"Model Calibration" article in Encyclopedia of Quantitative Finance states that
. . . a common approach for selecting a pricing measure Q is to choose, given a set of liquidly traded derivatives with (discounted) terminal payoffs (Hi)i∈I and market prices (Ci)i∈I, a pricing measure Q compatible with the observed market prices
where Q denotes
a probability measure on the set Ω of possible trajectories (St)t∈[0,T] of the underlying asset such that the asset price StNt discounted by the numeraire Nt is a martingale.
But we know that market prices (Ci)i∈I are generated by fallible human beings! Each of them has rather limited knowledge about "possible trajectories (St)t∈[0,T] of the underlying asset". Otherwise they wouldn't need the model we are trying to calibrate, would they?
So The Calibration Process receives some prices (Ci)i∈I, some arbitrarily choosen mathematical model (i.e. Heston) and produces as an output the calibrated model which supposedly able to give us predictions about the future (St)t∈[0,T]
Why do we believe that The Calibration Process is different from GIGO process?
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