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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 below). 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.

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

Year
Survey
Age in years
1999
Survey 2
73-78
2002
Survey 3
76-81
2005
Survey 4
79-84
2008
Survey 5
82-87
2011
Survey 6
85-90
Deceased5491237228936315294
Frailty (e.g. dementia, stroke)95303525594789
Withdrawn5631090135913671336
Total ineligible12072630417355927419
Contacted but did not return survey481861592640862
Unable to contact participant31029550964096
Total non-respondents791115611011280958
Respondents completed survey104348646715855604055
Eligible at current survey112259802825968405013
Retention rate as % eligible93.0%88.2%86.7%81.3%80.9%
Data known as at 2nd July 2021

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. The Table below shows the numbers of eligible participants and respondents at the end of each six month period. Response rates have been consistently above seventy percent.

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

6M period endingMay 2012Nov 2012May 2013Nov 2013May 2014Nov 2014May 2015Nov 2015May 2016Nov 2016May 2017Nov 2017May 2018Nov 2018May 2019Nov 2019May 2020Nov 2020May 2021July 2021*
Deceased55455940624166356962730376337980828186128896923194759774100161027710466106581076810772
Withdrawn2322235123962333228021672087195118571721165715101428128711821048979893870879
Total ineligible78678291863789689242947097209931101381033310553107411090311061111981132511445115511163811651
Non-respondent11358819539911072998986977932852817712653604598527497477463673
Respondent34303260284224732118196417261524136212471062979876767636580490404331108
Total eligible4565414137953464319029622712250122942099187916911529137112341107987881794781
Response rate as % of eligible75.1%78.7%74.9%71.4%66.4%66.3%63.6%60.9%59.4%59.4%56.5%57.9%57.3%55.9%51.5%52.4%49.6%45.9%41.7%13.8%
Status of 12,432 women from the 1921-26 cohort and at each 6MF questionnaire from November 2011 onwards. Using 6MF questionnaires logged by 2nd July 2021. Response rate based on eligible participants only.
*This 6MF collection period is still open for respondents.