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 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.

 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

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.

1973-78 cohort

Among the 1973-78 cohort, 69% responded to Survey 2 in 2000 and 65% to Survey 3 in 2003, 67% responded to Survey 4 in 2006 and 61% to Survey 5 in 2009 (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 was that the research team was unable to contact the women (21% of eligible women at Survey 2, 28% at Survey 3, 21% at Survey 4 and 23% at Survey 5), 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.

 

Table 2: Participation and retention of the 1973-78 cohort

 

Survey 2

Survey 3

Survey 4

Survey 5

Age in years

22-27

25-30

28-33

31-36

Eligible at previous survey

14247

14116

13886

13557

Ineligible

 

 

 

 

Deceased between surveys

22

10

15

8

Frailty (e.g. intellectual disability)

3

6

4

3

Withdrawn before mailout survey date

106

213

311

209

Total ineligible

131

229

330

220

Eligible at current survey

14116

13887

13557

13337

Non-respondents

 

 

 

 

Withdrawn from the project

124

200

171

113

Contacted but did not return survey

1332

653

1371

1994

Unable to contact participant

2972

3953

2870

3030

Total non-respondents

4428

4806

4412

5137

Respondents

 

 

 

 

Completed survey

9688

9081

9145

8200

Retention rate as % eligible

68.6%

65.4%

67.5%

61.5%


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%).

1946-51 cohort

Retention has been much higher among the 1946-51 cohort; 91% responded to Survey 2 in 1998, 84% responded to Survey 3 in 2001, Survey 4 in 2004, Survey 5 in 2007 and 82% responded to Survey 6 in 2010 (See Table 3). The major reasons for non-response among the 1946-51 cohort was that the research team was 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 9% of eligible women at subsequent surveys) 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).Women in the 1946-51 cohort 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.

Table 3: Participation and retention of the 1946-51 cohort

 

Survey 2

Survey 3

Survey 4

Survey 5

Survey 6

Age in years

47-52

50-55

53-58

56-61

59-64

Eligible at previous survey

13715

13605

13310

12979

12694

Ineligible

 

 

 

 

 

Deceased between surveys

50

65

88

99

119

Frailty (e.g. dementia, stroke)

7

14

14

19

28

Withdrawn before mailout survey date

53

216

229

167

277

Total ineligible

110

295

331

285

424

Eligible at current survey

13605

13310

12978

12694

12270

Non-respondents

 

 

 

 

 

Withdrawn from the project

155

155

136

226

201

Contacted but did not return survey

254

998

886

995

1153

Unable to contact participant

858

931

1052

935

905

Total non-respondents

1268

2084

2074

2056

2259

Respondents

 

 

 

 

 

Completed survey

12338

11226

10905

10638

10011

Retention rate as % eligible

90.7%

84.3%

84.0%

83.8%

81.6%

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 the 1946-51 cohort 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.

1921-26 cohort

Of women from the 1921-26 cohort, 91% responded to Survey 2 in 1999, 85% to Survey 3 in 2002, 84% to Survey 4 in 2005 and 79% to Survey 5 in 2008 (see Table 4). Among the 1921-26 cohort 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 could 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.

 

Table 4: Participation and retention of the 1921-26 cohort

 

Survey 2

Survey 3

Survey 4

Survey 5

Age in years

73-78

76-81

79-84

82-87

Eligible at previous survey

12432

11537

10185

8530

Ineligible

 

 

 

 

Deceased between surveys

529

570

770

869

Frailty (e.g. dementia, stroke)

106

265

379

322

Withdrawn before mailout survey date

260

517

506

338

Total ineligible

895

1352

1655

1529

Eligible at current survey

11537

10185

8530

7001

Non-respondents

 

 

 

 

Withdrawn from the project

313

384

269

159

Contacted but did not return survey

481

860

592

640

Unable to contact participant

309

294

511

642

Total non-respondents

 

 

 

 

Respondents

 

 

 

 

Completed survey

10434

8647

7158

5560

Retention rate as % eligible

90.5%

84.9%

83.9%

79.4%


Demographic characteristics (country of birth, marital status, education and lone person household) of the 1921-26 cohort 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.

 

Table 2: Participation and retention of the 1973-78 cohort

 

Survey 2

Survey 3

Survey 4

Survey 5

Age in years

22-27

25-30

28-33

31-36

Eligible at previous survey

14247

14116

13886

13557

Ineligible

 

 

 

 

Deceased between surveys

22

10

15

8

Frailty (e.g. intellectual disability)

3

6

4

3

Withdrawn before mailout survey date

106

213

311

209

Total ineligible

131

229

330

220

Eligible at current survey

14116

13887

13557

13337

Non-respondents

 

 

 

 

Withdrawn from the project

124

200

171

113

Contacted but did not return survey

1332

653

1371

1994

Unable to contact participant

2972

3953

2870

3030

Total non-respondents

4428

4806

4412

5137

Respondents

 

 

 

 

Completed survey

9688

9081

9145

8200

Retention rate as % eligible

68.6%

65.4%

67.5%

61.5%