In general, the probability of HPV-negative people developing cervical cancer is so low that it can be considered almost impossible to develop cervical cancer. Several studies since 1974 have reported that German scientists have found that 99.7% of cervical cancers are directly related to HPV infection, so HPV-negative people hardly ever get cervical cancer. Current international and national standards advocate cytology to screen for cervical cancer for people aged 21-29 years, and HPV screening for people aged 30 years and older, usually every 3-5 years, which is usually safer if they are HPV-negative.