Examination year (response rate, %) | P value (2000) | P value (2005) | P value (2009) | P value (2015) | P value (2018) |

2000 (69.5) | – | 0.013 | <0.000 | 0.003 | <0.000 |

2005 (64.1) | – | 0.151 | 0.381 | <0.000 | |

2009 (61.1) | – | 0.690 | <0.000 | ||

2015 (62.0) | – | <0.000 | |||

2018 (51.2) | – |

Statistical analysis: to test differences between groups, χ

^{2}test was used.