A researcher collected data on childhood accidents from the historical medical records maintained by a health authority. The table below summarises the number of medically attended injuries for 599 children in the age periods "4 to 7 years" & "8 to 11 years". For example, 67 of the 599 children had no medically attended injury when they were aged 8 to 11 years even though they had one injury during 4 to 7 years of age. Note that the same child's medical record was examined chronologically.

Number of injuries in the period 8 to 11 years of age S 1 2 3 24 sum S 101 76 35 15 7 234 Number of 1 67 61 32 14 12 186 injuries in the 2 24 36 22 15 6 103 period 4 to 7 10 19 10 5 2 46 years of age 24 4 9 8 7 2 30 sum 206 201 107 56 29 599 Glen D. Mellinger, David L. Sylwester, William R. Gaffey, Dean I. Manheimer