
of the cohorts are reasonably representative of the general
population of women of the same age in Australia (Table 1).
There is some response bias in terms of overrepresentation
of women with tertiary education and underrepresentation of
some groups of immigrant women.
The above
table and information are taken from:
Brown, W. J., Dobson, A. J., Bryson, L., & Byles, J. E.
(1999). Women's Health Australia: on the progress of the main
cohort studies. Journal of Women's Health & Gender-Based
Medicine, 8(5), 681-688.
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Sample
for the longitudinal study
Some
women only completed Survey 1 in 1996 and did not provide
any contact details (532 women from the 1973-78 cohort, 383
women from the 1946-52 cohort and 508 women from the 1921-26
cohort). Hence the numbers of women actually enrolled in the
Australian Longitudinal Study on Women’s Health (ALSWH)
were 14,247 women in the 1973-78 cohort, 13,716 women in the
1946-51 cohort and 12,432 women in the 1921-26 cohort.
1973-78
cohort
| Table
2: Participation and retention of the 1973-78 cohort |
| |
Survey
1 |
Survey
2 |
Survey
3 |
Survey
4 |
| Age
in years |
18
- 23 |
22
- 27 |
25
- 30 |
28
- 33 |
| Eligible
at previous survey |
|
14247 |
14116 |
13886 |
| Ineligible |
|
|
|
|
| |
deceased between surveys |
|
22 |
10 |
15 |
| |
frailty
(e.g. intellectual disability) |
|
3 |
6 |
4 |
| |
withdrawn
before mailout survey date |
|
106 |
213 |
311 |
| Total
ineligible |
|
131 |
229 |
330 |
| Eligible
at current survey |
|
14116 |
13887 |
13557 |
| Non-respondents |
|
|
|
|
| |
withdrawn
from the project |
|
124 |
200 |
171 |
| |
contacted
but did not return survey |
|
1332
|
654 |
1372 |
| |
unable
to contact participant |
|
2972 |
3952 |
2869 |
| Total
non-respondents |
|
4428 |
4806 |
4412 |
| Respondents |
|
|
|
|
| |
completed
survey |
14247 |
9688 |
9081 |
9145 |
| Retention
rate as % eligible |
|
68.6% |
65.4% |
67.5% |
| |
Among
the 1973-78 cohort, 69% responded to Survey 2 in 2000 and
65% to Survey 3 in 2003 and 67% have responded to Survey
4 in 2006 (Table 2). This retention compares well with other
surveys of this highly mobile age group. The major reason
for non-response among the 1973-78 cohort was that the research
team was unable to contact the women (21% of eligible women
at Survey 2 and 28% at Survey 3 and 21% at Survey 4), despite
using all possible methods of maintaining contact. Women
in their twenties are characterised by high levels of mobility,
change of surnames on marriage, often not having telephone
listings and not being registered to vote and making extended
trips outside Australia for work, education or recreation.
Demographic characteristics (country of birth, marital status,
education, employment and lone person household) of these
respondents at Survey 1 (1996) and Survey 2 (2000) were
compared with those of women of the same age in the Australian
population, using data from the 1996 and 2001 Censuses respectively.
The comparisons revealed few differences however there was
some under-representation of women from non-English language
countries at both surveys, a not unexpected finding given
that Medicare routinely excludes overseas students. The
disparity in education increased between 1996 and 2001.
Whereas at the 1996 Census almost 70% of young women had
no post school qualifications (ALSWH and the general population),
31% and 49% had no post school qualifications in the ALSWH
sample in 2000 and the 2001 Census respectively. Some of
these differences will be due to overseas graduates returning
home and Australian graduates working overseas. ALSWH women
were less likely to be employed compared to women of the
same age in the 1996 Census (52% versus 60%) but more likely
to be employed than women of the same age in the 2001 Census
(85% versus 67%).
