Interesting work by scientists at Microsoft that shows that using mobile phone social contacts is strongly correlated to the perceived mood.
They have designed a mobile application that enables collection of immense amount of data of user interaction with the phone and parameters such the number and time of calls made, the SMS sent and other social interactions. These interactions are then collected and analyzed in the cloud. Each of the parameters are then accumulated and assigned a weight. Once that is done, the study tried to find correlation between the user self assessment and compare that to assessment by the program.
They found a good correlation among users and when the data was compared to the same person then the correlation was higher than 90%.
This illustrates that it is possible to measure even difficult parameters quantitatively, even those which do not have a numerical value.