Kxsiii Tit, VAftiuLvI.Y MQT NGHIEN CtfU TH^C NGHigM VE NHU CAU THjT, CA d KHU V^C THANH THj VA NONG THON Vl|T NAM Pham Thanh Thai * Ngay nhan: / / ^ © M a s d : 1.TKPT Jit khda: He soco dan MarshalTian, Hickslan, he thdng ham cau, hoi quy kiem duyet, mo hinh LA/AIDS, phan tich nhu cau thit, ca, thi, ndng thdn, ViPt Nam /p viet bao cao ket qua nghien cdu nhu cau tieu ddng cac san pham thit va ca cho khu vuc (/ > thi va ndng thdn Viet Nam bang md hinh AIDS (Almost Ideal Demand System) Nghien cdu sd dung dO li$u thd cap loai dO li$u cheo duac thu thap td cupc dieu tra ve mdc song cua hP gia dinh d VIPt Nam (VHLSS 2008) bdi Tong cue Thong ke Viet Nam nam 2008 Phuang phap hoi quy kiem duyet (cencored regression) cho he thP'ng cac phupng trinh dUpc sd dung He thd'ng ham cau hai buPc dupc ddc luang Trong giai doan thd nhat ty le Mill nghich dao (Inverse Mill Ratio - IMR) ddPc udc lupng bang each sd dung md hinh hoi quy probit Ben giai doan thd hai bien IMR tinh d giai doan mpt dupc them vao mo hinh AIDS de udc lupng dp co dan cua cau cho cac mat hang thit va ca Miic tieu cua nghien cdu nham uPc lapng cac loai he soco dan cua cau khac cho cac mat hang thit va ca d khu vuc thi va ndng thon Viet Nam Ket qua nghien cdu cdng cd the cung cap mdt bang chdng thdc tien cho cac nha lam chinh sach de thiet ke chinh sach ve thuc pham d Viet Nam Cac tham so cua mdhinh udc lapng dapc sd diing de tinh cac he soco dan cua cau theo gia Hicksian va MarshalTian va he so co dan cua cau theo chi tieu Ket qua cho thay he sP'co dan theo chi tieu (thu nhap) cua thit Ipn va thit ga khu vuc ndng thdn nhd han so vdi khu vUc thi nhdng he soco dan theo chi tieu (thu nhap) cua thit dd va ca khu vdc thi lai cao han so vdi khu vdc nong thdn DPI vPi ca hai khu vdc, thjt lan va thit ga la hang hoa thdng thudng, thit bd va ca lai la hang hda xa xl Gidi thi^u Phan tich nhu eau tieu dung la mdt nhQng chij de quen thudc nhat kinh te hpc Qng dung Cac nghien cQu trUdc day thudng sQ dung md hinh phuong trinh don de udc lupng nhu cau hang hoa ciJa ngudi tieu dung Hdn nOa eac dac trUng md hinh phuong trinh dPn dupc de cap ban dau chu yeu la de udc lupng dd co dan va danh mdt it su chu y den ly thuyet tieu dung (Deaton va Muellbauer 1980b) Trong nhQng thap nien gan day, phan tich nhu cau tieu dung da cd nhQng each tiep can mdi theo hudng md rdng mang tinh he thd'ng Cach tiep can dam bao he thd'ng cau la phu hop vdi ly thuyet tieu dung, vl eac ham cau dupc xay dung dua tren ly thuyet ve su lua chpn cua ngUdi tieu dung Mdt nhQng md hinh duoc Ung dung rpng rai dd'i vdi cae nha nghien ciXi !a md hinh AIDS (Almost ideal Demand System) cua Deaton va Muellbauer (1980a) 86 49/2012 Nghien cUu ve cau true cau thyc pham da dupc tien hanh ra't bien d tren the gidi, dac biet la d cac quoc gia phat trie'n nhung tai Viet Nam thi cd rat it cac nghien