Extra - Minimising bias

Question 1

Minimise selection bias

Answer 1

Take controls from community

Question 2

Minimise recall bias

Answer 2

Objective measurements
Reduce recall time
Ask relatives for confirmation

Question 3

Attrition bias

Answer 3

Objective measures such as record linage to hospital admissions
Reminder letters
Regular contact through christmas/birthday cards
Analyse all data on those lost to follow up

Question 4

Reporting bias

Answer 4

Objective measures
Blind study participants to main exposure of interest