McConnell, Brue va Flynn [4] cho riing: Cdu hang h6a la s6 lu(lng hang h6a hay djch v~ khac nhau ma nguai mua s&n sang mua va co kha nang mua tgi cac mire gia khac nhau trong m(Jt khoang thai gian xac djnh, gia djnh cac yiu 16 khac kh6ng thay d6i.
- S11 khac bift gifra ciu, lll'Qllg ciu, nhu ciu.
Cclu la s6 luQTig hang h6a hay dich V\l ma nguai mua s~n sang mua va c6 kha nang mua di~u d6 c6 nghia la S\f s~n sang d6 phai duc;rc h6 trc;r b&i m9t s6 ti~n ccln thiSt.
Theo Jeffrey M. Perloff [13], lu(lng cdu (Quantity Demanded - QD) la s6 lu(lng m(Jt loCJi hang h6a hay djch v~ ma nguai mua s&n sang mua va c6 kha nang mua tgi m(Jt mire gia xac dinh trong m(Jt khoang thai gian nhdt ajnh.
LuQTig cclu a m6i muc gia phai duc;rc tinh tren m6i dan vi thai gian C\l th~, m9t ngay, m9t tucln, m9t thang hay m9t nam. Vi d\l, t<;ti m9t muc gia ca phe C\l th~, nSu m9t nguai tieu th\l m9t tach ca phe m9t ngay thi luQTig cclu v~ ca phe cua nguai d6 c6 th~ la m9t tach ca phe m9t ngay ho~c 7 tach ca phe m9t tucln hay 365 tach ca phe m9t nam. NSu kh6ng xac dinh thai gian, kh6 xac dinh chinh xac luQTig cfiu hang h6a d6 16n hay nho.
Theo Philip Kotler va Gary Armstrong [ 1], nhu cdu la trgng thai con nguai cam thdy thiiu th6n m(Jt cai gi d6 va cdn ilu()'C thoa man. Chung bao g6m nhu cdu v(jt chdt ca ban, nhu cdu xii h(Ji va nhu cdu thi hi¢n ban than.
2.1.1.2. Biiu cfiu, iluimg cfiu va ham sJ cfiu
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Bieu cau
N6i dSn luQTig cclu ta kh6ng chi xac dinh m9t luQTig cclu d6i v6i hang h6a hay dich V\l t<;ti m9t muc gia C\l th~ ma ta con c6 th~ xac dinh duc;rc m9t di~m tren duang cclu. Bi~u cfru se li~t ke tfrt ca cac luQTig cfiu cua hang h6a hay dich V\l tuang ung v6i cac muc gia khac nhau.
V~y, bi~u cclu la m9t bang s6 trinh bay s6 luQTig hang h6a ho~c dich V\l ma nguai mua s~n sang mua tuang ung v6i cac muc gia khac nhau [ 4].
Vi dZ;J 1: Gia sir co s6 li~u v~ lugng cdu thit heo t~i Thanh ph6 H6 Chi Minh trong m(H nam tuong ung v6i cac mire gia khac nhau duqc thS hi~n qua biSu cdu sau:
Bang 2.1: Bilu cdu vJ thtt heo tQi Thanh ph6 H6 Chi Minh trong m(Jt nam
Gia (P) LU'Q'Dg ciu thjt heo (Qo)
(nghin d6ng/kg) (nghin kg)
100 5
90 10
80 15
70 20
60 25
Bi~u cdu a bang 2.1 cho thAy m6i quan h~ giua cac mire gia khac nhau cua thjt heo v6i s6 lugng thit heo ma nguai tieu dung co th~ va sin sang mua a m6i mire gia nay. N~u gia thit heo la 60 nghin d6ng/kg thi lugng cdu v~ thit heo la 25 nghin kg. N~u gia thit heo la 70 nghin d6ng/kg thi lugng cfiu v~ thit heo la 20 nghin kg. Tuong tv a cac mire gia la 80 nghin d6ng/kg, 90 nghin d6ng/kg hay 100 nghin d6ng/kg thi lugng cfiu tuong ung la 15 nghin kg, 10 nghin kg hay 5 nghin kg.
DU'ong ciu
Duong cdu la duong bilu diin tren d6 tht m6i quan hi giua gia va lu(Jng cdu [14].
