A new treatment is available for a chronic lung disease and we are trying to determine if the treatment is effective in preventing mortality. We conduct a study in which we identify a large group of people with this chronic lung disease and determine whether or not they have received the new treatment. We then compare their outcomes.

1a. The initial results of our study are below. Calculate the risk ratio for mortality comparing those who did and did not receive the new treatment. Interpret this risk ratio. (5 points)

Treated?	Died	Did not die	Total
Yes	856	1,438	2,294
No	400	8,190	8,590
Total	1,256	9,628	10,884

The researchers are concerned that disease severity may be confounding the relationship between receipt of treatment and mortality. They split the people in the study into two groups, one with severe disease and the other with mild/moderate disease. They then examine the relationship between receipt of the new treatment and mortality in these two groups separately. The results are below.

Severe disease

Treated?	Died	Did not die	Total
Yes	830	770	1,600
No	100	100	200
Total	930	870	1,800

Mild/moderate disease

Treated?	Died	Did not die	Total
Yes	26	668	694
No	300	8,090	8,390
Total	326	8,758	9,084

1b. Is receipt of the new treatment associated with disease severity? Make a brief calculation to justify your answer. (2 points)

1c. Is disease severity related to mortality? Make a brief calculation to justify your answer. (2 points)

1d. Using the data in the tables above, calculate the risk ratio of mortality comparing people who received the new treatment to people who did not receive the new treatment in the severe disease group and mild/moderate disease group, separately. (5 points)

Hint:You should calculate two different risk ratios here, one for the severe disease group and the other for the mild/moderate disease group.

1e. Based on the results above, does disease severity confound the relationship between the new treatment and mortality? Briefly justify your choice. (4 points)