Sample

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 non-respondents 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 non-respondents’ 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 non-respondents, with non-respondents 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 non-respondents 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.

Table 1: Socio-demographic characteristics for the young, mid-age and older respondents and for women of the same age in the general population (ABS)
 
Young
(18-23 years)
WHA         ABS
%    %
Mid-age
(45-50 years)
WHA   ABS
  %   %
Older
(70-75 years)
WHA   ABS
  %   %
Number
14,762
759,680
14,072
734,155
12,804
377,152
             
Main current employment status
           
Employed full-time
31.3
32.4
36.1
36.0
NA
Employed part-time
19.2
26.4
30.1
28.5
NA
Worked (without pay)/ employed (other)
1.9
1.3
7.0
2.0
NA
Unemployed
6.4
10.5
1.9
4.0
NA
Total not in labour force
39.4
26.3
21.6
27.0
NA
Not stated
1.8
2.7
3.3
2.5
NA
             
Highest qualification completed
           
No post school qualification
69.8
69.3
63.1
61.8
79.8
70.4
Trade/Apprenticeship
2.4
7.9
3.5
7.0
3.7
2.7
Certificate/Diploma
15.1
6.0
15.9
8.7
7.3
3.3
University Degree
12.1
7.7
16.3
11.6
4.0
2.4
Other (not stated, inadequately described)
0.6
9.1
1.2
10.8
5.2
21.2
             
Aboriginal/Torres Strait Islander
           
Non indigenous
97.9
94.9
98.1
96.7
91.6
93.7
Aboriginal or TSI
1.6
2.7
0.8
1.1
0.3
0.4
Not stated
0.5
2.5
1.1
2.1
8.1
5.9
             
Country of birth
           
Australia
88.6
77.8
69.0
62.6
68.5
66.4
Other English speaking
3.5
4.1
13.9
11.6
12.4
11.0
Other Europe
1.3
1.6
8.7
11.0
9.7
12.7
Asia
3.6
10.6
4.3
8.2
1.8
3.3
Other/not stated
3.0
6.0
4.2
6.5
7.6
6.5
             
Present marital status
           
Married
8.2
9.0
75.1
73.0
54.7
48.9
Separated/divorced
0.0
1.1
13.2
18.7
6.3
6.8
Widowed
0.0
0.2
2.1
2.7
35.2
39.9
Never married
79.0
89.8
3.9
5.6
3.2
4.4
De Facto (not collected by ABS)
12.0
-
5.7
-
0.6
-
             
Present housing situation
           
House
74.3
79.4
84.7
89.2
76.7
79.3
Flat/apartment/unit
20.0
14.0
7.1
6.5
19.4
12.9
Other
5.7
6.6
8.2
4.3
3.9
7.9

The above table and information are taken from:
Brown, W. J., Dobson, A. J., Bryson, L., & Byles, J. E. (1999). The Australian Longitudinal Study on Women’s Health: on the progress of the main cohort studies. Journal of Women's Health & Gender-Based Medicine, 8(5), 681-688.

Sample for the longitudinal study of the 1973-78, 1946-51 and 1921-26 cohorts

Some participants 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). Also, a very small number of women have alerted the study that they were not eligible by their birth date and they have been removed. 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, 13,714 women in the 1946-51 cohort and 12,432 women in the 1921-26 cohort (Lee, C., Dobson, A. J., Brown, W. J., Bryson, L., Byles, J., Warner-Smith, P., & Young, A. F. (2005). Cohort Profile: the Australian Longitudinal Study on Women's Health. International Journal of Epidemiology, 34(5), 987-991).

1973-78 cohort

Among the 1973-78 cohort, 69% responded to Survey 2 in 2000, 66% to Survey 3 in 2003, 68% responded to Survey 4 in 2006, 62% to Survey 5 in 2009 and to Survey 6 in 2012, and 57% to Survey 7 in 2015 (See 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 has been that the research team has been unable to contact the women (between 21% and 28% of the cohort at subsequent surveys), 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, not being registered to vote, and making extended trips outside Australia for work, education, or recreation.  Despite these losses, modelling has shown that there is no serious bias in estimates of associations between risk factors and health outcomes in longitudinal models (Powers, J., & Loxton, D. (2010). The Impact of Attrition in an 11-Year Prospective Longitudinal Study of Younger Women. Annals of Epidemiology, 20(4), 318-321).

