Kessels and Ruiter BMC Public Health 2012, 12:352 http://www.biomedcentral.com/1471-2458/12/352 Methods Participants and design In total, 66 Dutch psychology undergraduate students took part in the experiment, 25 daily smoking students (16 women) and 41 non-smoking students (31 women). Normal or corrected-to-normal vision was used as an eligibility criterion. Participants were 18 to 24 years of age (Smokers: M = 20.69; SD = 1.99, Non-Smokers: M = 19.73; SD = 1.82). Participants were recruited in vari- ous public places at the university campus. They took part for course credits or received a gift voucher of 12 Euros. The experiment varied the content of the health text (high risk vs. coping) and photo (high threat vs. low threat) as within-subjects factors and smoking status (smoker vs. non-smoker) as between-subjects factor. The study was approved by the Ethical Committee Psychology of the School of Psychology and Neurosci- ence, Maastricht University, that functions in accordance with the Helsinki Declaration. Stimulus materials The stimulus materials were sixteen self-created pictures of cigarette packages (size, H × W: 11.22 × 7.87 in.). We used the basic format of a cigarette package and added a smoking related photo and health text. The upper part of each cigarette package was held constant, with the texts ‘cigarettes’ and ‘brand’ printed in a red and a white rectangle, respectively. The upper part of the package (‘cigarettes’ and ‘brand’) was 50% of the total picture (H × W: 5.61 × 7.87 in.). In the middle part of each cigarette package one of sixteen different low threat (8) and high threat (8) smoking-related coloured photos was presented (40% of the total picture; H × W: 4.53 × 7.87 in.). Below the picture we varied the content of the health texts (10% of the total picture; H × W: 1.10 × 7.87 in.). In total, we presented sixteen smoking- related health texts in white letters on a black background. The health texts and photos on the cigarette packages were selected from two pilot studies. In the first pilot study, we tested twenty-four smoking-related health texts that were partly based on existing texts on cigarette packages (http://www.tobaccolabels.ca/). Sixteen smok- ing and fifteen non-smoking undergraduate students (not tested for main study) rated each text on 7-point scales measuring whether the text contains helpful infor- mation regarding quitting smoking (1 = not at all, 7 = very much) and the perceived level of threat of the text (‘This photo is threatening’; 1 = not at all, 7 = very much). Eight coping texts and eight high risk texts were selected. The selected coping texts were believed to con- tain more helpful information about quitting smoking (M = 3.50; SD = 1.21) than the high risk texts (M = 1.64; SD = .84), t(30) = 6.69, p < .001, whereas the high risk Page 3 of 9 texts were evaluated as more threatening (M = 2.09; SD = .63) than the coping texts (M = 1.58; SD = .50), t (30) = 4.99, p < .001. An example of the selected coping texts is “You can do it, your doctor or pharmacist can help you stop smoking”. An example of the high risk texts is “Smoking clogs the arteries and causes heart attacks and strokes”. In the second pilot study, thirty-five smoking-related photos were tested that were found on the website of the European Community presenting the cigarette packages and via a search through the internet using search terms like ‘smoking’, ‘cigarette’, or ‘causes smok- ing’. On a 7-point scale sixteen smoking and sixteen non-smoking undergraduate students (not tested for main study) rated the perceived level of threat (‘This photo is threatening’; 1 = not at all, 7 = very much) for each photo. We made a selection of eight low threat photos with a maximum score of 2 on the perceived level of threat scale and eight high threat photos with a minimum score of 3.5 on the perceived level of threat scale. The high threat photos contained pictures with black lungs, skeletons with a smoking cigarette and other images that illustrate the negative health conse- quences of smoking. The low threat pictures showed smoking cigarettes, people smoking a cigarette, and other images that do not directly focus on the negative health consequences of smoking. For creating the sixteen cigarette packages for the main experiment each low threat and high threat photo was paired to one risk or coping text with the result that each of the sixteen images used to represent a cigarette package contained a unique combination of photo and health text (risk or coping text). Within each text*photo condition four combinations were presented. All other features of the cigarette packages (e.g., colour, font type) were kept constant across conditions. The final set of stimulus materials is available from the first author on request. Apparatus Eye movements were registered by the EyeLink I eye- tracker from SensoMotoric Instruments (SMI, Germany) and SR Research (Canada), with a 250 Hz temporal reso- lution, a 0.005˚ gaze and eye position resolution, and a gaze position accuracy with 0.5−1.0˚ average error. It is a headband-mounted infrared video-based tracking sys- tem that can track both eyes. The system corrects for head-motion by means of an additional infrared camera. The participant’s head rested on a chin-rest at an ap- proximate distance of 57 cm of the computer monitor (19” flat panel Dell monitor). Monitor resolution was set to 1024 * 768 pixels. The cigarette packages were pre- sented in colour format and sized 7 by 13 cm. The pres- entation of stimuli and analysis of the eye tracking was