472 SAMJ VOL 79 20 APR 1991 Epidemiology of non-fatal injuries due to external causes in Johannesburg-Soweto Part 11. Incidence and determinants D. YACH, D. S. O. BROWN, K. JOHNSON V. NELL, B. RADEBE, A. BUTCHART, A. ANDERSON, Summary A total of 3535 trauma cases were enumerated in Johannes- burg-Soweto 1989 and 1990 in the course of 271 hospital ward rounds and 43 casualty watches. The overall trauma incidence was 2886 new cases per annum per 100000 population, rising to 19872 for coloured males aged 20 - 24 years and to 8761 for black males aged 20 - 24 years. Overall the malelfemale ratio was 2,9 rising to 6 or more in adoles- cence (15 - 19) for blacks andcoloureds. There were some 156 new resident cases of trauma daily; half these were victims of interpersonal violence, and coloureds constituted 22% of this group, although forming only 8% of the denomi- nator population. Witluegards to cause, most trauma among blacks and coloureds arose from interpersonal violence and significantly less from transport accidents. Among blacks injured in transport accidents (the majority of which involved motor vehicles) most were pedestrians, whereas most whites injured in such accidents were occupants of vehicles. For all groups trauma was most likely to be incurred 'in the street' although for white and coloured women the home was most dangerous. The implications of these and related findings for treatment and prevention and briefly reviewed. factors of its people. Thus, despite the fanciful statutory elaboration of race distinctions in South Africa, the concept of race has become a social reality. The sample During the 271 ward rounds and 43 casualty watches conducted between 8 June 1989 and 24 August 1990, 3535 patients were sampled. Of these, 99 were excluded because they did not meet the diagnostic criteria and/oT were interviewed more than 24 hours post-trauma. Of the 3436 eligible cases, 754 (21,9%) were non-resident, giving an overall resident/non- resident ratio of 4,6. Of patients identified' during casualty watches, the percentages of Asian and white non-residents (34,6% and 32,4% respectively) were substantially higher .than the percentages of coloured (20,5%) and black (17;6%) non- residents. Among patients sampled during ward rounds, 42,6% of those classified as white were non-residents, whereas only approximately 20% of the patients in each of the other three race categories were non-residents. .. S Air Med J 1991; 79: 472-479. Incidence rates for non-fatal injuries The methodology and materials used in this prospective hospital-based survey of non-fatal injuries due to external causes in Johannesburg-Soweto were outlined in part I of this article (p. 466).1 Here the results are presented. The emphasis is on using descriptive statistics to give a breakdown of trauma by ·incidence rates and causes, and an overview of the distri- bution of trauma in relation to various demographic, temporal and geographical variables. Most results are presented for the four 'official' South African race categories, Asian, black, coloured, and white. These categories have neither the ethnic nor cultural substance the state claims for them. None the less, they have become central to the ideology of all South Mricans and thus powerfully shape the social, economic and political institutions of the country and also the daily experience and consequent risk Health Psychology Unit, University of South Africa, Pretoria A. BUTCHART, B.A. HONS, M.A. (CLIN. PSYCJ;I0L.) V. NELL, B.A. HONS, MA, D.LITT. ET PHIL. D. S.O. BROWN, MA B. RADEBE, BA K. JOHNSON, BCUR Centre for Epidemiological Research in Southern Africa of the South African Medical Research Council, Parowvallei, CP D. YACH, M.B. CH.B., B.Se. HONS (EPIDEMIOL.), M.P.H. Institute for Biostatistics of the South African Medical Research Council, Johannesburg A. ANDERSON, NAT. DIP. MED. MICROBIOL. Crude incidence Raw case counts obscure the relative proportions between groups defmed in terms of age, sex and race. It was therefore necessary to calculate crude incidence rates per 100000 popu- lation. As this requires that all cases be drawn from a target population of known size, the 754 cases not resident in the catchment area were excluded. Also excluded. from these calculations were 51 cases where the relevant data were missing, and all 17 cases (of which 3 were Asian, 3 coloured and 11 white) identified during casualty watches at private hospitals - the very small number- of sampling days on which these cases were found artificially inflated annualised estimates for the race, sex and age groups in which they appeared. Crude incidence rates for all trauma were therefore based on a total of 994 resident cases identified during casualty watches at state hospitals) and for crude inpatient incidence rates on a total of 1 620 resident admissions seen' during ward rounds at both state and private hospitals. All trauma. Table I presents estimates by age, sex' and race of the annualised incidence and crude incidence rates per 100000 population for all trauma, using data from the casualty watches. Table I shows that the estimated total of new trauma cases resident in Joharmesburg-Soweto during the survey year was 56892 (95% confidence interval (Cl) ± 798), the· majority being males aged between 25 and 34 years. The overall incidence rate for all trauma was 2886/100 000 population. The highest incidence rate for all trauma was for coloured men aged 20 - 24 years. Overall, incidence followed an age gradient, rising to a peak at age 20 - 24 years, gradually declining, and again rising sharply after 75 years. In interpreting these