Table 2: Participation and retention of 14,247 women in the 1973-78 cohort of women who were 18-23 years old at the first survey in 1996*

Survey
Age in years

Survey 2
22-27

Survey 3
25-30

Survey 4
28-33

Survey 5
31-36

Survey 6
34-39

Survey 7
37-42

     Deceased

22

33

50

58

77

102

     Frailty (e.g. intellectual disability)

3

9

12

15

16

16

     Withdrawn

230

518

800

951

1157

1436

Total ineligible

255

560

862

1024

1250

1554

             

Contacted but did not return survey

1332

653

1371

1994

1455

1399

Unable to contact participant

2972

3953

2869

3030

3533

4108

Total non-respondents

4304

4606

4239

5024

4988

5507

Respondents completed survey

9688

9081

9145

8199

8009

7186

Eligible at current survey

13,992

13,687

13,385

13,223

12,997

12693

Response rate as % eligible

69.2%

66.3%

68.3%

62.0%

61.6%

56.6%

*data known as at July 2018

1946-51 cohort

Retention has been much higher among the 1946-51 cohort of women; 92% responded to Survey 2 in 1998, 85% responded to Survey 3 in 2001, Survey 4 in 2004 and Survey 5 in 2007, and 83% responded to Survey 6 in 2010, 81% to Survey 7 in 2013 and 80% to Survey 8 in 2016 (See Table 3).  Women in the 1946-51 cohort are currently responding to Survey 7.  The major reasons for non-response among the 1946-51 cohort has been that the research team has been unable to contact the women (6% to 8% of eligible women between Survey 2 and Survey 6), and non-return of questionnaires by women who could be contacted (2% at Survey 2 and 7% to 10% of eligible women at subsequent surveys). 

Table 3: Participation and retention of 13,715 women in the 1946-51 cohort of women who were 45-50 years old at the first survey in 1996*

Survey
Age in years

Survey 2
47-52

Survey 3
50-55

Survey 4
53-58

Survey 5
56-61

Survey 6
59-64

Survey 7 62-67

Survey 8
65-70

     Deceased

50

119

216

328

474

673

871

 

     Frailty (e.g. dementia, stroke)

7

23

34

51

70

100

120

     Withdrawn

209

424

622

870

1108

1651

2005

Total ineligible

266

566

872

1249

1652

2424

2996

               

Contacted but did not return survey

254

997

886

995

1148

1051

714

     Unable to contact participant

856

925

1051

832

903

1088

1382

     Total non-respondents

1110

1922

1937

1827

2051

2139

2096

Respondents completed survey

12,338

11,226

10,905

10,638

10,011

9,151

8,622

Eligible at current survey

13,448

13,148

12,842

12,465

12,062

11,290

10,718

Retention rate as % eligible

91.7%

85.4%

84.9%

85.3%

83.0%

 81.1%

80.4%

*data known as at July 2018

1921-26 cohort

Of women from the 1921-26 cohort, 93% responded to Survey 2 in 1999, 88% to Survey 3 in 2002, 87% to Survey 4 in 2005, 81% to Survey 5 in 2008 and to Survey 6 in 2011 (See Table 4).  Among the 1921-26 cohort the major reason for non-response was non-return of the questionnaire, rising from 4% at Survey 2 to 17% at Survey 6 although up to 9% of participants could not be contacted. Non-respondent women tended to report poorer self-rated health at Survey 1 than respondents. The effects of these losses were evaluated in terms of losses due to death and non-death (Brilleman, S. L., Pachana, N. A., & Dobson, A. J. (2010). The impact of attrition on the representativeness of cohort studies of older people. BMC Medical Research Methodology, 10. doi: 7110.1186/1471-2288-10-71). Brilleman et al. concluded that non-death losses were potentially a greater source of bias than effects of death.

