The overall impact of chemotherapy on cancer survival can be difficult to estimate, since improved cancer screening, prevention (e.g. anti-smoking campaigns), and detection all influence statistics on cancer incidence and mortality. In the United States, overall cancer incidence rates were stable from 1995 through 1999, while cancer death rates decreased steadily from 1993 through 1999.[13] Again, this likely reflects the combined impact of improved screening, prevention, and treatment.