Giả Thiết Của OLS
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Transcript of Giả Thiết Của OLS
Gi thit ca OLSBn chtNguyn nhnHu quPht hinKhc phc
Y = beta1 + beta2*X + U (1)
E(U)0- SSNN (U) i din cho tt c cc yu t ngoi X c nh hng n Y- GT ca PP OLS l E(U) = 0, tc l ng vi gi tr X=Xi, c nhiu gi tr U khc nhau nhng trung bnh bng 0 (tc l v s cc yu t trong U khng nh hng mt cch h thng ln Y)- Khi E(U) 0 tc l gi thit ca PP OLS b vi phm2 nguyn nhn c bn- M hnh thiu bin quan trng (c bin Z no c tc ng n bin Y v c tng quan vi X)
- Dng hm ca MH sai (c th c bc cao hn ca X trong MH hoc mt dng hm khc phn nh MQH gia y v X)Nu E(U)0 th cc c lng thu c bng PP OLS s l cc c lng chch do cc suy din thng k s khng cn ng tin cy (cc thng k T khng phn phi Student)C nhiu kim nh pht hin ra MH thiu bin quan trng v dng hm sai nhng Kim nh Ramsey l mt kim nh ph bin v hiu qu- Cp gi thuytH0: M hnh c ch nh ng (Dng hm ng/MH khng thiu bin)H1: MH c ch nh sai (dng hm sai/thiu bin)- Cc bc kim nh+ Hi quy MH (1) thu c Y^+ Hi quy MHY=beta1+beta2*X+beta3*(Y^)^2+ beta4*(Y^)^3+v+ Kim nh cp gi thuytH0: beta3=beta4=0 (M hnh c dng hm ng v khng thiu bin)H1: c t nht 1 hs khc 0 (MH c dng hm sai, thiu bin)+ S dng kim nh F-Statistic hoc gi tr P-valueTy theo nguyn nhn m c cch khc phc ph hp- Nu thiu bin c sn s liu th b sung thm vo MH- Nu dng hm sai th i dng hm ph hp (cn c vo l thuyt kinh t, vo kinh nghim,.)- Nu thiu bin m khng c s liu th s dng bin i din (Proxy variable) thay th (cn c vo thc th, kinh nghim, v d: sd bin lng thay th cho bin thu nhp khng c s liu)
Y = beta1 + beta2*X + beta3*Z+ U (2)
Var(U/Xi) Var(U/Xj)- SSNN (U) i din cho tt c cc yu t ngoi X, Z c nh hng n Y- GT ca PP OLS l Var(U) = sigma^2 (l mt hng s), tc l Phng sai l mt hng s vi cc gi tr khc nhau ca Xi - Khi Var(U/Xi) Var(U/Xj) tc l gi thit ca PP OLS b vi phm- Do bn cht ca s liu (s liu cho thng c hin tng PSSS thay i)- Do m hnh thiu bin quan trng hoc dng hm sai (kim nh Ramsey pht hin ra vn ny)- Cc c lng OLS vn l cc c lng khng chch nhng khng phi l cc c lng tt nht (Phng sai ca cc c lng khng phi l nh nht). Do KTC v kim nh gi thuyt v cc hs hi quy khng cn gi tr sd (khng cn hiu lc)
pht hin s dng gi tr phn d. C nhiu kim nh nhng kim nh white l kim nh ph bin v hiu qu- Cp gi thuytH0: MH c PSSS ng uH1: MH c PSSS thay i- Cc bc kim nh+ HQ (1) thu c phn d (E)+ HQ 1 trong 2 m hnh sau:MH1: E^2 = = beta1 + beta2*X + beta3*Z + beta4X^2+beta5Z^2+beta6X*Z+ UMH2: E^2 = = beta1 + beta2*X + beta3*Z + beta4X^2+beta5Z^2+ V+ Kim nhH0:beta2=beta6=0H1: c t nht 1 hs khc 0+ Sd kim nh F-Statistic hoc P-Value (nu c)
- Ty thuc nhn nh v vic PSSS thay i ph thuc vo cc bin c l nh th no m ta c cch khc phc tng ng- Gi s Var(U)=a*X^2, khi khc phc bng vic chia c 2 v ca MH (1) cho X ta c MH mi(Y/X) = beta1/X + beta2*X/X + beta3*Z/X+ U/X (2)Khi MH (2) c Var(U/X) = a (l mt hng s, tc l PSSS ng u)- Nh vy, thay v L MH(2) l MH c PSSS thay i (khng tt) tm cc c lng cho cc beta, ta L MH (2) l MH c PSSS ng u (tt hn)(Phng php ny gi l PP sai phn tng qut)
Y = beta1 + beta2*X + U (3)
U khng phn phi chun
