General Study Design
Two studies were conducted. Twenty-nine participants took part in the first study (three groups of approximately 10 each), and 20 in the second (two groups of 10). In each, participants came to the study site in separate groups of two to three on two consecutive days. Participants were told that they would be fed two meals the first day and then return the second day to respond to some questions about foods people eat. They were not told that they would be asked about portion sizes consumed. To approximate real-life eating, participants served themselves rather than being provided standard portions for each food. On the first day participants signed an informed consent and then, for both breakfast and lunch, served themselves nine foods from five food categories in a buffet line: amorphous/soft foods, single unit foods, small pieces, spreads, and shaped foods. ( lists the foods served within each category for each study.) Subjects were instructed to take and consume at least some of each food at each meal. A room monitor ensured that no foods were spilled, exchanged, or thrown out. Trained research assistants weighed the serving containers unobtrusively using the UltraShip digital scale, model UL-35 (My Weigh/GKI Technologies, Vancouver, BC, Canada), (accurate to 2 g) before and after each participant selected food and left the serving area. The assistants recorded weights in grams into a spreadsheet that calculated a gram weight for the amount self-served by each participant for each food. Any food remaining on the plate was also weighed so that a final determination of how much was consumed could be made. Two staff members independently recorded the weights for each item. When observations differed, the item was reweighed and discrepancies reconciled. On the second day, participants used a computer-based application, developed specifically for this study, to view a series of screens that varied by presentation factors described below and selected the portion sizes they estimated to be closest to what they had eaten the day before. For each screen presentation, participants were offered the option to select “less than” or “more than” the amount that was presented by the images to best represent what they had consumed the previous day. After estimating their portion sizes, participants completed a paper questionnaire to rank their preferences among the various screen presentations.
Studies 1 and 2: Controlled feeding study menu items by food categories for five groups of participants completing a computer portion-size application.
Westat staff used an internal database of research volunteers, including Westat employees, to identify potential participants for each study. Potential volunteers agreed to attend sessions on two contiguous days and take and eat at least some of each food offered at each meal. Participants were recruited to represent a range of demographic characteristics, including sex, race/ethnicity, age, and educational status via a telephone-administered questionnaire. The goal was to recruit about one third with only a high school education for purposes of evaluating the usefulness of the application with lower literacy participants. Participants were required to be familiar with a computer. Westat employees (N = 12) received $90 as an incentive for participation; others, because they had to travel to the location of the study for 2 days, received $120. The Institutional Review Boards of both Westat and National Cancer Institute approved this study.
A computer software program was developed to display portion-size images used for estimating amounts consumed. The application presented participants with different screens displaying pictures of various portions sizes of each food that had been served the previous day. Images showed the food on or in a plate, bowl, or other container with a knife and fork on either side that served as a point of reference. No other information (eg, cups, ounces) regarding portion size was provided on the images. Participants selected a picture that represented the portion size they judged to be closest to the amount they had eaten the day before. For each test, the computer program randomly assigned one of three orders in which differing screen layouts were presented; and, within a food, images appeared in a sequence of smallest to largest portions. Participants completed all judgments for all foods they consumed using one screen layout before moving to the next layout. All subjects ate and reported the portion size for all nine foods within the five food categories leading to 30 to 51 judgments per subject (depending on the study and group described below) for each relevant combination of the factors of food, type of image, and method of presentation (simultaneous vs sequential).
Study 1 had two objectives. The first was to determine the accuracy of portion size estimates using four different types of images: aerial photographs, photographs shot at a 45° angle, images of household measures such as cups and spoons, and images of food mounds used in National Health and Nutrition Examination Surveys (30
). The portion size ranges displayed were based on the 5th to the 95th percentiles of reported amounts from National Health and Nutrition Examination Survey 2003-2004. For example, for scrambled eggs, the 5th percentile is slightly less than ¼ c and the 95th percentile is approximately 1 c. Therefore, the range of photos shown was ¼ to 1 c in ¼-c increments for four images. provides example screen shots used in Study 1.
