Samkange-Zeeb et al. BMC Public Health 2011, 11:727 http://www.biomedcentral.com/1471-2458/11/727 (66.6% of respondents aware), measured in a different population, was observed in the second recent study on HPV [48]. Two factors appeared to have influenced awareness. The first was of a methodological nature and related to the fact whether an open or closed question was posed. Of the studies included in the review which assessed awareness, all but one used closed-form questions only. The adolescents either had to identify sexually trans- mitted diseases from a given list of diseases, or the ques- tion was in a yes/no format. Initially, Höglund et al. asked participating adolescents to list all STDs known to them and then later on, if they had ever heard of HPV. Only one participant (0.2%) mentioned HPV as one of the STDs known to them, but later, 24 (5.4%) reported to have heard of HPV [47]. In comparison to open-form questions, closed questions are not only more practical and easier to respond to, but also easier to code and analyse. One of the arguments raised against closed questions, especially where a list of possi- ble answers is given, is the risk of guesswork. It can not be ruled out that some participants, unable to answer the question, will select answers at random [50,51]. In the study by Garside et al. for example, among year 9 pupils, 14.5% incorrectly identified plasmodium, and 20.6% filariasis from a given list as STDs [42]. Open questions have been recommended for surveying partici- pants with unknown or varying knowledge/awareness [50] as these questions provide a more valid picture of the state of knowledge [51]. To a lesser extent, gender also appears to have influ- enced knowledge and awareness, especially for HPV [46,48]. Significant gender differences were observed, with females having better awareness and knowledge than males. Although the data are limited as not all stu- dies reported results separately for males and females, these findings, could be reflective of the way awareness campaigns, for example on HPV, have been targeted more at females than at males. The studies on HIV included in our review generally reported high awareness of the protective effect of con- doms among adolescents [36,41,43,47,49]. One study included in the review however observed that adolescents seem to regard condoms primarily as a method of contra- ception and not as a means of protection against sexually transmitted diseases (40). In this study, 19 out of 20 female adolescents who reported more than 4 sexual partners at the age of 18 reported intercourse without a condom in relationships of less than 6 months’ duration. The majority of them were, however, convinced that they had neither acquired (96%) nor transmitted (93%) an STD at last unprotected intercourse [40]. Other studies also indicate that consistent condom use is generally low among adolescents [27,52-55]. Page 10 of 12 Where reported, participation rates were generally high, probably due to the fact that the adolescents were recruited in schools. In some instances however, the number of participants was low even though the participation rate was reported as high. In the study by Tyden et al. for example, the study sample consisted of 213 pupils, 12% of the 1830 students in the first form of upper secondary school in Uppsala [38]. The authors base the participation rate of their study (98%) on the 12%, without explaining how it came about that only 213 pupils were considered for parti- cipation. The one study which recruited participants per post had a very low participation rate of 21.5% [45]. Never- theless, the study had more participants than others with comparatively higher participation rates. Bias related to selective participation is an issue that needs to be consid- ered on a study by study basis, and reporting on response proportions should be considered essential for all studies. Study strengths and limitations To our knowledge no systematic reviews of published lit- erature on knowledge and awareness of sexually trans- mitted diseases among school-attending adolescents in Europe have been conducted to date. The current review confirms that there are considerable gaps in knowledge and awareness on major STDs in European adolescents. Our results underline the importance of the objectives set for adolescents’ sexual and reproductive health in Europe, the first of which foresees that adolescents be informed and educated on all aspects of sexuality and reproduction [31]. We could not identify many studies on knowledge and awareness of sexually transmitted diseases among school- attending adolescents in Europe. This could be due to the fact that knowledge has been shown to have little impact on behaviour change, and prevention interventions have generally moved away from a focus on knowledge and awareness as key mediators. Another possible reason is that schools are not always willing to participate in such studies due to competing demands of other school activ- ities or because of the subject content [16,28-30]. One limitation of our review is that the 15 studies included did not all focus on the same sexually trans- mitted diseases. The four studies conducted in Eastern Europe were all on HIV/AIDS knowledge and awareness only, whereas Western European studies were on STDs in general or on HPV. Furthermore, the formulation of the questions used to assess awareness and knowledge varied between studies, making it difficult to directly compare the findings of individual studies. Another potential limit- ing factor is the age variation of participants in the studies included in the review, especially as all but one study did not clearly investigate the association between age and awareness or knowledge. Due to the afore-mentioned fac- tors and the small number of studies available, it was not possible to perform a meta-analysis of the study findings.