The mean period of smoking was 7 years ( std. dev = 5.4 ) .
The mean number of cigarettes smoked daily was 11.5 ( std. dev = 6.9 ) .
The mean period of smoking was 7 years ( std. dev = 5.4 ) and the mean number of cigarettes smoked daily was 11.5 ( std. dev = 6.9 ) .