Study 1: Example screen shots of aerial, angled, mounds, and household measures images used to aid portion size estimation for scrambled eggs for participants completing a computer portion-size application.
The second objective was to evaluate two different screen presentation methods for each type of photograph: simultaneous—participants were presented a screen showing all portion sizes at once—and sequential—participants were presented with a screen with one photograph of an empty plate and selected buttons to display pictures depicting a sequential increase/decrease in portion sizes.
Types of images tested for each food category are shown in . Some photographs were not relevant for certain foods (such as images of mounds for bread) and were, therefore, not tested.
Study 1: Type of image tested for each food category in a computer portion-size application.
For Study 1, two groups of 10 subjects and one group of nine participants participated in each 2-day cycle. In each group, participants were offered different foods within each food category (shown in ) to allow for testing a variety of foods within a food category. Evaluation of image type and screen presentation method in Study 1 was conducted by displaying images combining simultaneous or sequential presentation with each of the following image types: aerial photographs, angled photographs, images of mounds, and images of household measures.
The purpose of Study 2 was to assess the effects of both the size and number of portion-size images on the accuracy of estimates. For size, participants were shown either large (1 7/8-in×2½ in) or small (1 5/16-in×1 7/8-in) images. For number, participants were shown either four or eight images. Similar to Study 1, the 5th to 95th percentiles of portion sizes from National Health and Nutrition Examination Survey 2003-2004 were used. The size of the increments varied among the various presentations. For example, for scrambled eggs, the increment between photos was ¼ c when four pictures were shown and ⅛ c when eight pictures were shown. provides example screen shots used in Study 2.
Study 2: Example screen shots of large, small, four, and eight images used to aid portion size estimation for corn chips for participants completing a computer portion-size application.
Twenty participants in two groups of 10 participated in Study 2. As in Study 1, subjects were randomly assigned to one of three orders of viewing the screens. Each of these three orders started with a different image size and number combination (eg, large size, four photographs). Based on findings of Study 1 (presented below), all portion sizes were shown simultaneously.
Most portion sizes of food displayed in the images were weighed to determine their gram weight. For some foods, such as honey, butter, and jam, for which there is little variability in volume weights, gram weights for images of the various portion sizes were assigned using standard weights from the Food and Nutrient Database for Dietary Studies (version 1.0, 2004, US Department of Agriculture Agricultural Research Service, Food Surveys Research Group, Beltsville, MD). In this way, each image in the database had an associated gram weight.
Accuracy of participants' estimates was computed as the absolute difference between the gram weight consumed and the gram weight assigned to the selected image. Therefore, lower values indicate greater accuracy.
In Study 1, estimates were missing for three foods because one participant did not take one food and two others took amounts too small to register (<2 g) on the scale. In Study 2, judgments were missing for one food. The univariate distributions of gram-weight differences were inspected to identify outliers, and no consistent patterns were found. Nine estimated amounts and four estimated amounts in Studies 1 and 2, respectively, were found to be more than 10 times the amount consumed. These estimates represented <1% of the possible total 2005 estimates across both studies. Since these values distorted the overall analyses, they were excluded. Thus, the total numbers of estimates used for analyses were 1,413 for Study 1 and 592 for Study 2.
Repeated measures analysis of variance was used to determine whether presentation factor was significantly associated with accuracy. An interaction term formed by each of the independent variables also was included in the model. These analyses were conducted for each food and each food category. An F
test was used to test for significant effects, and multiple comparisons were conducted using the Tukey-Kramer (32
) (Tukey JW. The Problem of Multiple Comparisons, 1953, unpublished manuscript) method. Because this was an exploratory study with a small sample size, the ability to detect meaningful differences was poor. The power to detect a 20% difference in mean accuracy for a single food was 9% to 86% with coefficients of variation ranging from 1.0 to 0.2, respectively.
Means of the rankings were computed from preference questionnaires. The means were used to determine the preferred image type for each food and for each food category.