Consider this: If you were to rate your life in terms of happiness, how much would that be? In fact, how would you actually come up with a system that rates it properly over your entire life?
If you were to try a 0-10 scale now, the rating would surely be different on a day you're feeling like shit, and vastly contrasting with one of those "best day of your life" days (face it, we actually have more than one "best day of your life" since best is only referring to "best yet").
But supposed if these kinks were worked out, and everyone could obtain a life rating for his/her own life, it will become another measurement of success. Currently, many people judge other people's lives using mostly wealth, job, and health. Satisfaction should be another factor, and while wealth, health, and job are more stable in the long term, happiness is not, and can swing back and forth with a wide variance. In fact, it's almost impossible to predict how good or bad you would feel with great accuracy over an event that has yet to take place.
So, you can't just measure current levels of happiness due to its fluctuating nature (which could even dependent on the time of the day). As you might think, averaging it out is the key. And that's where we suffer another problem. Our memories. For most of us, recent happenings are easier to recall than past, and our memories tend to get more sparse the further back in time we probe into our minds. We also have a tendency to forget certain events and remember others. This skews our perception of happiness.
You may argue that perception of happiness is everything, but then, I am intending this rating to give a kind of idea of what it will be like to live as another person, and also as a means of comparison between people. (Comparison has an importance. I'll come to that in a while)
So, to average it out, let's say, I'll give you a scale of 0-10, and since the time you can understand enough to start doing your own rating, every day, you'll rate your day's happiness on a 0-10 scale (10 being the happiest day in your life). A problem surfaces immediately. Remember what I said about "best days in your life"? Well, while we have a few (and possibly many) of them, none of them are equal. Right? It's always the "best day yet". But if we were to just average out these numbers, then, each best day would be equated to each other, and that's not right is it?
Since, as we grow older, we experience more, and are able to better judge current events, then we should make age a deciding factor as well. As simple and crude as it may be, since it's practically impossible to do the math for this, I shall assume that experience and age are linear, and correlated. So, instead of a fixed scale of 0-10, let's make it 0-(your age). I understand that the "best day of your life" when you are younger may trump many of the other better days in your life when you're older, I think this might not be common, since, with age, you usually gain independence, capability, power, and freedom, which tends to allow you to seek happiness, and very more likely find it, to a higher level as well.
So using this rating scale, you add them in fractional form. 6/10 for one day, 5/10 for the next makes 11/20 in total, and so on. The percentage will then be your "life happiness rating". But we shouldn't stop here, because there are many problems with this system.
One of them is that, let's say if there was an "absolute value" of happiness for each event that you experience, then for one who has experienced a really really high absolute value of happiness relatively early in life, against another who didn't, then the life rating for the one who did would be lower, simply because now his "100% standard" (best day of his life) would be raised far above the rest of his life.
People tend to normalise their own happiness over time too. Let's say you got into a really really great job, or school (basically position in life), and you're extremely elated to take it on. First day on the job/in school, you're over the moon, and your rating is very high. But as the weeks go by, the months, you tend to find it normal. The saying goes "you don't know what you've got till it's gone" expresses this effect in simple terms. We normalise things if we find that they're happening on a regular basis. This means that although two people may have the same "life rating" they could be on extremely different levels of "absolute happiness".
So how to solve these problems? Somewhat of a solution is to use more statistics. Get the variance or standard distribution, get the peak to peak difference, get the occurrence rate of a certain level, say, >80% or >90%. These will all indicate aspects that will give you an idea of a person's life. E.g. the occurrence rate of >80% or >90% will give an indication of how often that person experiences great joy. The variance will tell you how "spicy" their life is, and the peak to peak difference will tell you the range of happiness they have experienced.
The best part, however, is when there are numbers that differ from person to person, it will be compared. More so if the numbers matter. But what does this do? Well, if you do realise, no one wants to lose out on this, cos, well, having a low number would definitely suck. Thus people will try to rate their numbers higher, and then feel good about it. This... produces happiness in itself, muahaha! So it actually gets everybody to feel better about his/her life. Now isn't that advantageous?
Interesting idea, and just pure thought entertainment. Wish it could happen though.