How can a survey be conducted without a sampling frame? For demographers and epidemiologists alike, some subpopulations, denned by a health problem or a specific situation, are difficult to study, because there is no sampling frame for conducting a survey. In some cases, at the cost of considerable effort, the census can serve as a starting point. For example, 360,000 persons were interviewed for the "Vie quotidienne et santé" (Daily Life and Health, VQS) survey conducted alongside the 1999 census. On the basis of this very large sample, it was possible to select the persons eligible to participate in the "Handicaps, incapacités, dépendance" (Disability, Impairments and Dependence, HID) survey and 16,900 respondents were interviewed (Mormiche, 2000). In other cases, administrative files may provide access to the targeted population: for example, the survey launched in late 2004 of cancer patients still alive two years after diagnosis of the disease was based on the health insurance files of patients with long-term illnesses.
For other subpopulations, the most reasonable method is to contact respondents via facilities that provide them with specific services. For example, the various surveys conducted among homeless persons in France used shelters and distribution sites for food or hot meals as sampling frames, along with day centres or other mobile services (Firdion et al., 1998; Firdion et al., 2001; Brousse et al., 2002). It is even possible, if différent types of service or site are used, to attempt to enumerate the subpopulation in question by identifying duplications in the sample (capturerecapture method): this technique was used to estimate the number of drug users at local level (Chevallier, 2001)O. For this type of survey, special attention must be paid to the definition of the target population, to the hidden population (persons who do not visit the sites used as the sample frame(2)), to the unequal individual probabilities of being included in the sample (due to varying frequency of visits to the sampling sites), but also to the numerous potential sources of data collection bias, for example because the interview does not take place in the respondent's home(3', or because an intermediary is required to establish contact.
The HIV-positive population: a subpopulation that is difficult to reach
Introduced in 1996, combination antiretroviral therapy (ART) has transformed the lives of persons infected with the HIV virus, at least in developed countries where these treatments are available and widely accessible. This treatment does not provide a cure, but it keeps the disease at bay and considerably increases the life expectancy of HIV-positive persons. In France, the number of deaths due to AIDS has considerably decreased in recent years (Nizard, 2000; Delfraissy, 2004). HIV infection has become a chronic disease, around which infected persons must learn to (re)build their lives.
To ensure adequate and effective medical care for patients, it has become crucial to study the living conditions of HIV-positive persons: the physical and psychological consequences of the infection on their daily lives, their social and labour force integration, their emotional and sex lives and their plans for the future. Existing surveys, whether cohort follow-up studies or cross-section surveys, do not explore all the different implications of the "chronicization" of AIDS, either because they are too old (Schwoebel, 1997), or because they are limited to specifie groups such as persons infected through drug use (Moatti et al. 2000), or yet again because they are centred on bioclinical aspects (Meyer et al. 2003) or on a particular theme (ART monitoring, see Spire et al. 2002; "needs" surveys of the hospital administration, INVS, 2001). Moreover, in France, the dynamics of the HIV-positive population over time have varied according to the origin of infection (male homosexuals and bisexuals, intravenous drug users, migrants from sub-Saharan Africa, general heterosexual population), and from one region to another. …