3- Chi tieu cOng - 4- Dau tic
Hirsh 6.4. S6 nhan - gia t6c:
Tac Ong cna sir thay ail tint ngnt len dttOng can 3
Bien dO cua cac Wen (long phu thuOc vao t6c dO dieu chinh dau to theo cau va phu thuOc vao t6c do san xuat theo cac thanh phan to chu cua cau. Khi dau to &rot dieu chinh nhanh chong theo su bien thien A cua cau thi bien dO dao dOng cua chu trinh (do thi) có the ducyc bung no: tac dOng cua he s6 tang t6c lam phat sinh nhiing bien Ong rat mph cua cau. Khi dau tu ducyc dieu chinh cham thi bien do dao Ong cua do thi bi thu hep (hinh 6.4 the hien dOng thai cua he só tang t6c trong mO hinh kinh to vi mO - Daleau, Malgrange, Muet[1981]). De cac gia tri ve thed han dieu chinh tieu diing va dau tu có the sir dung cluqc thi bien dO dao Ong dm chu trinh (do thi) diroc sinh ra tir mot cu s6c dung tiled diem dia cau se bi thu hep lai rat nhieu (hinh 6.4a). Thad han dieu chinh dau tu can phai rat ngan, do vay tac dOng cua he s6 tang t6c o ky ngdn han de sinh ra mot chu trinh dirot danh dau bai mot cu s6c thing thin diem la rat 16n (16n hcrn 3), hinh 6.4b.
Ngoai ra, khi dau tu den nhanh hon chu trinh san xuat va khi tac
clOng dm he s6 tang t6c la dong th6i (hinh 6.3), thi dac tinh tien nhanh
cua dau tu se bi giarn rat nhanh khi to tinh den thu han dieu chinh cua dau tu (hinh 6.4).
Theo al-1h chat tuyen tinh, he so tang t6c chi co the sinh ra chu trinh deu khi nhcing thay doi ngau nhien ma phan b6 theo du6ng cau duqc the hien du'Oi clang dac tinh chu trinh (he s6 tuong quan to dOng cua cac sai s6). Do dO, trong nhang nam 1950, nhang may do dao Ong kh6ng tuyen tinh da duoc phat tridn rat manh (Goodwin [1951]).
6.2.4. Hong thai cling 101 nhuan - tich 1u9 tic bin va he s6 gia t6c Ve ban chat cac mO hinh chu trinh he s6 tang t6c khac nhau deu Ve ban chat cac mO hinh chu trinh he s6 tang t6c khac nhau deu co chung mot coy sa lY thuyet. Tuy nhien trong tirng mO hinh van ton tai mot s6 rieng biet mang al-1h dac thii. VI du mO hinh Kalecki [1935], dau tu phu thuOc vao dO chenh lech gida lgi nhuan du kien va gia tri tu ban duqc tich luy:
Khi phan lgi nhuan trong san xudt khOng doi thi phuong trinh dau to khOng c6 sir khac biet so vgi mO hinh nang lkrc san xudt da trinh bay truck day va Ong thai cita mO hinh Kalecki IA met clang he s6 tang t6c3.
Khi dau to phu thugc vao lgi nhuan, deng thai cua viec tich luy to ban se bao g6m hai dac tit-1h co hoc sau:
• D6ng thai did niat dm clang he s6 tang t6c la ket qua tit skr anh hugng cac bien dgng ve tY le sir dung nang ltrc san xudt tren tY sudt led nhuan (de phan chia tien luong - lgi nhuan khong d6i, cac bien dgng ye tY sudt lgi nhuan se tac Ong den tY le sir dung nang luc san xudt);
• D6ng thdi thd hai la ket qua tir sir phan chia tien luvng - Icri ninic:in ma nguyen matt cUa clang nay la mO hinh Goodwin [1967] trong do thi tragng hang hod duoc can bang.
