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.

 

 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.