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1946-51
cohort
| Table
3: Participation and retention of the 1946-51 cohort |
| |
|
Survey
2 |
Survey
3 |
Survey
4 |
Survey
5 |
Age
in years |
47
- 52 |
50-55 |
53-58 |
56-61 |
| Eligible
at previous survey |
13715 |
13605 |
13310 |
12979 |
| Ineligible |
|
|
|
|
| |
deceased
between surveys |
50 |
65 |
88 |
99 |
| |
frailty
(e.g. dementia, stroke) |
7 |
14 |
14 |
19 |
| |
withdrawn
before mailout survey date |
53 |
216 |
229 |
167 |
| Total
ineligible |
110 |
295 |
331 |
285 |
| Eligible
at current survey |
13605 |
13310 |
12978 |
12694 |
| Non-respondents |
|
|
|
|
| |
withdrawn
from the project |
155 |
155 |
136 |
226 |
| |
contacted
but did not return survey |
254 |
998 |
886 |
995 |
| |
unable
to contact participant |
858 |
931 |
1052 |
835 |
| Total
non-respondents |
1268 |
2084 |
2074 |
2056 |
| Respondents |
|
|
|
|
| |
completed
survey |
12338 |
11226 |
10905 |
10638 |
| Retention
rate as % eligible |
90.7% |
84.3% |
84.0% |
83.8% |
|
|
Retention
has been much higher among the 1946-51 cohort; 91% responded
to Survey 2 in 1998 and 84% responded to Survey 3 in 2001
and Survey 4 in 2004 (Table 3) and survey 5 in 2007. The major
reasons for non-response among the 1946-51 cohort was that
the research team was unable to contact the women (6%, 7%,
8% and 7% of eligible women at Survey 2, Survey 3 and Survey
4 respectively) and non-return of questionnaires by women
who could be contacted (2%, 8% and 7% of eligible women at
Survey 2, Survey 3, Survey 4 and Survey 5 respectively) and
non return of questionnaires by women who could be contacted
(2%, 8%, 7% and 8% of eligible women at the second, third,
fourth and fifth surveys). Middle aged women typically lead
busy lives often working, as well as caring for parents and
their children. The women who could not be contacted were
more likely to be separated, divorced or widowed.
Data from
the 1996 and 2001 Censuses were used to compare demographic
characteristics (country of birth, marital status, education,
employment and lone person household) of women of the same
age in the Australian population with respondents at Survey
1 (1996) and Survey 3 (2001). There were few differences,
however there was some under-representation of women from
non-English language countries and women who were separated
or divorced at both surveys.
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1921-26
cohort
| Table
4: Participation and rentention of the 1921-26 cohort |
| |
|
Survey
1 |
Survey
2 |
Survey
3 |
Survey
4 |
Survey
5 |
| Age
in years |
70
- 75 |
73
- 78 |
76
- 81 |
79
- 84 |
82-87 |
| Eligible
at previous survey |
|
12432 |
11535 |
10184 |
8528 |
| Ineligible |
|
|
|
|
|
| |
deceased
between surveys |
|
529 |
570 |
770 |
848 |
| |
frailty
(e.g. dementia, stroke) |
|
106 |
265 |
379 |
326 |
| |
withdrawn
before mailout survey date |
|
262 |
516 |
507 |
336 |
| Total
ineligible |
|
897 |
1351 |
1656 |
1510 |
|
Eligible at current survey |
|
11535 |
10184 |
8528 |
7018 |
| Non-respondents |
|
|
|
|
|
| |
withdrawn
from the project |
|
311 |
383 |
267 |
161 |
| |
contacted
but did not return survey |
|
481 |
860 |
592 |
642 |
| |
unable
to contact participant |
|
309 |
294 |
511 |
655 |
| Total
non-respondents |
|
1101 |
1537 |
1370 |
1458 |
| Respondents |
|
|
|
|
|
| |
completed
survey |
12432 |
10434 |
8647 |
7158 |
5560 |
| Retention
rate as % eligible |
|
90.5% |
84.9% |
83.9% |
79.2% |
| |
Of the
1921-26 cohort, 91% responded to Survey 2 in 1999 and 85%
to Survey 3 in 2002, 91% to Survey 4 in 2005 and 79% to Survey
5 in 2008 (Table 4). Among these women the major reason for
non-response was non-return of the questionnaire (4%, 8%,
7% and 9% of eligible women at Surveys 2, 3, 4 and 5 respectively)
although increasingly the participant can not be contacted
(3% at Surveys 2 and 3, 6% at Survey 4 and 9% at Survey 5).
Non-respondent women tended to report poorer self-rated health
at Survey 1 than respondents.
Demographic
characteristics (country of birth, marital status, education
and lone person household) of the respondents at Survey 1
(1996) and Survey 3 (2002) were compared with those of women
of the same age in the Australian population, using data from
the 1996 and 2001 Censuses respectively. Comparisons showed
few differences. There was some under-representation of women
from non-English speaking countries in the ALSWH sample at
both surveys. Comparisons are difficult for marital status
and educational qualifications due to the high level of missing
data in the Census.
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See
also:
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