A study conducted in Amboseli National Park in Kenya observed a baboon troop for several days, recording the time at which half of the troop had descended from the trees in which they slept. On some days the observers arrived early enough to observe this exactly. Other times they arrived after this median descent time, so the day's observation was left censored.
Create a Nelson-Aalen model for the left-censored data and estimate the median.
Compute the probability that the median descent time is greater than nine hours.
Visualize a 95% simultaneous band for the cumulative descent probability.