Algebra U = ∑ a(t) Y(t) E{U} = c Y ∑ a(t) cov{U,V} = ∑ a(s) b(t) c YY (s-t)
description
Transcript of Algebra U = ∑ a(t) Y(t) E{U} = c Y ∑ a(t) cov{U,V} = ∑ a(s) b(t) c YY (s-t)
Algebra
U = ∑ a(t) Y(t) E{U} = c Y ∑ a(t)
cov{U,V} = ∑ a(s) b(t) c YY(s-t)
U is gaussian if {Y(t)} gaussian
Some useful stochastic models
Purely random / white noise (i.i.d.)
(often mean assumed 0)
cYY(u) = cov(Y(t+u),Y(t)} = σY2 if u = 0
= 0 if u ≠ 0
ρYY(u) = 1, u=0
= 0, u ≠ 0
A building block
Random walk
Not stationary, but
∆Y(t) = Y(t) – Y(t-1) = Z(t)
Y(t) = Y(t-1) + Z(t), Y(0) = 0
Y(t) = ∑i=1t Z(i)
E{Y(t)} = t μZ
var{Y(t)} = t σZ2
Moving average, MA(q)
Y(t) = β(0)Z(t) + β(1)Z(t-1) +…+ β(q)Z(t-q)
If E{Z(t)} = 0, E{Y(t)} = 0
cYY(u) = 0, u > q
= σZ2 ∑ t=0
q-k β(t) β(t+u) u=0,1,…,q
= cYY(-u) stationary
MA(1). ρYY(u) = 1 u = 0
= β(1)/(1+ β(1) 2), k = ±1
= 0 otherwise
Backward shift operator remember translation operator TuY(t)=Y(t+u)
Linear process. )(MA
jtt
j XXB
0iitit ZX
Need convergence condition, e.g. |i | or |i |2 <
q
q
t
q
q
tt
BBB
ZBB
ZBX
qMA
...)(
)...(
)(
)(
10
10
BjY(t) = Y(t-j)
autoregressive process, AR(p)
first-order, AR(1) Markov
Linear process invertible
For convergence in probability/stationarity
1||
tt
tptptt
ZXB
ZXXX
)(
...11
ttt ZXX 1
...
)(
2
2
1
21
ttt
tttt
ZZZ
XZZX
(**)
a.c.f. of ar(1) from previous slide (**)
||
22||
)(
,...2/,1/,0 ),1/()(k
Z
k
k
kk
p.a.c.f. using normal or linear definitions
corr{Y(t),Y(t-m)|Y(t-1),...,Y(t-m+1)}
= 0 for m p when Y is AR(p)
Proof. via multiple regression
ρYY
In general case,
Useful for prediction
tptptt ZXXX ...11
tystationarifor 1||in 0(z) of roots need
)(
z
ZXB tt
Yule-Walker equations for AR(p).
Sometimes used for estimation
Correlate, with Xt-k , each side of
tptptt ZXXX ...11
0 ),(...)1()( 1 kpkkk p
ρYY
ARMA(p,q)
qtqtttptptt ZZZXXX ...... 11011
(B)Yt = (B)Zt
ARIMA(p,d,q).
0)ARIMA(0,1,
)1(
walkRandom
1
tt
ttt
ZX
XBXX
q)ARMA(p, stationary a is t
d X
Xt = Xt - Xt-1 2Xt = Xt - 2Xt-1 + Xt-2
arima.mle() fits by mle assuming Gaussian noise
Armax.
(B)Yt = β(B)Xt + (B)Zt
arima.mle(…,xreg,…)
State space.
st = Ft(st-1 , zt ) Yt = Ht(st , Zt )could include X
Next i.i.d. → mixing stationary process
Mixing has a variety of definitions
e.g. normal case, ∑ |cYY(u)| < ∞, e.g.Cryer and Chan (2008)
CLT mY = cY
T = Y-bar = ∑ t=1T Y(t)/T
Normal with
E{mY} = cY
var{mY} = ∑ s=1T ∑ t=1
T c YY(s-t)
≈ T ∑ u c YY(u) = T σYY if white noise
OLS.
Y(t) = α + βt + N(t)
b = β + ∑ (t - tbar)N(t) /∑ (t - tbar)2
= β + ∑ u(t) N(t)
E(b) = β
Var(b) = ∑ ∑ us ut cNN(s-t)
Cumulants.
cum(Y1,Y2, ...,Yk )
Extends mean, variance, covariance
cum(Y) = E{Y}
cum(Y,Y) = Var{Y}
cum(X,Y) = Cov(X,Y)
DRB (1975)
Proof of ordinary CLT.
ST = Y(1) + … + Y(T)
cumk(ST) = T κ k additivity and imdependence
cumk(ST/√T) = T–k/2 cumk(ST) = O( T T–k/2 ) → 0 for k > 2 as T → ∞
normal cumulants of order > 2 are 0
normal is determined by its moments
(ST - Tμ)/√ T tends in distribution to N(0,σ2)
Stationary series
cumulant functions.
cum{Y(t+u1 ), …,Y(t+u k-1 ),Y(t) } = ck(t+u 1 , … ,t+u k-1 ,t) = ck(u1 , .., uk-1)
k = 2, 3,, 4 ,…
cumulant mixing.
∑ u |ck(u1 , ..,uk-1)| < ∞ u = (u1 , .., uk-1)