Kessels and Ruiter BMC Public Health 2012, 12:352 http://www.biomedcentral.com/1471-2458/12/352 performed with Matlab 6.5. Although viewing was bin- ocular, only fixations and saccades of the right eye were monitored. Procedure The experiment took place in a dimly lit, sound-attenu- ating room. After explaining the procedure of the ex- periment, participants signed an informed consent, and the Eyelink system was installed on the head. Partici- pants were seated at a table in front of a computer screen, with their head resting on a chin-rest. After cali- brating and validating the Eyelink system, the experi- ment started. Each of the sixteen cigarette packages was randomly presented for 10 seconds. Before a package appeared on the screen, a dot was presented at the centre of the screen. Participants were asked to focus on the dot, so that each participant would start viewing every package from the same position on the screen. During exposure to the stimuli, eye movements were registered. After presentation of the sixteen different packages, the head- set was removed and the participant started with filling out the self-report questionnaires to evaluate the stimuli. Eye movement registration We extracted the eye tracking data from Data Viewer (SR Research software) to SPSS. For analysing the eye- movements we created two areas of interests (i.e. two rectangles) for each cigarette package. The first area of interest contained the health text (coping vs. high risk) and the second area of interest contained the photo. The number of fixations and dwell time for the area of inter- est of each health text were divided by the number of syllables per text. The mean number of syllables was not different across conditions, Fs < 1.53, ns, η2p < .05, (cop- ing text – low threat photo: M = 14.5; SD = 3.11; range: 12–19; high risk text – low threat photo: M = 14.5; SD = 1.91; range: 12–16; coping text – high threat photo: M = 14.75; SD = 3.59; range: 12–20; high risk text – high threat photo: M = 16; SD = 5.35; range: 10–21). Finally, for each area of interest we compared the number of fixations and dwell time for all different con- ditions after averaging for each condition the responses to the four stimuli within that condition. Self-report measures To measure participants’ evaluation of the health texts and the photos, we used self-report items with 7-point Likert scales. Five items that measured clarity, credibil- ity, interest, usefulness and acceptability of each of the presented risk and coping texts were combined to meas- ure message acceptance of the health texts (α’s > .88; 1 = not at all, 7 = very much). Novelty and perceived per- sonal relevance of the text were both measured with one Page 4 of 9 item (1 = not at all, 7 = very much). Furthermore, per- ceived level of threat was assessed for each health text and each photo (‘This text/photo is threatening’; 1 = not at all, 7 = very much). Data-analysis Mixed analysis of variances (ANOVA) were conducted to test the effects of within-subjects factors health text (coping vs. high risk) and photo (low threat vs. high threat) and the between-subjects factor smoking status (smokers vs. non-smokers) on the number of fixations and dwell time. Similar analyses were conducted for the self-report measures. Significant interactions between health text, photo and smoking status were followed-up by mixed analyses of variance (ANOVA) for each level of photo. To control for sphericity violations in the mixed ANOVAs, we report probability values with Greenhouse-Geisser correction for F tests with more than one degree of freedom in the numerator. The reported estimates of the effect size are the partial eta squared (η2p) for the analyses of variance. Only significant effects are reported (p < .05), whereas lower order effects in the omnibus analyses are only reported in case no sig- nificant higher order effects were found. In case of main effects of health text, photo and smoking status overall means are provided in the text. Results Sample characteristics No differences between smokers and non-smokers were observed in terms of age and gender, ps > .06 (see Table 1 for demographics). Smoking participants smoked some- what more than six cigarettes per day on average (M = 6.40; SD = 4.30). The data sets of two smoking par- ticipants could not be used because of technical pro- blems. Therefore 64 participants remained in the analyses. Eye tracking Table 1 shows the mean number of fixations and dwell- time for each area of interest (health text and photo), for each experimental condition, and for the two smoking status groups. Area of interest – health text Fixations. The number of fixations per syllable for the health texts revealed a significant 3-way interaction be- tween health text, photo and smoking status, F(1, 62) = 8.38, p < .01, η2p = .12. Separate analyses for each level of photo showed for the health messages combined with a high threat photo a significant interaction effect between health text and smoking status, F(1, 62) = 11.00, p < .01, η2p = .15. Paired- samples t-tests for each level of smoking status showed