There is considerable concern about consent issues in relation to groups perceived as 'vulnerable', i.e., where individuals have difficulties in giving initial and continued informed consent because of issues of 'competence'. Groups who are perceived as vulnerable include children and young people, people with mental health problems and people with learning disability. Ethics Committees generally ask that special consideration is given to the ways in which 'vulnerable groups' are accessed and give consent to participate in research to ensure that they understand what participation involves and are not coerced into taking part. The expectation is generally that the researcher should justify the importance of the research and the need to include 'vulnerable' populations and should identify the means whereby informed consent will be obtained - in many cases there will be an expectation that proxy consent (from a parent or relative) may be used to supplement the consent or assent from the individual who is not seen as competent to give consent in their own right (Baez 2002).
It has been argued that researchers should seek consent each time they collect data from a study participant to ensure that they are aware that data are being collected and that they are willing to continue participating in the study.