6MOTIVATING EXAMPLES
1.4 Clinical trials of exercise as an aid to smoking cessation in
women: the Commit to Quit studies
1.4.1 Studies and data
The Commit to Quit studies were randomized clinical trials to examine the
impact of exercise on the ability to quit smoking among women. The women
in each study were aged 18 to 65, smoked five or more cigarettes per day for
at least 1 year, and participated in moderate or vigorous intensity activity for
less than 90 minutes per week.
Thefirst trial (hereafter CTQ I; Marcus et al., 1999) enrolled 281 women
and tested the effect on smoking cessation of supervised vigorous exercise
vs. equivalent staff contact time to discuss health care (henceforth called the
‘wellness’ arm); the second trial (hereafter CTQ II; Marcus et al., 2005) en-
rolled 217 female smokers and was designed to examine the effect of moderate
partially supervised exercise. Other analyses of these data can be found in
Hogan et al. (2004b), who illustrate weighted regression using inverse propen-
sity scores; Roy and Hogan (2007), who use principal stratification methods
to infer the causal effect of compliance with vigorous exercise; and Liu and
Daniels (2007), who formulate and apply new models for the joint distribution
of smoking cessation and weight change.
In each study, smoking cessation was assessed weekly using self-report, with
confirmation via laboratory testing of saliva and exhaled carbon monoxide.
As is typical in short-term intervention trialsforsmoking cessation, the target
date for quitting smoking followed an initial ‘run-in’ period during which the
intervention was administered but the participants were not asked to quit
smoking. In CTQ I, measurements on smoking status were taken weekly for
12 weeks; women were asked to quit at week 5. In CTQ II, total follow-up
lasted 8 weeks, and women were asked to quit in week 3.
1.4.2 Questions of interest
In each study, the question of interest was whether the intervention under
study reduced the rate of smoking. This can be answered in terms of the
effect of randomization to treatment vs. control, or in terms of the effect of
complying with treatment vs. not complying (or vs. complying with control).
The former can be answered using an intention to treat analysis, contrasting
outcomes based on treatment arm assignment. The latter poses additional
challenges but can be addressed using methods for inferring causal effects; we
refer the reader to Roy and Hogan (2007) for details.
In our analyses, we frame the treatment effect in terms of either (a) time-
averaged weekly cessation rate following the target quit date or (b) cessation
rate at the final week of follow-up. More details about analyses are given
below.