cQu ve van de Mdt sd nghien cdu gan day sd dung dQ lieu cheo phan tich nhu eau thUc pham d Viet Nam bao gdm Linh Vu Hoang (2009), Canh Quang Le (2008), Haughton & ctg (2004), Thang va Popkin (2004), Benjamin va Brandt (2002) Minot va Gdetti (2000) Figule (2003) analyses vegetable consumption behaviour in Vietnam, while FAQ (1999) considers the relationship between food consumption and nutntionNghien cUu sd dyng md hinh AIDS de ude lupng he thd'ng ham cau thjt va ca cho khu vuc thi va ndng thdn Viet Nam, thong qua * ThS Trudng Oai hoc Nha Trang ^iiiitfng n/lai is K I M I I E VA ^AX J.Y dd ude luong eac he so co dan cua cau theo gia, theo thu nhap va so sanh he s d co dan gida khu vuc thi vdi ndng thdn Ket qua nham cung cap cac ket qua mang tinh thuc tien de giup cae nha hoach dmh chinh sach cd eo sd khoa hoe hon vi#c thiet ke cac chinh sach lien quan Cd sd ly thuyet va phUdng phap nghien ctiu Md hinh dupc ap dung nghien cQu la md hinh AIDS (Almost Ideal Demand System) cua Deaton va Muellbauer (1980a) Md hinh AIDS la mdt nhQng dang ham bien nhat dUpe sd dung de Udc lupng he thdng ham cau Mdi phUPng trinh ham cau he ham cau AIDS cd the dupc viet nhu sau: = a,-hZ/.jlnpj- Mn -U, (1) Trong dd: W| la ty phan chi tieu cho mat hang i, Pj la gia eua mat hang j , x la tong chi tieu cua cac mat hang co he thdng, y la he so cua bien gia, p la he sd cua bien chi tieu (thu nhap) va P la chi sd gia dupc dmh nghTa d phUdng trinh (2) \nP = ao + S a , In;?, -i SZr,j ^np, [npj (2) De dam bao tinh ben vQng ve mat iy thuyet cho ham cau, Deaton va Muellbauer (1980a, 1980b) da nghien cCiu va dua cae rang buoc cho md hinh AIDS, cu the la: Tinh cpng don: Z«,-^Zr,-0'ZA = o t3) Tinh ddi xdng: /i, ^ Vj, (4) Tinh ddng nhat: Y./,,=^ (5) He sd CO dan theo ehi tieu (thu nhap) dupc tinh theo cdng thUc sau: A, = \ + p,l\\, (6) He so CO dan^ theo gia rieng dUpc bieu dieh theo cdng thQc sau: E„ = -\+yJw,-/^_ De tinh he so co dan mo hinh ;* = /:„ + « ,.-), (7) Va he sd co dan theo gia cheo dUpc xac dinh theo cdng thdc sau: £",j=0',j-w,>fii)/w, (8) Ham cau AIDS la tuyen tinh ngoai trd dang ham translog cua chi s d gia InP Trong hau het cac nghien cdu Qng dyng, dd khac phuc van de phi tuyen va lam giam cac anh hudng cua hien tuong da cdng tuyen md hinh cac nha nghien cUu thudng sU dung ehi so gia Stone {\nP = Y,,w, li) nham tao he thdng tuyen tinh (Cach lam cung dupe de nghi bdi Deaton va Muellbauer, 1980a, 1980b) Md hinh AIDS vdi chi sd gia In P - X , w, In p, dUdc goi la md hinh AIDS xap xi tuyen tinh va dUpc ky hi^u la LA/AIDS (Linear Approximate Almost Ideal Demand System) Tuy nhien, ehi so gia Stone khdng thda man tinh chat CO ban cua sd chi sd, bdi vl ehi sd gia Stone la khac cd su thay ddi cac ddn vi ludng eua gia ea, Mdt nhQng giai phap de hieu chinh don vi cua sai s d d o ladng la chuIn hda gia ea bang each chia gia ca