Groeneveld et al. BMC Public Health 2011, 11:836 http://www.biomedcentral.com/1471-2458/11/836 who performed the data analyses was blinded to the group allocation. Due to the study design, the interven- tion providers and participants could not be blinded. The sample size was based on PA; one of the main outcome measures of the study. To detect a 10% difference between the control and intervention group in the pro- portion of participants meeting none of both Dutch guidelines for moderate and vigorous intensity PA after 6 months, i.e. 38% in the control group and 28% in the intervention group, with a power of 80% and a 95% confi- dence interval (a = 0.05), 692 persons were needed at the first follow-up measurement. Intervention and control condition Over a period of 6 months, each participant in the inter- vention group had three 45-60 minute face to face and four 15-30 minute telephone contacts with an occupa- tional physician or occupational nurse. This counselor applied a client-centered counseling style using MI techni- ques such as asking open questions, summarizing, care- fully listening, supporting, and raising ambivalence. In the first session, a stepwise protocol had to be followed. First, the participant’s CVD risk profile was presented and his current health status was discussed. Second, the partici- pant decided to aim at PA and diet, or smoking. Third, the participant was encouraged to indicate advantages and disadvantages of current and ‘desired’ behavior. Fourth, the participant was asked to indicate his willingness, readi- ness, and perceived confidence in his ability to change on 10-point scales. Last, the participant set long- and short- term goals, and formulated implementation intentions [25]. In the following counseling sessions, progress and barriers were discussed. The participants in the control group received usual care, consisting of brief oral or writ- ten information from the occupational physician about their risk profile, based on the periodical health screening results. To all participants of both intervention and control group, brochures were provided containing information on PA, healthy eating, smoking cessation, and CVD. Outcome measures At baseline and after 6 and 12 months, a questionnaire on PA, diet, and smoking was filled out. For measuring PA, we used the fairly reliable (rspearman 0.58) and valid (rspear- man 0.45) Short QUestionnaire to ASsess Health enhancing PA (SQUASH)[26]. By means of this questionnaire, we determined the number of minutes per week spent on two domains of PA, i.e. leisure time PA (walking, cycling, doing odd jobs, gardening), and sports activities. In addi- tion, the total weekly amount of energy expended due to the activities in those two domains was estimated, by mul- tiplying the total number of minutes spent on each activity by its metabolic equivalent- (MET-) value [27] and sum- ming all MET-minutes. With regard to diet, in line with Page 3 of 9 the Short Questionnaire for Measuring Fruit and Vegeta- ble Intake, the average weekly intake during the past month was determined for fruit (pieces) and vegetables (heated and raw; tablespoons). Moreover, consumption of alcohol (glasses), and snacks were assessed. The latter food group was defined as the sum of sweet (e.g. piece of pie), cold salty (e.g. handful of crisps), and warm salty snacks (e.g. piece of egg roll), eaten outside the regular meals. The food questionnaire was not validated but tested for face validity by an expert in nutrition and lifestyle change, and for comprehensibility by two construction workers. Current smoking status was defined as ‘smoker’ or ‘non- smoker’. Furthermore, the participant was asked whether he had used nicotine replacement therapy or medication in case he had succeeded in smoking cessation. At base- line, the possible confounding variables age (years) and BMI were determined at the occupational health service. For determining BMI, body height (meters) without shoes was determined with the participant in standing position, his heels and head against the wall and his face in a hori- zontal plane. Body weight (kilograms) was measured with- out shoes and jacket, using a digital balance. Data analyses Data were analyzed using SPSS (Version 15.0, Chicago Ill). In the baseline questionnaire, all participants had indicated whether they would prefer to improve their dietary or PA behavior, or to quit smoking. Those who had indicated to prefer improving diet or PA (energy balance-related beha- viors; EB) were analyzed separately from the ones who had indicated to prefer smoking cessation (SC). As a result of self selection, baseline differences between the intervention and control group may have arisen, possibly leading to confounding. Therefore, we checked for baseline differ- ences between intervention and control group in age, BMI, smoking status and all of the outcome measures, within both the EB group and the SC group. To determine the effects at 6 months, linear and logistic regression ana- lyses were done with the variable of interest as the out- come, and group allocation (intervention vs. control) as the independent variable, adjusted for the baseline value. The effects at 12 months were evaluated using the same method. In both the EB and SC group we checked for confounding by age and BMI, and for effect modification by variables that theoretically could modify the effect on the outcome measure of interest, i.e. age, BMI, smoking, work type, and marital status (partner/no partner). Effect modification was concluded in case the p-value of the interaction term was <0.1. Only participants for whom data were present on all three time points were included in the analyses. Additionally, in order to assess the inter- vention effects among participants who had adhered to the protocol, linear and logistic regression analyses accord- ing to the ‘per protocol’ principle were done, for the effects