Of particular importance to the decision to immunise are studies describing the operation of omission bias and choices involving ambiguous situations [33] . Omission bias describes a preference for taking no action if the action might cause harm, even if there is a greater risk of harm by 'doing nothing'. Ambiguity describes the decision-maker's feeling that there is missing information relevant to outcome or choice [33] . The effect of ambiguity on choice is to reduce people's willingness to act or to postpone the action until the missing information can be obtained [33] . These studies used hypothetical scenarios, making decisions about others, and participants were generally tertiary students and sometimes, parents.