The study focuses on the cantons of Geneva, Vaud, Basel-City, Bern, Zurich and Ticino. Potential participants receive a letter of invitation and a brochure outlining the study's purpose, procedures and team. They are informed that we will call them the following week. If the centenarians cannot be reached by telephone, we contact their relatives by telephone, letter or visit. If they live in an institution, we contact the centenarian's relatives via the administration. There are no exclusion criteria, but centenarians or their families can refuse to take part.

There are different levels of participation, depending on the cognitive state of the centenarian. If relatives indicate that the centenarian's cognitive functioning and state of health are too limited to participate in the study, they may nevertheless agree to be enrolled as proxy informants. If relatives refuse to participate, they will be asked for basic information to determine the selectivity of the sample (for example, the centenarian's health, cognitive state, reasons for non-participation). If they are able to provide reliable information, the centenarians will be asked whether they approve of the agents' participation. Once participation has been accepted, interviews are scheduled within two weeks. Because of the potential health problems of the population studied, the interviews will be conducted in the participant's home by well-trained interviewers.

Information on each centenarian will be gathered from three sources:

• the centenarian him/herself (if he/she has the cognitive capacity for reliable self-reporting, see below)
• the proxy respondent
• the interviewer

During the first visit, the interviewer will explain the nature and aims of the study to the centenarian in the presence of a close relative, a carer or a member of the institution's staff. Participants will be able to ask questions. Written informed consent is obtained from participants if they are deemed competent; if not, proxy consent is obtained from the next of kin.

Two sessions of two hours each (including flexible breaks) are then organised to gather basic information. The first session involves the assessment of demographics and life achievements, physical health, cognition, quality of life and well-being. Session 2 measures focus on psychopathology and psychological characteristics, including personality, psychological strengths, activities, life events, current challenges and goals, and attitudes towards death/preparation for end of life. Biological indicators such as blood pressure, temperature, blood samples, etc. are also measured during these sessions.

As the SWISS100 study takes into account the whole life course, information is collected on changes in vulnerability, such as changes in well-being or transitions in care. Follow-up interviews with the centenarian and proxy (i.e. a 1.5-hour face-to-face session) take place every six months. In addition, we will conduct brief telephone interviews with the proxy (30 minutes) between follow-up sessions (i.e. approximately 3 months after each follow-up).