Akl et al. BMC Public Health 2011, 11:244 http://www.biomedcentral.com/1471-2458/11/244 Results including prevalence results (ever, current, regular) of waterpipe only smoking, waterpipe smok- ing (regardless of other tobacco products use), cigar- ette smoking, and factors significantly associated with waterpipe smoking. Data analysis We calculated the kappa statistic to evaluate the agree- ment in assessing full texts for eligibility. We stratified data analysis by country and age group. We present the results by the region of the world. Results Description of included studies Figure 2 shows the study flow and reasons for study exclusions. We considered a total of 64 studies. Of these we excluded 26 studies for the following reasons: conve- nience sampling (n = 9) not representative sampling (n = 7); did not measure prevalence (n = 6) waterpipe smoking not reported separately from other forms of smoking (n = 3) and qualitative study of waterpipe smo- kers (n = 1) (Additional file 2). We included a total of 38 studies. The countries in which the studies were Page 3 of 12 conducted were: Lebanon (n = 10), Arabic Gulf coun- tries (n = 8), USA (n = 6), Pakistan (n = 4), Egypt (n = 4), Syria (n = 3), Australia (n = 1), UK (n = 1), and Esto- nia (n = 1). The target populations in the identified stu- dies were middle or high school students (n = 11), university students (n = 8), adults (n = 17), and preg- nant women (n = 3). Only 26 of the 38 papers reported the year of data collection. All of the papers were pub- lished after 1998. The studies varied by whether they reported the pre- valence of waterpipe only smokers (n = 5), all waterpipe smokers (n = 37) (i.e., irrespective of whether they smoked other forms of tobacco), and by the types of measure of waterpipe smoking: ever smoker (n = 16), current smoker (n = 34), and regular smoker (n = 3). Five studies reported on group waterpipe smoking while 35 studies reported on individual smoking. Methodological quality of included studies Ten studies reported sample size calculation. The instruments used to measure waterpipe use were: self developed instrument with no validation reported (n = 15); self developed instrument based on previously vali- dated instruments, with no validation of the new Figure 2 Flow of studies considered in the systematic review.