Monday, October 19, 2015

probability - Quantile normal and lognormal


Let's assume we have a normal distribution XN(μ,σ2). In a normal distribution the quantile can be calculated as follows:


Φ1X(p)=μ+σ2erf1(2p1)


If we want to calculate the value in the future of a stock we map it as:


Y=exp(X)

Which means: log(Y)N(μ,σ2)



I would like to know that if the function of the quantile can be calculated based directly on:


Φ1Y(p)=exp(μσ/2+σ2erf1(2p1))


The part of the equation σ/2 is extracted from îto calculus, however, I cannot find anywhere the correctness of this equation (I deduced it). I think the function exp is monotonic, so, it should preserve the value for the quantiles, but I'm not certain. One of my certainties is that μ changed to μσ/2, I have no idea if that modifies in some way the calculation of Φ1Y(p), or if σ also changed.



Answer



Quantiles are preserved under monotonic transformations, hence the quantile for Y is simply the exponential of the quantile of X, no need for corrections whatsoever (see here for instance).


Put otherwise, let q denote the quantile α of X i.e. P(Xq)=α

then P(Xq)=P(exp(X)Yexp(q)Q)=α


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