t.from t.to circumcision n.at.risk n.seroconversions  person.years  
  0    0.5        1        2263            14            1172.1  
  0    0.5        0        2319            19            1206.7
  0.5  1.0        1        2235             5            1190.7
  0.5  1.0        0        2229            14            1176.3
  1.0  2.0        1         964             3             989.7
  1.0  2.0        0         980            12            1008.7