The term 'dietary survey' refers to a group of methods that are used to collect food consumption data to study the diets of individuals or groups. Common methods in dietary surveys are food frequency questionnaires, food diaries, and 24-hour recalls. These are often combined with the weighing and analysis of food to determine its nutritional properties. Dietary surveys are sometimes combined with other methods (e.g. the measurement of BMI or blood values) to study the relations between certain dietary patterns and health outcomes. National dietary surveys, conducted and commissioned by national and international health authorities, study national dietary patterns and are often an important input for the development of food policy.