Exercise :
The relation between length of lactation period and breast cancer was investigated in randomly selected 270 healthy females and 135 breast cancer patients. The length lactation period was divided into 4 groups, ‘less than 3 months’, ‘3 to 6 months’, ‘6 to 9 months’, and ‘9 to 11 months’, and the results are shown below. Analyze whether the length of lactation period is related to breast cancer risk. (C Hirose: Statistical Data Analysis, 1983.)