eho gia tri trung binh mau cua nd (Chern & ctg, 2003) Moschini (1995) de nghi sd dung chi sd gia Laspeyres vdi muc dieh khac phuc van de sai sd ludng Chi sd gia Laspeyres cd the dUpc bie'u dieh nhU sau: ln(P^) = Xw,ln(p,) (9) Trong dd: w , ia phan chi tieu trung binh ciia hang hda i Nhu vay nghien cQu chi so gia Laspeyres duoc sCf dung de thay the cho chi so gia Stone udc lUpng md hinh LA/AIDS Phuong phap Udc lUOng dung nghien la SUR (Seemingly Unrelated Regression) de dat dupc tinh hidu qua va bao ham kha nang cd tUdng quan dong thdi glQa cac sai sd ng^u nhien he thd'ng cae phUdng trinh cau, vi dieu kien cdng ddn tao mot ma tran hiep phUdng sai khac thudng Do vay, mdt phuong trinh nao dd phai dupc loai bd tu he thdng ham cau trude udc lapng (trong nghien cCiu phupng trinh ham cau cho mat hang ca dupc loai bd) Cae rang budc cdng ddn td (3) dupc sd dung de tim cac tham sd ham cau Hicksian chtjng ta sd dung phuong trinh Slutsky nhU sau (Eij" CO dan Hicksian, Ely co dan Marshaltian) KHOA HOC 16 Tliiitfng n/lai' So 49/2012 HUSH 1^' T A qjuhi phudng trinh ham cau cho mat hang ca bi loai bd Cac he s d co dan cua cau theo chi lieu (thu nhap), theo gia neng va theo gia cheo dUpc tinh theo cac cdng thQc (6), (7) va (8) Tnang nghien cQu nay, cac hd s d eo dan deu dupc tinh tai diem trung binh mail DOIi$u nghien cOu va kyihu$t udc lapng mo hinh DQ lieu cho nghien cUu la dO iieu thU cap loai dO lldu cheo dupc thu thap tQ cudc dieu tra ve mQc sdng cua hd gia dinh d Viet Nam (VHLSS2008) Tac gia sd dung ma'u "thu nhap va chi tidu" gdm 9.189 hd gia dinh cupc khao sat de phan tich vi d day cd day du dQ lldu ve tat ca c^c bien can thiet cho yeu cau cua nghien cQu DQliau dUpc thu thap nam mdt l i n Tdng cue Thdng ke thue hien Dd Udc lupng hS sd co dan cua cSu theo thu nhap (ehi tieu), dQ lidu vi md d mQc dd hd gia dinh la thich hpp hPn vl cd the tranh dupc van de cdng gijp cac hang hda udc lUpng Tuy nhien, viec sd dung dO li&u hd gia dinh cho muc dich ude lapng nhieu loai m§t hang rieng le cd the dan tdi mpt van de khd khan udc lupng mdt so hd gia dinh cd mdc tieu dung bang (khdng mua) Van de bat ngudn td thuc te rang mdt sd hd khdng tieu dQng mdt sd loai m5t hang nhat dmh suot thdi gian khao sat Trong mat hang thit va ca cua nghien cQu nay, van de tieu dung bSng dSc bldt nghiem trpng doi vdi hai mat hang la thjt bd va thit ga vdi ty le hd gia dinh khdng mua thit bd va thit ga lan iQpt Id 40,94% va 53,38% (tinh toan cua tac gia tQ bd dQ lieu VHLSS2008) Cac nghien cQu trudc chi rang, neu chung ta chi sd dung dO li#u tieu dung dapc quan sat cd gia tri dUPng de udc lapng hanh vi tieu dung bang phaong phap binh phUdng be nhat thdng thudng (OLS) se cho ket qua la cac udc lupng OLS bi Chech va khdng nhat quan va'n de thien lech chon mau vl vay lam giam kha nang du bao cua md hinh Bien phu thudc la phan ngan