Incomplete outcome data (attrition bias) All outcomes
High risk
Intervention group: 86.2% of the original group had completed data immediately af- ter intervention (n 2344). At end of one yr 62.3% completed follow up (n 1694) No differential attrition an Control group: 65.5% of the original co hort completed the 12m follow-up (n 4198) No explanation of low levels of response at 12m.