Marjolein Lugtenberg drafted the manuscript, performed the statistical analyses, and contributed to all other aspects of the study. Phil JM Heiligers contributed to the acquisition of the data in 1996 and in 2004 and was involved in drafting the manuscript. Judith D de Jong contributed to the acquisition of the data in 2004 and critical revision of this manuscript. Lammert Hingstman contributed to the acquisition of the data in 1996 and in 2004 and was involved in drafting the manuscript. All authors have given final approval of the submitted manuscript.

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