Table 4: Participation and retention of 12,432 women in the 1921-26 cohort of women who were aged 70-75 years at Survey 1 in 1996*

Survey
Age in years  

Survey 2
73-78

Survey 3
76-81

Survey 4
79-84

Survey 5
82-87

Survey 6
85-90

     Deceased

549

1237

2287

3623

5282

     Frailty (e.g. dementia, stroke)

95

303

525

596

791

     Withdrawn

563

1090

1361

1372

1345

Total ineligible

1207

2630

4173

5591

7418

     contacted but did not return survey

481

861

592

640

862

     unable to contact participant

310

295

509

641

97

     Total non-respondents

791

1156

1101

1281

959

Respondents completed survey

10,434

8,646

7,158

5,560

4,055

Eligible at current survey

11,225

9,802

8,259

6,841

5,014

Retention rate as % eligible

93.0%

88.2%

86.7%

81.3%

80.9%

*data known as at April 2018

Six-month follow up surveys of the 1921-26 cohort

From November 2011, shorter surveys containing a set of core questions were mailed to the 1921-26 cohort every six months after the return of the previous survey. Table 5 shows the numbers of eligible participants and respondents at end of each six month period. Response rates have been consistently above seventy percent.

Table 5: Participation in 6-month follow up surveys of the 12,432 women in the 1921-26 cohort (from November 2011 onwards)*

 

May 2012

Nov 2012

May 2013

Nov 2013

May 2014

Nov 2014

May 2015 Nov 2015 May 2016 Nov
2016
May
2017
Nov
2017
May
2018

July
2018*

Deceased

5,532 5927 6,227 6,620 6,947 7,288 7,617 7,962 8,261 8,591 8,873 9,203 9,302 9304

Withdrawn

 2,334 2,363 2,409 2,347 2,294 2,181 2,102 1,967 1,875 1,740 1,678 1,534 1,546 1570

Total ineligible

7,866 8,290 8,636 8,967 9,241 9,469 9,719 9,929 10,136 10,331 10,551 10,737 10,848 10874

Non-respondent

1,136

882

954

992

1,073

999

987 979 934 854 819 716 707 1155

Respondent

 3,430 3,260 2,842 2,473 2,118 1,964 1,726 1,524 1,362 1,247 1,062 979 877 403

Total eligible

 4,566 4,142 3,796 3,465 3,191 2,963 2,713 2,503  2,296 2,101 1,881 1,695 1584 1558

Response rate as % of eligible

75.1% 78.7% 74.9% 71.4% 66.4%  66.3% 63.6% 60.9% 59.3% 59.4% 56.5% 57.8% 55.4% 25.9%

* using 6MF questionnaires logged by 31st July 2018; this 6MF collection period is still open for respondents.

Comparison of the cohorts with Australian Censuses in 2001, 2006 and 2011

Demographic characteristics (Indigenous status, country of birth, marital status, lone person household, education, and employment) of respondents at each of the surveys have been compared with those of women of the same age in the Australian population, using data from the Australian Census conducted closest to the survey (see (1) Powers, J. (2004) Comparison of the Australian Longitudinal Study on Women’s Health cohorts with women of the same age in the 2001 Census. Technical Report. Newcastle: ALSWH.  At: WHAvsCensus2001Feb2004; (2) Australian Longitudinal Study on Women’s Health (2009). Comparison of the ALSWH cohorts with women of the same ages in the 2006 Census and the 2004/2005 National Health Survey. In: Technical Report No. 32. 111-142; (3) Australian Longitudinal Study on Women’s Health (2013). Comparison of the ALSWH cohorts with women of the same ages in the 2011 Census. In: Technical Report No. 36. 107-135. As was found previously, there was some over-representation of Australian-born women and of women with tertiary education. In addition, ALSWH participants were more likely to be married, be employed and work longer hours than women of the same age in the general population. Some of these differences will be due to differences in the questions asked and the sampling frame i.e. to be selected for ALSWH women must have a Medicare card.

