The first wave of the Chinese longitudinal survey on healthy longevity was conducted in 1998. We randomly selected 50 percent of the counties and cities in 22 provinces. The 22 surveyed provinces are: Liaoning, Jilin, Heilongjiang, Hebei, Beijing, Tianjing, Shanxi, Shaanxi, Shanghai, Jiangsu, Zhejiang, Anhui, Fujian, Jiangxi, Shandong, Henan, Hubei, Hunan, Guangdong, Guangxi, Sichuan, and Chongqing, which have a total population of 985 million. The nine provinces of Xinjiang, Qinghai, Ningxia, Inner Mongolia, Tibet, Gansu, Yunnan, Guizhou, and Hainan, all of which have a high proportion of inhabitants belonging to ethnic minorities, are not included in this study mainly due to potentially inaccurate age reporting.

We tried to interview all centenarians who voluntarily agreed to participate in the survey in the randomly selected half of the counties and cities of the 22 provinces. For each centenarian, one octogenarian (aged 80-89) who live nearby and one nonagenarian (aged 90-99) who live nearby, with pre-designed age and sex were matched and interviewed. We tried to have approximately equal numbers of male and female octogenarians and nonagenarians at each age from 80 to 99. We over-sampled extremely old persons, and over-sampled male oldest old persons, given the fact there are in fact fewer persons at more advanced ages, and fewer males than females.

We did not follow the procedure of proportional sampling, in order to avoid the errors of random fluctuation due to too small sample size at more advanced ages, especially for males. Consequently, appropriate weights should be used to compute the overall average for the oldest old persons age 80+ and the averages of the age groups (e.g. 80-89 and 90-99), but no weights are needed when computing average for the centenarians. The method for computing the age-sex and rural-urban specific weights and the discussions are presented in Chapter Five.

We also tried to include in the sample surviving siblings who were living close by and were themselves over 80 years old. Extensive questionnaire data were gathered (92 questions on 180 items), and a basic health examination was carried out for each interviewee by a medical doctor or a nurse. The contents of the individual questionnaire included questions concerning family households, activities of daily living (ADL), lifestyle, diet, psychological characteristics (disposition), economic resources, family support and medical care services, etc. Some demographic, socioeconomic, and environmental data on the 666 sampled counties and cities were also collected. A total of 9,073 valid individual questionnaires were completed in our 1998 survey. The participation rate was 88 percent if we include those who were deceased or too sick to be interviewed or who migrated before the interview as non-participants. The participation rate was 98 percent if we exclude these people. Among the 9,073 interviewees 92.8 percent are Han Chinese and the remaining 7.2 percent belong to minority ethnic groups; 245 of those interviewed are 80+ year-old surviving siblings.