- SSNN phn phi chun l gi thit v phn phi ca U lm c s cho gi thit cc beta^ phn phi chun v do c thng k (T, F phn phi student v Fisher)- Nu U phn phi khng chun th gi thit ca OLS b vi phm- C th do dng hm sai, m hnh thiu bin quan trng (kim nh Ramsey)- Do bn cht ca s liu sn c SSNN khng phn phi chunDo SSNN khng phn phi chun nn cc beta^ cng khng phn phi chun nn cc suy din thng k khng ng tin cy (khi kch thc mu l khng ln)S dng thng tin t phn d v tin hnh kim nh Jacque Bare- Cp gi thuytH0: SSNN phn phi chunH1: SSNN khng phn phi chun- Cc bc + HQ MH(3) thu c phn d (E)+ Sd phn d tnh cc hs bt i xng (Skewness) v hs nhn (Kurtosis)+ Tnh thng kJB=n(S^2/6 + (K-3)^2/24)+ Vi mc ngha anpha cho trc tm c gi tr thng k Khi bnh phng vi 2 bc t do+ Kt lun v vic bc b hay chp chn H0Thng th nu khc phc c hin tng dng hm sai, Mh thiu bin th SSNN cng phn phi chun
Y = beta1 + beta2*X + beta3*Z+ U (4)
X = a + bZ+v- CT l hin tng xy ra vi MH hi quy bi (c hn 1 bin c lp) khi cc bin c lp c quan h tuyn tnh vi nhau- CT l hin tng xy ra ph bin cc MH do ngi ta thng quan nim CT mc no- Do bn cht ca cc bin s vn c quan h (s ngi v thu nhp;)- Mu khng mang tn i din MH c CT cao lm cho cc phng sai ca c hs beta^ ln do cc KTC v K v cc hs beta khng ng tin cy- S dng hs tng quan cp gia cc bin c lp- S dng MH hi quy ph (p dng khi MH c nhiu bin c lp) v xem xt hs xc nh ca MH hi quy ph ny- CH :Trong MH m kim nh F-statistic cho thy MH Hi quy ph hp nhng cc kim nh T cho thy cc hs beta^ c lng c l khng c ngha thng k. Y L DU HIU CA MH C HIN TNG CT TRM TRNG- B bt bin nu c th (y l phng php cc oan)- Tng kch thc mu (thu thp thm quan st)- i dng hm
Y = beta1 + beta2*X + U (5) Cov(U, U(-P))0- TTQ l hin tng xy ra khi HQ MH vi S liu theo thi gian, trong gi tr ca SSNN ti cc thi k khc nhau c quan h tng quan vi nhau- Bc ca TTQ+ Nu U v U(-1) c quan h TQ vi nhau th gi l TTQ bc 1+ Nu U v U(-p) c quan h TQ vi nhau th gi l TTQ bc p
- Do bn cht ca s liu- Do dng hm sai, MH thiu binCc kt lun t bi ton L KTC v K khng ng tin cy- TTQ bc 1, s dng K DW+ iu kin p dng: (1) MH khng c tr ca bin ph thuc vi vai tr l bin c lp trong MH; (2) MH khng b mt quan st+ Cc bc(1) HQ MH (5) thu c phn d (E)(2) Sd gi tr phn d tnh gi tr DW (gi tr ny Eviews tnh t ng)(3) vi mc ngha cho trc, kch thc mu, v s hs ca MH ta xc nh c 1 lc v TTQ(4) So snh gi tr DW vi lc a ra kt lun- TTQ bc p, s dng K BG(1) HQ MH (5) thu c cc phn d (E)(2) HQ MH: E = beta1 + beta2*X +a1*E(-1)+a2*E(-2)++apE(-p)+V(3) K: H0: a1==ap = 0(4) kt lun (sd KDD F-Statistic hoc P-Value)Phng php sai phn tng qut+ Xc nh c lng hs TTQ t thng k DW+ Phng php L hs TTQ theo nhiu bc(GT trang 309 317)
3 CU HI THNG GP I VI PHN KIM NH V LA CHN M HNH Gi tr F-Statistic/Khi bnh phng tng ng vi cc K (Ramsey; JB; White; BG) c xc nh ntn? Cn nu cc bc theo phn pht hin Gi tr ny cho sn trong bng kt qu Da vo gi tr F-Statistic/Khi bnh phng tng ng vi cc K (Ramsey; JB; White; BG) kt lun g v MH Vit cp gi thuyt tng ng Kt lun v hin tng Cc k qu v vic tm KTC, K gi thuyt da trn cc hs hi quy c lng c t MH c ng tin cy hay khng? Ch da vo nhng thng tin trong bng kim nh cc khuyt tt ca MH (khng suy din t nhng thng tin khng c trong bng) Ch cn c t nht 1 gi thit OLS khng c tha mn th a ra kt lun theo hu qu li ca gi thit b vi phm nh phn trn
M HNH VI S LIU THEO THI GIAN V D BO MH c th c bin tr ca bin ph thuc v bin c lp vi vai tr l bin c lp ca m hnh MH c th c bin xu th (trend) thng k hiu l bin T, bin ny nhn cc gi tr theo th t t 1 ng vi quan st u tin v n vi quan st th n D bo Da vo gi tr Y^ (y l gi tr tnh c khi bit gi tr ca cc bin c lp bng cch thay vo hm SRF). Y^ l gi tr c lng im ca E(Y/X) Da vo KTC ca E(Y/X) v Y theo cng thc ti trang 158, 159 gio trnh KTL