Longitudinal sample for the 1989-95 cohort

In 2012 and 2013, 17011 18-23 year old women were enrolled in the 1989-95 cohort. Women were mainly recruited using the internet and social media platforms. Consistent with the other cohorts, women were required to have a Medicare card. Women completed the online survey and provided consent to linkage of survey data with administrative databases such as Medicare.

Due to this different method of recruiting it was not possible to provide a response rate. Therefore it was necessary to compare the representativeness of the sample with women of the same age in the closest Australian Census (see Table 6). The 1989-95 cohort were broadly representative in terms of area of residence, State and Territory distribution, marital status and age distribution. Women with tertiary education were over-represented.

Table 6: Comparison of sociodemographic characteristics of the 1989-95 cohort of women aged 18 to 23 in 2013 with women of the same age range in the 2011 Australian Census

Table 6.1: Age

Age

1989-95 cohort 2013 (n=17,069)

%

2011 Census (N=847,042)

%

 

18

15.2

16.0

 

19

17.5

16.2

 

20

17.2

16.8

 

21

16.5

17.1

 

22

16.9

17.0

 

23

16.7

17.0

Table 6.2: State/territories

 

1989-95 cohort 2013 (n=17,069)

%

2011 Census (N=847,042)

%

 

New South Wales

27.8

31.0

 

Victoria

24.0

25.4

 

Queensland

22.3

20.6

 

Western Australia

11.0

10.6

 

South Australia

7.6

7.3

 

Australian Capital Territory

3.3

2.0

 

Tasmania

2.9

2.1

 

Northern Territory

0.8

1.0

Table 6.3: Area of residence

 

1989-95 cohort 2013 (n=17,069)

%

2011 Census (N=847,042)

%

 

Major city

75.3

74.5

 

Inner regional

16.6

16.0

 

Outer regional

6.7

7.2

 

Remote

0.8

1.1

 

Very remote

Migratory/no usual address

0.3

N/A

0.9

0.3

 

Missing

0.3

-

Table 6.4: Highest Qualification

 

1989-95 cohort 2013 (n=17,069)

%

2011 Census (N=847,042)

%

 

Less than Year 12

7.4

14.9

 

Year 12 or equivalent

43.0

46.1

 

Certificate / Diploma

25.9

21.7

 

University degree

22.5

9.4

 

Missing/not stated/inadequately described

1.2

7.8

Table 6.5: Aboriginal or Torres Strait Islander origin*

 

1989-95 cohort 2013 (n=17,069)

%

2011 Census (N=847,042)

%

 

No

97.3

91.9

 

Yes

2.7

3.4

 

Missing

0.0

4.7

Table 6.6: Marital Status

 

1989-95 cohort 2013 (n=17,069)

%

2011 Census (N=847,042)

%

 

Never married

95.5

94.5

 

Married

3.0

4.9

 

Separated/divorced/widowed

0.4

0.6

 

Missing

1.2

-

*n=14,909 as this question was only asked in a later version of the survey

Table 7: Participation and retention of 17,011 women in the 1989-95 cohort of women who were aged 18-23 years at Survey 1 in 2013*

Survey
Age in years  

Survey 2
19-24

Survey 3
20-25

Survey 4
21-26

Survey 5
22-27

     Deceased

1 6 8 13

     Frailty (e.g. mental impairment)

1 1 1 1

     Withdrawn

681 694 1744 1943

Total ineligible

683 701 1753 1957

     contacted but did not return survey

2362 3879 1850 1813

     unable to contact participant

2622 3470 4400 4745

     Total non-respondents

4985 7350 6250 6558

Respondents completed survey

11,344 8,961 9,007 8495

Eligible at current survey

16,328 16,310 15,257 15053

Response rate as % eligible

69.5% 54.9% 59.0% 56.4%

*data known as at July 2018