Sonja Lyubomirsky: We let people define happiness for themselves. There's no happiness thermometer. No one else can tell you how happy you are. It's a subjective phenomenon. No one but you knows, or should tell you, how happy you truly are.
Jim Harter: We ask people to rate the quality of their overall life today on a 0–10 ladder of life developed by Hadley Cantril of Princeton, and what they think it will be in the next five years—to tap into their "reflecting" self. The good news is that most people have a more positive view of the future than the present...maybe this keeps us striving for something better. We use responses to questions to categorize people as "thriving," "struggling," or "suffering." We also ask people to recall their experiences from the previous day. This allows us to tap into the "experiencing" self or how much positive and negative emotions and experiences people have on a typical day. These are both important aspects of well-being...the evaluating self and the experiencing self.
Ruut Veenhoven: In my definition, happiness is how much one likes the life one lives. So if people say they are happy they are happy, unless they are lying.
Ed Diener: The key is that each person is making the evaluation of his or her life—not an expert's, philosopher's, or somebody else's. Thus, the person herself or himself is the expert: Is my life going well, according to the standards that I choose to use?