The bacteria Pseudomonas aeruginosa is the primary cause of lung infections in people with cystic fibrosis (CF). Babies with CF are born without Pseudomonas in their lungs, but they may quickly become infected from sources in their environment. West et al. (2002) investigated the times from birth for this infection to establish in babies with CF. They followed 63 babies with CF from birth to 120 months of age, and their results are shown in the following figure.

a. Visually estimate the median time to first infection by Pseudomonas.

b. The small crosses on the curve (“+”) represent babies who were withdrawn from the study at the times indicated. Because of these withdrawals, what term do we apply to these data?

c. What is missing from this graph that, if added, would allow viewers to assess the uncertainty of the estimates of survival probability?

d. For this survival curve, what is the probability that the time to established Pseudomonas infection occurs more than 60 months after birth? Estimate by eye.