Tu biSu cfiu trong bang 2.1, chung ta co thS ve m<)t duang cdu (D) tuong ung tren d6 thi, v6i lugng cfiu (Qo) cua thit heo duqc thS hi~n tren tq1c hoanh, gia (P) cua thit heo duqc thS hi~n tren tf\lc tung nhu hinh 2.1 du6i day:
p
100 A
90 ããã-ãããã-ãããããããã•ãããã-ãããããã-ãã-ãã
i l
l I
70 ããã-ãããããããããã•ãããããã .. ãã..._- - ,i---
80 60
0 5 10 15 20 25 Q
Hinh 2.1: D6 thi auang cdu
Tuong ung v6i tung muc gia khac nhau la cac luQ'Ilg cfiu khac nhau.
Duong thing di qua cac di~m A B C D E th~ hi~n cac luQ'Ilg cfiu tuong ung v6i cac muc gia dugc gQi la duong cfiu, ky hi~u la (D). Trong hinh 2.1, duong cfiu (D) di qua 5 di~m v6i gia va luQ'Ilg thit heo tuong ung v6i bang 2.1.
Duong cfiu c6 th~ di~n ta m6i quan h~ gifra luQ'Ilg cfiu va gia cua m9t hang h6a, trong khi cac y~u t6 khac nhu thu nh?p cua ngucri tieu dung, gia ca m~t hang lien quan, ... khong d6i.
Dva vao d6 thi trong hinh 2.1 cho thfry, 6 muc gia 100 nghin d6ng/kg thit heo thi ngucri tieu dung sin sang mua v6i s6 luQ'Ilg la 5 nghin kg. Nhung khi muc gia giam xu6ng 80 nghin d6ng/kg thi luQ'Ilg cfiu cua ngucri tieu dung la 15 nghin kg. Ta nh?n thfry, khi gia cang giam thi luQ'Ilg cfiu cang tang, va ngugc l~i, n~u gia cang tang thi luQ'Ilg cfiu cang giam. Vi th~ duong cfiu luon d6c xu6ng. N ghia la, gifra gia va luQ'Ilg cfiu c6 m6i quan h~ nghich bi~n. M6i quan h~ nghich bi~n d6 chinh la quy lu?t cfiu.
Quy loft ciu
Colander, David C [ 14] da k~t lu?n: Lu(Jng cdu vJ hang h6a hay djch v~ c6 m6i quan h¢ ngu(lc chiJu vai gia ca. Niu gia hang h6a ho(lc djch v~ tang, cac yiu t6 khac khong a6i, thi lu(Jng cdu vJ hang h6a ho(lc djch v~ se giam. Va ngu(lc l<;zi, niu gia hang h6a ho(lc dtch v~ giam, cac yiu t6 khac khong a6i, thi lu(Jng cdu vJ hang h6a ho(lc dich v~ a6 se tang
/en. M6i quan hf nghjch bidn gifta gia ca va lu(fflg cdu chinh la quy lu<jt cdu. '
Lugng cAu c6 xu hu6ng giam khi gia tang ho~c ngugc l~i. Di~u nay c6 thS dugc giai thich thong qua hi~u (mg thu nh~p va hi~u ung thay th~.
Hi~u ung thu nh~p chi ra ring, khi gia hang h6a giam lam tang sue mua d6i v6i thu nh~p cua ngucri tieu dung cho phep h9 mua nhi~u san phfim hon tru6c, ngugc l~i, khi gia hang h6a tang len, ngucri tieu dung cam thfiy minh ngheo hon tru6c nen lugng cAu hang h6a d6 giam. Vi d\l, n~u gia thjt heo tang gfip doi, ngucri tieu dung cam thdy thu nh~p thgc t~ tro nen it hon va lugng tieu th\l thjt heo giam di.
Hi~u ung thay th~ chi ra ring khi gia hang h6a nay giam di va gia cac hang h6a khac khong d6i, thi hang h6a nay tro nen hfip d~n hon va ngucri tieu dung se tieu dung hang h6a nay nhi~u hon do n6 re hon m9t each tuong d6i so v6i cac hang h6a khac. Vi d\l, khi gia thjt heo tang len ngucri tieu dung c6 xu hu6ng thay th~ thjt heo bing thjt ga, thjt bo, ho~c ca, ...
Chung ta c6 thS tom tit quy lu~t cAu nhu sau:
P1' => Qow Pw => Qo 1'
Ham s6 ciu
Qua hinh 2.1 chung ta thfiy duang cAu cua m9t m~t hang thuang co d~ng d6c xu6ng ( co thS la duang th~ng ho~c duang cong) v~ hen phai, thS hi~n m6i quan h~ nghjch bi~n gifra gia va lugng cAu.
