Animal welfare is the well-being of animals. The standards of "good" animal welfare vary considerably between different contexts. These standards are under constant review and are debated, created and revised by animal welfare groups, legislators and academics worldwide. Research on aspects of animal welfare has also focused on the body, using physiological measures, such as endorphins, plasma cortisol, and heart rate to examine how the animal is coping with its environment. It is an offence under the Prevention of Cruelty to Animals Ordinance Cap.169 to treat animals cruelly and cause unnecessary pain or suffering to them. Under the Ordinance, an animal is defined as any mammal, bird, reptile, amphibian, fish or any other vertebrate or invertebrate whether wild or tame. It is useful in ensuring that an ensuring an animal is not treated cruelly or caused unnecessary pain or suffering. The various components which are used to assess animal welfare are the use of the five freedoms, the assessment of welfare inputs and welfare outputs, inputs being the factors that affect the animal’s welfare and outputs being the actual impact of these factors on the animal’s welfare and the quantification of welfare problems found or measured using severity, duration and number of animals affected. Animal welfare is often confused with animal rights. Animal welfare denotes the desire to prevent unnecessary animal suffering and wants to ensure a good quality of life and humane death. Animal rights denote the philosophical belief that animals should have rights, including the right to live their lives free of human intervention. Animal rightists are philosophically opposed to the use of animals by humans.