Haraldsdóttir J, Tjønneland A, Overvad K
Research Department of Human Nutrition, Royal Veterinary and Agricultural University, Copenhagen, Denmark.
Int J Epidemiol. 1994 Aug;23(4):786-96. doi: 10.1093/ije/23.4.787.
Dietary surveys often rely upon subjects' ability to estimate their portion sizes instead of weighing them, but information on the validity of such estimates is lacking. This study investigated the validity of estimated portion sizes from a self-administered food frequency questionnaire including photos.
Validity was evaluated by comparison with portion sizes from 14-day weighed food records. Eight sets of photos were included in the study (three meat dishes, one fish dish, three vegetables, potatoes), each set with four options. Subjects (59 men, 85 women) were 40-64 years.
The majority of subjects (85-95%) selected the most correct photo or a neighbouring photo. However, regression analyses showed that the relationship between the estimated and the measured portion sizes was relatively weak for most of the foods tested, and correlations were only significant for three of the eight foods. This apparent contradiction was partly due to the pattern of the error: subjects selecting small portions had underestimated their actual portions while subjects selecting large portions had overestimated them. This error pattern could not be explained by the theoretical error resulting from the use of four discrete portion sizes instead of continuous weights, neither was it a simple consequence of the unidirectional errors at the extreme ends. Actual portion sizes varied from day to day, with intra-individual coefficients of variation of 34-40%.
The photos used in the present study were of limited value for ranking individuals correctly according to their actual portion sizes. Whether this is a common feature of photos used in food frequency questionnaires remains to be answered.
饮食调查通常依赖受试者估计食物分量大小的能力，而非称重，但缺乏有关此类估计有效性的信息。本研究调查了一份包含照片的自填式食物频率问卷中估计食物分量大小的有效性。
通过与14天称重食物记录中的食物分量大小进行比较来评估有效性。研究中包括八组照片（三道肉类菜肴、一道鱼类菜肴、三种蔬菜、土豆），每组有四个选项。受试者（59名男性，85名女性）年龄在40至64岁之间。
大多数受试者（85%-95%）选择了最正确的照片或相邻照片。然而，回归分析表明，对于大多数测试食物，估计的和测量的食物分量大小之间的关系相对较弱，并且相关性仅在八种食物中的三种上显著。这种明显的矛盾部分归因于误差模式：选择小分量的受试者低估了他们的实际分量，而选择大分量的受试者高估了他们的实际分量。这种误差模式既不能用使用四个离散的分量大小而非连续重量所产生的理论误差来解释，也不是极端情况下单向误差的简单结果。实际食物分量大小每天都有所不同，个体内变异系数为34%-40%。
本研究中使用的照片对于根据个体实际食物分量大小正确排名的价值有限。这是否是食物频率问卷中使用照片的共同特征仍有待回答。
