Explicit consent. Participants give consent by answering a specific question about their willingness to participate. This may be done in written (consent form) or oral form.
To be informed, consent must be given by persons who are competent to consent, have consented voluntarily, are fully informed about the research, and have comprehended what they have been told. Unless they are emancipated minors, individuals under 18 may never give consent. Also question the legal competence of people affected by mental illness, or institutionalized in the prison system. If a person is not legally competent to give consent, a parent or legal guardian has to give it. The participant may still give assent.
Some participants, although not legally competent, are able to make judgment about their participation themselves. This is the case for teenagers and some mentally impaired individuals, for instance. The assent formgives them an opportunity to express their agreement to participate in research in writing, beyond the consent given by a legal guardian or parent. Although the assent is not legally binding, and does not dispense a researcher from obtaining consent from a parent or legal guardian, it is advisable as sound ethical practice. It reinforces the voluntary nature of participation.
Written consent. Participants give their consent by filling out a consent form. Written consent guarantees active and explicit consent, thus offering the highest guarantees to the participant. It is most appropriate in studies that contain some level of risk, but also in many studies with no risk above those of daily life, when participants disclose personal or sensitive information, when they are exposed to deception, or any experimental treatment. Experiments and in-depth interviews in particular should consider written consent.