2.  A probability derived from an
individual's personal judgment
about whether a specific outcome
is likely to occur. Subjective
probabilities contain no formal
calculations and only reflect the
subject's opinions and past
experience.

3.  Di Finetti Game:
 Say, your friend tells to you, coming out of exam
hall, that he is getting a 100. How much is that
certain now? To measure this, all one have to do
is to ask few questions. Tell him that, in a bag,
there are hundred balls. Out of which 98 are
white and 2 are black. Given a chance, like, a
thousand rupees bet, for either drawing a white
ball from the bag or wait till exam results and on
getting the score as 100. Assume that the answer
of your friend as “draw the balls”. Now, increase
the black ball count to 20 (80 White + 20 Black).
And ask him to pick between drawing a ball from
the bag or wait till the exam results.

4.  If he chooses to wait for the exam result for
the thousand rupee bet, increase the white
ball count to 90 (90 White + 10 Black) and
repeat the same test. If he wants to draw,
then increase the black count to 15 (85 White
+ 15 Black) and repeat the experiment. End
this game with a sufficient interval. Say,
between 85 and 88. This means, that the
subjective probability of your friend getting a
centum is any where between 85 ~ 88.

5.  Now, let us analyse the rationale behind the game.
For the first time, when given a chance between 98:2
and “waiting till results”, your friend goes for the
draw.
 This means that he thinks drawing the white ball is
easier than getting a 100 in exam. So, the subjective
probability of his 100 is lesser than 98% (or .98). This
is the upper limit of his chances of getting 100.
 When you change the balls in the bag to 80:20 and
ask him for a pick, he chooses to wait for the result.
Meaning that, he thinks the probability of getting 100
is more than choosing the ball, which was 80% (or .8).
This serves the lower limit.
 For the third time, when you repeat the same
experiment, you are trying to find the finer details.