ALSWH aims

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

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). 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 (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 (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 2

Survey 3

Survey 4

Survey 5

Survey 6

Age in years

22-27

25-30

28-33

31-36

34-39

     Deceased

22

33

49

57

76

     Frailty (e.g. intellectual disability)

3

9

12

15

16

     Withdrawn

230

518

800

951

1157

Total ineligible

255

560

861

1023

1251

 

 

 

 

 

 

Contacted but did not return survey

1332

653

1371

1994

3604

Unable to contact participant

2972

3953

2870

3030

1474

Total non-respondents

4304

4606

3041

5024

5078

Respondents completed survey

9688

9081

9145

8200

8010

Eligible at current survey

13,992

13,687

13,386

13,224

12,996

Retention rate as % eligible

69.2%

66.3%

68.3%

62.0%

61.6%


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 (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 who were aged 45-50 years at Survey 1 in 1996

 

Survey 2

Survey 3

Survey 4

Survey 5

Survey 6

Age in years

47-52

50-55

53-58

56-61

59-64

     Deceased

50

119

216

327

472

     Frailty (e.g. dementia, stroke)

7

20

31

48

67

     Withdrawn

209

427

625

873

1112

Total ineligible

266

566

872

1248

1651

 

 

 

 

 

 

Contacted but did not return survey

254

997

886

995

1148

     Unable to contact participant

857

926

1052

834

905

     Total non-respondents

1111

1923

1938

1829

2053

Respondents completed survey

12,338

11,226

10,905

10,638

10,011

Eligible at current survey

13,449

13,149

12,843

12,467

12,064

Retention rate as % eligible

91.7%

85.4%

84.9%

85.3%

83.0%

 

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 2

Survey 3

Survey 4

Survey 5

Survey 6

Age in years

73-78

76-81

79-84

82-87

85-90

Deceased

551

1240

2290

3622

5250

     Frailty (e.g. dementia, stroke)

95

302

523

597

801

     Withdrawn

563

1089

1361

1372

1371

Total ineligible

1209

2631

4174

5591

7422

     contacted but did not return survey

481

860

592

640

862

     unable to contact participant

308

294

508

641

93

Total non-respondents

789

1154

1100

1281

955

Respondents completed survey

10,434

8,647

7,158

5,560

4,055

Eligible at current survey

11,223

9,801

8,258

6,841

5,010

Retention rate as % eligible

93.0%

88.2%

86.7%

81.3%

80.9%

 

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. The May 2014 six month follow-up survey is not yet complete.

Table 5: Participation in 6-month follow up surveys of the 12,432 women in the 1921-26 cohort

 

May 2012

Nov 2012

May 2013

Nov 2013

May 2014

Deceased

5529

5929

6223

6620

6900

Withdrawn

2349

2373

2415

2351

2340

Total ineligible

7878

8302

8638

8971

9240

Respondents

3431

3299

2853

2478

2107

Total eligible

4554

4130

3794

3461

3192

Response rate

75.3%

79.9%

75.2%

71.6%

66.0%

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, 17069 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

 

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

%

2011 Census (N=847,042)

%

Age

   
 

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

State/Territories

   
 

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

Area of residence

   
 

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

-

Highest qualification

   
 

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

Aboriginal or Torres Strait Islander origin*

   
 

No

97.3

91.9

 

Yes

2.7

3.4

 

Missing

0.0

4.7

Marital Status

   
 

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