Dang Ong thai thin hai nay se dugc xem xet ky trong chu trinh Bang gia ca - san luong dugc trinh bay tiep sau day.
6.3. PHI CAN BANG TH! TRUONG HANG HOA VA HONG THAI CUA MO H1NH GIA CA - SAN WONG MO H1NH GIA CA - SAN WONG
Nhu trong cac mO hinh phi can bang, cac chu trinh clang gia ca - san lugng chU yeu tac deng len sir can bang cUa thi truong trong n6n kink to dang hien (c6 nghia la sir can bang nay khOng duoc thkrc hien bgi met nha cung cap ma nha cung cap nay se dua ra thong bao mot day cac loai gia lien tiep nhau cho den khi cung duoc (lieu chinh theo cau).
Khi can bang 6n Binh, qua trinh nay clan den phuong thdc to chit theo huOng can bang thi trugng. Chang ta bat dau bang viec gi6i thieu mO hinh n6i tieng nhat: mO hinh mang hay mO hinh Cobweb truec khi de cap den nguyen ly chung nhfing clOng thai cUa mO hinh gia ca - san Wong.
6.3.1. MO hinh Cobweb
M6 hinh nay mO to tinh hu6ng dua tren yeti t6 chinh IA thgi han san xudt, c6 nghia la viec quyet dinh san xudt cua ky sau dim tren tinh hinh san xudt dang quan sat 614 hien tai. Vi du ta co the ldy hai thi trugng: thi trugng nOng nghiep va thi trugng truyen th6ng Mit Icrn. Hinh 6.5 mO ta
cdc dao dOng gia ca va san lugng quay quanh diem can bang E, giao diem cua dugng cung va dugng cau. Gia thiet rang gid ban dau P„ cao hon so
vii gid can bang. Dtrong cung hang hod 6 thi kY 1 qua cao Q,, va gia P1 dieu chinh diem can bang mai dm dugng cung va dung cau i th6i ky 1 la yen hon so vii gid can bang. Nha vay, gia P1 nay se thap hon gid can bang (duoc xdc dinh theo dugng cau). Gia P, 614 sau se tao ra cung hang hod rat yeu Q2, nhu vay gid can bang giira clueing cung va cau P2 se rat cao. Hinh 6.5 va mO hinh dudi day se khao sat ye chu trinh hOi to theo huong can bang neu do d6c cua diking cung thSp hon d6 d6c cua dugng cau va trong truing hgp ngugc lai.
Chu trinh clang gid ca - san luyng ce, hai dac tinh:
• Gid ca va san luting 1a hai dai luting tY le nghich: san luting cang lan thi gia cang thgp va ngugc lai, sari luting cang thilp thi gia cang cao;
• MOt ky cua chu trinh along throng vdi hai kY san xuat (th6i han giira viec quan sat gia va cung hang hod).
MO hinh cua chu trinh cho phep neu dtra vao d6 thi to c6 the du kien mgt cach don gian gia ca phat sinh tir cdc hoat dOng san xuat i ky t dua tren gia quan sat 6 kY t-1. Nhang du kien phU hop vii thirc tin hon (Nerlove [1958] se dan den mot Ong thdi 6n dinh hon neu viec dieu chinh nhang du kien la du cham, va cling se clan den mot sir hOi to don dieu ye diem gid can bang.
D(Ing HIM cua nni hinh clang gia ca - san lining
Ta coi duong cung va clueing cau tren thi trutrng kin luvt thoa man diet' kien sau:
< 0 va Es>0
(Ed va s. ldn luot la dO d6c cua duong cung va dugng cau).
Bo qua yen to ve dg tre, phtrong trinh dung cung va cau &roc via thanh:
P, = P S (Q,)= 1 (Q, - a)Cung (6.18)
ss
a) b) QI Qz P2 PO Q Q2 Qd P, PO
Htnh 6.5. Chu trinh gia ca - san lacing chu trinh mang nhO