2.4.1. Outcome Variables
SECONDARY
The primary outcome was a holistic measure of diet quality: the average NS points of the shopping basket, weighted by the number of servings. Because the NS points range from −15 to 40, to ease the interpretation of the results, we reversed and shifted the NS point such that it lies in between 0 and 55, with 0 being the least healthy score and 55 the healthiest. For instance, if a participant’s grocery basket contains beverage A (1 serving, NS point 13) and food B (3 servings, NS point 30), then we compute the weighted average of NS points by using the following formula: The secondary outcome measures of diet quality focused on (1) total and (2) weighted (by the number of servings) average calories, sugar, sodium, and saturated fat per serving of the grocery baskets. The per-serving measures were calculated in the same way as for the primary outcome variable. The reason we are interested in both total and per-serving changes is to examine whether the labels induce people to purchase fewer nutrients or energy per serving but more total nutrients or energy, which has been shown to occur for some labels [