Suppose the level of pollution has nothing to do with the incidence of heart attack. How many heart attacks would be expected to occur where the pollution level on the index date is higher than the pollution level on the control date? (Ignore cases where the pollution level on the index and control dates are the same.)

Environmental Health

Some previous studies have shown that mortality rates are higher on days with high pollution levels. In a follow-up on this observation, a group of 50 nonfatal heart attack cases were ascertained over a 1-year period. For each case, the level of pollution (total suspended particulates) was measured on the day of the heart attack (index date) and also 1 month before the heart attack (control date).

The results shown in Table 5.4 were obtained:

Table 5.4: Comparison of pollution levels on index date vs. control date

______________________________________________ n

Pollution level on index date > pollution level 

on control date ………………………….……............ 30

Pollution level on control date > pollution level

on index date …………………………............……… 15

Pollution level the same on ………………......…….. 

both days ……………………..………………................. 5

Total …………………………..………………................. 50