Reliability and validity of food portion size estimation from images using manual flexible digital virtual meshes.

OBJECTIVE: The eButton takes frontal images at 4s intervals throughout the day. A three-dimensional manually administered wire mesh procedure has been developed to quantify portion sizes from the two-dimensional images. The present paper reports a test of the inter-rater reliability and validity of use of the wire mesh procedure. DESIGN: Seventeen foods of diverse shapes and sizes served on plates, bowls and cups were selected to rigorously test the portion assessment procedure. A dietitian not involved in inter-rater reliability assessment used standard cups to independently measure the quantities of foods to generate the 'true' value for a total of seventy-five 'served' and seventy-five smaller 'left' images with diverse portion sizes. SETTING: The images appeared on the computer to which the digital wire meshes were applied. SUBJECTS: Two dietitians and three engineers independently estimated portion size of the larger ('served') and smaller ('left') images for the same foods. RESULTS: The engineers had higher reliability and validity than the dietitians. The dietitians had lower reliabilities and validities for the smaller more irregular images, but the engineers did not, suggesting training could overcome this limitation. The lower reliabilities and validities for foods served in bowls, compared with plates, suggest difficulties with the curved nature of the bowls. CONCLUSIONS: The wire mesh procedure is an important step forward in quantifying portion size, which has been subject to substantial self-report error. Improved training procedures are needed to overcome the identified problems.