Exchangeable random variables
Prediction without covariates
Lemma 1
Suppose Z1,⋯Zn are exchangeable random variables. For any α∈{0,1}, P[Zn≤Q^n(α)]≥α
Moreover, if Z1,⋯Zn are a.s distinct, P[Zn≤Q^n+1(α)]≤α+n1
key proof: using the uniform distribution or these order statistics?
Lemma 2
Suppose Z1,⋯Zn+1 are exchangeable random variables. For any α∈{0,1}, define αn as αn=(1+n1)α. Then, P[Zn+1≤Q^n+1(αn)]≥α
Moreover, if Z1,⋯Zn+1 are a.s distinct, P[Zn+1≤Q^n+1(αn)]≤α+n1.
Key of the proof:
use lemma 1 and change n points to (n+1) points of these order statistics
and you cannot jump for two points since you only added one point into the empirical distribution
One-sided prediction interval without covariates