Twin cross-trait correlations of sleep problems and gastroesophageal reflux disease.

aPolychoric correlation.bAsymptotic error of the polychoric correlations.cThe insomnia index was constructed by combining the three different sleep problem questions and 0 points were given for “seldom”, 1 point for “sometimes” and 2 points for “often”. The highest scores, 4–6 points or if the participant had answered “often”, were then classified as “often having sleep problems”, 1–3 points as “sometimes having sleep problems” and 0 as ”seldom having sleep problems”.a. 多列相关系数（Polychoric correlation）。b. 多列相关系数的渐近误差（Asymptotic error）。c. 失眠指数由三项不同睡眠问题条目整合构建，其中"很少"计0分，"偶尔"计1分，"经常"计2分。得分处于4~6分区间，或受访者选择"经常"作答的情况，被归类为"经常存在睡眠问题"；1~3分归类为"偶尔存在睡眠问题"；0分则归类为"很少存在睡眠问题"。