In an open formula, ingredients, and possibly actual nutrient profiles, change depending on availability and market prices. In a fixed formula, combinations of ingredients and nutrient profiles do not change regardless of fluctuating market prices of the ingredients. Product formulations can be either fixed or open. This kind of information is useful when evaluating animals that are having an adverse reaction to a pet food storage container, possibly due to an allergy or intolerance to one or more ingredient sources such as beef, poultry, rice, corn, etc. The value of this list is limited to determining the sources of the proteins and carbohydrates for dogs or cats. No reference to quality or grade of an ingredient is allowed to be listed; therefore, it is difficult to evaluate a product solely on the basis of the ingredient list.