Discussion
To our knowledge, this is the first study to compare individual estimates of amounts of served food with the actual weight of the food served in a laboratory setting, where less disruptive data collection techniques are applied. Other studies assessing the accuracy of estimated portion sizes have used photographs as the reference for the assessment (Huybregts et al, 2007, Turconi et al, 2005), fake food items (Bucher et al, 2011, Bucher et al, 2012, Bucher et al, 2013) or average portion size weights based on repeated measures (Ngigi, Verbeke, & Pérez-Cueto, 2011). In contrast, the present study was performed using actual food items. In addition, the use of the Intelligent Buffet allowed the exact weight of food served by each participant to be recorded in a relatively undisturbed manner.

There are a few potential sources of bias in this study, one being measurement bias. An attempt was made to limit this by calibrating the equipment used for data collection before use and by following standard laboratory procedures. Therefore, if any measurement bias were to occur, it would be systematic throughout all observations. Secondly, the sample size may have been insufficient. However, the calculated power of 0.99 for the observed mean differences of both vegetables and whole grains suggests that the sample was enough to support the rejection of the null hypothesis. Previous studies have used a similar sample size with sufficient power (Mekhmoukh, Chapelot, & Bellisle, 2012). Thus, we assume that the result of the Wilcoxon signed-rank test was accurate (absence of type II error), and that it was correct to reject the null hypothesis since there was a significant difference between the two variables – the estimated and actual amount of food served.

Our findings are in agreement with similar studies conducted in a number of different countries and settings, including Norway (Uglem et al., 2008), the Netherlands (Lechner et al., 1997), Australia (Pollard et al., 2008) and Denmark (Sørensen et al., 2013), suggesting that individuals commonly have difficulties estimating food portion sizes with accuracy. The findings of a statistically significant difference between participants’ self-estimation and the actual weight of the vegetables served led to the rejection of the null hypothesis (Table 1). The participants in this study were also asked to estimate the weight of a whole grain serving in a relatively complex meal. It would be of interest to determine whether the results would have been more accurate if a simpler meal had been served (e.g. a bowl of cereal). However, the everyday diet commonly consists of prepared meals or ready-to-eat foods and individuals are often not aware of the actual composition of the foods they consume. Therefore, serving a “complex meal” is comparable with a real life situation, where participants are expected to estimate the amount of food served (Combet et al, 2013, Loh et al, 2013).

This study presents the question of whether food intake estimations that are independent of actual weighing are valid, particularly in the evaluation of adherence to healthy eating recommendations (Pérez-Cueto et al., 2012). This also underscores the importance of using alternative equipment and techniques, such as an Intelligent Buffet, in order to obtain actual data in studies measuring factors such as food intake (Bucher et al., 2012). The Intelligent Buffet presents many opportunities to conduct other studies or further research on this particular topic. In combination with video cameras, this equipment could be used for choice architectural experiments researching behaviour related to a buffet setting. For example, it could be used to investigate whether individual consumption differs in relation to the order of food items or the size of serving trays.

As seen in Table 1, there is no statistical correlation between measured and estimated values; they are independent variables meaning that there is no predictive relationship between the two measurements.

Consequently, if authors were to compare the difference between means they would probably find that the estimations are so different from the actual intake that they are neither reliable nor valid, thus supporting the use of alternative techniques such as the Intelligent Buffet when accuracy is a requirement of the research.

Generalizability of the findings is limited due to the study design and sample size. The use of a convenience sample from a university setting introduced a sample mainly consisting of higher educated participants aged between 20 and 29 years. Consequently, the results measure perception of serving size against the weight of the food served for this survey sample. A larger sample size and a different study design would need to be undertaken to explore if these findings extrapolate to the general University student community or to specific socio-economic groups within that community. The literature and the current study findings suggest the likelihood of an overestimation of portion sizes. However, further studies should be carried out before making conclusions about whether the general population is capable of accurately estimating food portion sizes and about the effects this could have on the degree of validity in studies involving self-estimation as a data collection method.

Considering that the majority of participants were highly educated people and nevertheless unable to correctly estimate the amounts of vegetables and whole grain served, and previous research highlighting that lower socioeconomic groups may find it difficult to interpret serving size recommendations, these findings may be of concern for nutrition educators (Drewnowski, Specter, 2004, Kamp et al, 2010, Robinson, 2008, Sinclair et al, 2013). Studies show that individuals from higher socio-demographic groups have a greater knowledge and comprehension of nutrition than individuals from low socio-demographic groups (Drewnowski, Specter, 2004, Pollard et al, 2008, Sinclair et al, 2013).

Furthermore, this study raises the question about whether self-estimation of portion sizes would be an appropriate way to evaluate intake of vegetables and whole grain cereals, particularly in self-serving settings, such as University Canteens, since it appears that the participants had very little ability to estimate their own intake (estimated serving vs actual serving were independent measures). Probably, this will be applicable to most other food groups, and thus this study challenges the way for instance communication towards healthy eating is delivered. It also calls for innovative ways to make the population aware of what portions are if the intention is to rely on the general ability to estimate quantities of food served or eaten.

Conclusion
The present study was the first to assess the accuracy of self-estimates of vegetables and whole grain portion sizes in a self-served meal conducted in a laboratory setting using an Intelligent Buffet combined with a quantitative questionnaire. When comparing the self-estimated amount of vegetables and whole grains with the actual amount, the analysis revealed a significant difference between the means (P < 0.001) where students’ self-reported portions were about three times bigger than the actual servings.

The study supports the use of innovative techniques such as the Intelligent Buffet. Further it supports the use of triangulation in order to obtain more precise estimations on intake and to better evaluate the real adherence to nutritional recommendations.

The questionnaires provided information on the profile of the participants and it appeared that there was little variety within the sample. Further limitations could be linked to the sample size and the methodological approach, particularly the equipment used for the data collection and data management, which had never been utilised prior to the study.

This study suggests that at least for this survey sample individuals do not provide accurate estimates of the amount of vegetables and whole grains they serve themselves on a plate. Our study findings lead us to question individuals’ ability to assess the amounts of vegetables and whole grain they serve themselves from a buffet. The degree of underestimation of servings within this small experimental group is high. On average participants assessed that they served themselves about three times what they actually did. Although there was a high awareness of the Danish 6 A Day recommendation among study participants, their ability to accurately estimate their serving size appeared to be a limited. These findings highlight the need for further research into the accuracy of self-estimated intake across the population and the implications of the overestimation of current consumption for nutrition promotion interventions.

Based on the findings of this study it is recommended that policy makers and social marketing campaign creators consider the development of more specific serving size measurements or that explicit communications e.g. regarding serves of plant foods used in nutrition education are needed to assist consumers in estimating own portions sizes in relation to general health recommendations.