sach dOng cho chi tieu cac loai m3t hang dupc xac dmh eu the (vi dy, mSt hang i nao dd), bang neu hd gia dinh h khdng mua mat hang i va dUdng (+) neu cd mua Phan chi tieu bang se bi kiem duyet (censored) bdi mot bien tiem an khdng quan sat dUdc Trong nghien cQu nay, tae gia ap dung md hinh hai budc eua Heckman dd hieu chinh van de tieu dung bang Nghien cdu gia thiet rang So 49/2012 IX tieu dung bang la van de chpn ma'u, vay quy trinh hai bUdc cua Heckman se la md hinh phd hpp (Chern & ctg, 2003) Heckman (1979) da de xuat mdt phuong phap nham giai quyet van de tieu dung bang Ong xay dung md hinh ve quyet dmh tieu dung va sd dyng md hinh hdi quy Probit de xae dinh xac suat mua sam mpt san pham nhat dmh Cac udc lUPng tU mdhinh hdi quy Probit dupc dung detinh ty le IMR (Inverse Mills Ratio), la ty le cua cac gia tri Udc lupng dUPe eua ham mat dp chuan hda vdi cac gia tn udc lupng dupc cua ham phan phd'i tich luy chuan hda Ty le IMR dupc tinh cho moi quan sat bd dQ lieu, ve mat toan hpc thu tuc Heckman cd the dUpc md ta nhU sau: (10) (11) IMR, _^{xP) ^{xP) (12) Trong dd phUPng trinh (10) ludng bien tiem a'n p* bien nhi nguyen p cd gia tri bang ne'u p" > va bang neu p' < 0, x la tap hpp cac bien ddc lap, p la vector tham so thich hop Trong phUPng trinh (11), q* luu giQ thdng tin ve cac quan sat ed gia tri cua bien nhi nguyen bang Sau tinh ty le IMR, phuong trinh Udc lupng cudi cung dUPc phat trien de' bo sung them ty le IMR nham loai bd tinh thien lech chpn mau phuong trinh eau va dupc md ta theo ham sau day: w = f{xp)+X\USL (13) Trong dd,_/(x|3) la phUdng trinh de Ude lUpng va IMR duoc xem la bien cdng eu Trong Udc lUpng cudi cung, chi cac quan sat cd gia tn khdng gidi han mdi dupc sd dung Ty le IMR trd mpt bien ket ndi quyet dmh tham gia (cd tieu dung hay khdng) vdi phUdng trinh ma nd dai dien cho iupng cau Theo Heckman (1979), thien lech chpn mau xay neu tham so X cd y nghTa ve mat thong ke Helen va Wesseils (1990) da khai quat quy trinh hai budc cua Heckman de ket hpp ty le IMR cho cae quan sat ed gia tri bang bien phu thudc, tu dd sQ dung tat ca cac quan sat budc thQ hai Ty le IMR dUpc tinh cho moi hd gia dinh (h) va hang hda i, sd dung phaong phap adc lupng thich hpp cue dai (maximum likelihood) td TiiiAmg n/lai i? HJGVH TE VA ^AX J7£ tinh ddng nhat va tinh doi xUng dUdc ap dat de dam bao tinh ben vQng ve mat ly thuyet cau Cac h# so udc lupng cua mat hang ca thu dUPe bSng viec sd dung rang budc cdng don Ket qua cho (14) thay cac he sd hoi quy cua bien IMR deu cd y nghTa thong ke tat ca cae phUdng trinh ham cau thit va ca Dieu ed nghTa la neu ehung ta (15) bd qua van de tieu dung bang BangJ.; Cac tham so Udc lUdng cua md hinh LA/AIDS cho khu vdc ndng thdn md hinh hdi quy Probit va vay, bang ty le cua ham mat dd chuan tdc ( ^ tren ham phan phdi tich luy chuan tac (O): IMR,, va IMR,,, '0{xl3)