The U. S. National Toxicology Program (NTP) studies the toxicity and carcinogenicity of agents potentially causing a risk to human health. Electromagnetic fields around electrical installations are harmless in theory, but they might have a long‑term effect on health (for instance, in triggering tumors). Observational studies in human populations have suggested a potential association. An NTP study looked at the occurrence of cancers in otherwise healthy rats after two years of daily exposure to 60 ‑hertz electromagnetic fields of various intensities. Displayed are the numbers of male and female rats with a tumor after the two‑year study period. Note that two gauss is approximately 1000 ‑fold greater than what is considered high exposure for humans. Male rats Female rats Control: no exposure 16 ( n = 99 ) 19 ( n = 100 ) 0.02 ‑Gauss exposure 31 ( n = 100 ) 22 ( n = 100 ) 2 ‑Gauss exposure 30 ( n = 100 ) 22 ( n = 100 ) In rats not exposed to an electromagnetic field (sample size in parentheses), is there significant evidence of a difference in the tumor rates between males and females? Are the conditions for the z ‑test for comparing two proportions met?