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Using data from breast cancer screening programs to study the temporal trend of age at menopause is difficult since especially younger women in the same generational cohort have often not yet reached menopause. Deleting these younger women in a breast cancer risk analyses may bias the results. The aim of this study is therefore to recover missing menopause ages as a covariate by comparing methods for handling missing data. Additionally, the study makes a contribution to understanding the evolution of age at menopause for several generations born in Portugal between 1920 and 1970.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Methods<\/jats:title>\n                <jats:p>Data from a breast cancer screening program in Portugal including 278,282 women aged 45\u201369 and collected between 1990 and 2010 are used to compare two approaches of imputing age at menopause: (i) a multiple imputation methodology based on a truncated distribution but ignoring the mechanism of missingness; (ii) a copula-based multiple imputation method that simultaneously handles the age at menopause and the missing mechanism. The linear predictors considered in both cases have a semiparametric additive structure accommodating linear and non-linear effects defined via splines or Markov random fields smoothers in the case of spatial variables.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Results<\/jats:title>\n                <jats:p>Both imputation methods unveiled an increasing trend of age at menopause when viewed as a function of the birth year for the youngest generation. This trend is hidden if we model only women with an observed age at menopause.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Conclusion<\/jats:title>\n                <jats:p>When studying age at menopause, missing ages must be recovered with an adequate procedure for incomplete data. Imputing these missing ages avoids excluding the younger generation cohort of the screening program in breast cancer risk analyses and hence reduces the bias stemming from this exclusion. In addition, imputing the not yet observed ages of menopause for mostly younger women is also crucial when studying the time trend of age at menopause otherwise the analysis will be biased.<\/jats:p>\n              <\/jats:sec>","DOI":"10.1186\/s12874-022-01658-x","type":"journal-article","created":{"date-parts":[[2022,7,11]],"date-time":"2022-07-11T15:13:03Z","timestamp":1657552383000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Is age at menopause decreasing? \u2013 The consequences of not completing the generational cohort"],"prefix":"10.1186","volume":"22","author":[{"given":"Rui","family":"Martins","sequence":"first","affiliation":[]},{"given":"Bruno de","family":"Sousa","sequence":"additional","affiliation":[]},{"given":"Thomas","family":"Kneib","sequence":"additional","affiliation":[]},{"given":"Maike","family":"Hohberg","sequence":"additional","affiliation":[]},{"given":"Nadja","family":"Klein","sequence":"additional","affiliation":[]},{"given":"Elisa","family":"Duarte","sequence":"additional","affiliation":[]},{"given":"V\u00edtor","family":"Rodrigues","sequence":"additional","affiliation":[]}],"member":"297","published-online":{"date-parts":[[2022,7,11]]},"reference":[{"issue":"10204","key":"1658_CR1","doi-asserted-by":"publisher","first-page":"1159","DOI":"10.1016\/S0140-6736(19)31709-X","volume":"394","author":"Collaborative Group on Hormonal Factors in Breast Cancer","year":"2019","unstructured":"Collaborative Group on Hormonal Factors in Breast Cancer. 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Usage of data derived from the records is according to Portuguese and European laws and regulations. All women signed the informed consent prior to the screening procedure.","order":2,"name":"Ethics","group":{"name":"EthicsHeading","label":"Ethics approval and consent to participate"}},{"value":"All authors consent to this publication.","order":3,"name":"Ethics","group":{"name":"EthicsHeading","label":"Consent for publication"}},{"value":"The authors declare that they have no competing interests.","order":4,"name":"Ethics","group":{"name":"EthicsHeading","label":"Competing interests"}}],"article-number":"187"}}