Define a variable Smokchng that measures change in smoking status between baseline and follow-up, so that Smokchng equals 1 if a person changes from being a smoker to being an ex-smoker and equals 0 otherwise.

(a) Is there an association between Staget and Smokchng?

(b) What effects does Smokchng have upon survival? Is it significant? Is the effect the same if the smoking status variables are also included in the model as independent variables?