Jeffrey M. Perloff [ 13] chi ra ring: Ham s6 cAu la m9t ham nghjch bi~n va dS don gian h6a, ham s6 cAu thuang dugc SU dl;lng la m9t ham tuy~n tinh c6 d~ng t6ng quat nhu sau:
Qo = aP + b ( a < 0)
Dga vao vi dl;l trong bang 2.1, chung ta c6 thS vi~t ham s6 cAu (phuong trinh duang cAu) cho m~t hang thjt heo t~i Thanh ph6 Hf> Chi Minh nhu sau:
Ham s6 cAu t6ng quat c6 d~ng:
Qo = aP + b ( a < 0) H~ sf> g6c a duqc tinh nhu sau:
a= ~Qo I ~p
a= (Qo2 - Qo,) / (P2 - P1)
D\l'a vao bang 2.1 chung ta ch9n gia trj bdt ky, gia sir ch9n a hai mire gia P1 = 60 (nghin d6ng/kg) va P2 = 70 (nghin d6ng/kg) thi tuang ung la cac mire san lUQ'Ilg Qo I = 25 nghin kg va Qo2 = 20 nghin kg.
V~y a= (20 - 25) / (70 - 60) = - 5/10
Tinh gia trj b, chung ta the gia trj h~ s6 g6c a va s6 li~u trong bang 2.1 vao ham cdu t6ng quat Qo = aP + b thi chung ta c6 th~ tinh duqc gia trj b nhu sau:
b = Qo - aP (gia sir ch9n Qo = 10, P = 90 tu bang 2.1)
¢ b = 10 - (- 5/10). 90 = 55
V~y ham s6 cfiu thit heo cua Thanh ph6 H6 Chi Minh d6 la:
Qo=(-5/10)P+55 Hay: P = - 2Qo + 110
Qua ham sf> cfiu da tim, chung ta c6 th~ d\l' tinh duqc luQ'Ilg cdu tu cac mire gia cua thit khac cac mire gia trong bang 2.1. Vi d\l, khi gia thit la 85 nghin d6ng/kg thi luQ'Ilg cfiu se la: Qo = (-5/10).85 + 55 = 12,5 nghin kg thit heo.
Nhu v~y, cfiu m{>t lo~i hang h6a c6 th~ duqc bieu di€n thong qua bieu cfiu, duong cfiu va ham s6 cdu.
2.1.1.3. S{l' dich chuyin cua dirirng cdu
' , ? .,
Duong cau khong phai luc nao ciing co djnh ma c6 the thay doi khi c6 S\l' tac d{>ng cua cac yeu t6 khac ngoai gia, nhimg yJu t6 nay ciing anh huang den vi~c mua hang cua nguoi tieu dung. Luc d6 duong cfiu c6 the djch chuyen sang phai ho~c sang trai nhu bieu di€n trong hinh 2.2 du&i day:
p
(D'
0 Q
' '
Hinh 2.2: ( a) Cau tang, iluang cau (D) djch chuyen sang phai (D ') •
p
0
(D)
Q
Hinh 2.2: (b) Cdu giam, iluang cdu (D) djch chuyin
sang trai (D ')
Theo Mankiw [7]; McConnell, Brue va Flynn [4] va Colander,
? , ,
David C [14] thi: T~i m◊t muc gia khong doi, nhfrng yeu to tac d9ng len cfiu lam ducmg cfiu dich chuy~n bao g6m: ( 1) thu nh~p cua ngucri tieu dung;
(2) gia ca cac lo~i hang hoa co lien quan; (3) thi hi~u cua ngucri tieu dung;
(4) s6 lugng ngucri mua tren thi trucmg; (5) dv doan cua ngucri tieu dung
' ' ' ,
ve cac sv ki~n trong tuong lai va (6) dieu ki~n ve khi h~u, thcri tiet [10].
- Thu nh@p: N€u thu nh~p cua ngucri tieu dung thay d6i thi cfiu v€
m<)t s6 m~t hang cfing thay d6i nhu sau:
+ Khi thu nh~p cua ngucri tieu dung tang, gia cua hang hoa va cac y~u t6 khac khong d6i, thi ngucri tieu dung co th~ mua dugc nhi€u hang hoa hon. Do do cfiu cua nhi€u m~t hang se tang, ducmg cfiu cua cac m~t hang nay dich chuy~n sang phai (hinh 2.3a).
+ N gugc l~i, khi thu nh~p cua ngucri tieu dung giam, gia cua hang hoa va cac y~u t6 khac khong d6i, thi ngucri tieu dung co th~ mua dugc it
' ' '
hang hoa hon. Do do cau cua nhieu m~t hang se giam, ducmg cau cua cac m~t hang nay dich chuy~n sang trai (hinh 2.3b ).
p
(D')
0
Q