Schulz et al. BMC Public Health 2011, 11:108 http://www.biomedcentral.com/1471-2458/11/108 Background Cardiovascular diseases (CVD) and cancer are the two major causes of illness and death in the Netherlands as well as in other Western countries. Causal factors of these diseases are partly related to unfavourable lifestyle behaviours, such as smoking, high alcohol consumption, unhealthy eating habits and physical inactivity [1,2]. In the Netherlands, public health guidelines are defined for these health behaviours, i.e., non-smoking, not drinking more than one (women) or two (men) glasses of alcohol a day [3], eating two pieces of fruit per day [4], eating 200 grams of vegetables per day [4], and being moder- ately physically active for 30 minutes at least five days a week [5]. Approximately three quarters of the Dutch population eats too little fruit and vegetables, nearly half of the population does not meet the recommendation for physical activity, nearly one third smokes and it is estimated that a comparable percentage of people drinks too much alcohol [6,7]. No single behaviour can be ascribed as a sole causal factor in the development of diseases such as CVD or cancer. Recent research shows that about 5% to 10% of the population complies with all these public health guidelines and that 5% to 10% does not comply with any of the five guidelines [8]. Interventions are needed in order to encourage people to adopt a healthier lifestyle. Nowadays, 88.6% of the Dutch population have access to the Internet [9]. This implies that web-based lifestyle interventions have the potential to reach a large number of people [10,11]. Web-based computer tailoring (CT) is a relatively new effective technique for health education. CT is any com- bination of information or change strategies intended to reach one specific person, based on characteristics that are unique to that person, related to the outcome of interest, and have been derived from an individual assessment [12,13]. Positive aspects of web-based CT programmes are that these can be applied in privacy, for example at home, and at a time the respondent finds convenient [14], and that they can be integrated in other interventions [15]. A CT programme even has some key benefits in comparison to non-tailored materi- als. It can provide personalized feedback which attracts attention and consists of less unnecessary information than general material [16]. CT messages are usually bet- ter read, saved, remembered and discussed with others than non-tailored materials [15,17,18]. CT is cost- effective due to the fact that it can be provided via the Internet and can reach many people [15,19]. CT seems to be more effective in comparison to general messages about health beliefs and health behaviour [15,17,20] as shown by their effects on increasing smoking cessation [21], decreasing alcohol consumption [14,22], increasing fruit and vegetable intake [23-26] and increasing physi- cal exercise [27-30]. In the past, most CT programmes Page 2 of 9 addressed only one behaviour. By now, the focus on multiple behaviours has become increasingly popular [23,31-34]. However, the best strategy for offering health information to motivate the population to change multi- ple behaviours has not yet been determined. Two different types of strategies can be distinguished in this respect: a simultaneous behaviour change strat- egy in which personal advice on multiple lifestyle beha- viours is given at the same time; and a sequential behaviour change strategy in which personal advice on multiple lifestyle behaviours is provided consecutively at different points in time. Both strategies have potential advantages as well as disadvantages which may have influence on the effectiveness and appreciation of the tailoring programme. In the simultaneous behaviour change strategy, people receive a lot of information and they may be inclined to choose one behaviour from the range of options and to autonomously change or add to this choice at any time. However, people in the sequen- tial behaviour change condition may perceive a lower level of autonomy since they have to limit themselves to one single behaviour at first [35,36]. Due to the amount of information that is given in the simultaneous beha- viour change strategy, participants actually receive more options and choices and this strategy may possibly be more effective for people who are already motivated to change their lifestyle. On the other hand, it is conceiva- ble that individuals in this situation become over- whelmed due to the amount of information, leading to too few energy resources to change [37] and a more negative feeling regarding the programme. This may be particularly true for people who are not ready to make changes in their lifestyle. Recent research suggests that the probability of enhan- cing a second behaviour increases when an individual has successfully changed a first behaviour [38]. This means that, for example, an increase in physical activity may lead to improved eating habits [39]. Such synergis- tic or spill-over effects may occur in both strategies [40], but it is unclear in which strategy such effects are strongest. The tailoring programme used in the current study is theoretically based on the integrated model for explain- ing motivational and behavioural change (the I-Change model) [41]. The I-Change model builds on the Atti- tude-Social influence-Self-efficacy (ASE) model [42], which in turn integrates ideas from social-cognitive models, such as the Theory of Planned Behaviour [43], the Social Cognitive Theory [44], the Health Belief Model [45], and the Transtheoretical Model [46]. According to the I-Change model, the most proximal determinant of behaviour is the intention to perform this certain behaviour. If someone has the intention to perform a specific behaviour, barriers may hinder the