1. CHN MU V TNH TON C MU

2. Chn mu? Nu tt c cc c th ca mt qun th u ging nhau, chng ta c mt qun th thun nht (homogenous). Khi , c tnh ca mi c th cng chnh l ca qun th. Khng c s khc nhau/ giao ng tnh cht gia cc c th. 3. Chn mu? Khi cc c th trong mt qun th khc nhau, chng ta c mt qun th hn tp/ khng thun nht (heterogeneous). Khi c tnh ca mt c th bt k khng mang tnh i din cho c qun th. C s khc nhau/ giao ng gia cc c th. Nu mun m t c tnh qun th, khi khng th quan st c tt c cc c th, ngi ta phi chn mt s lng c th t hn trong kh nng, i din tt cho tt c cc c th ca qun th quan st. 4. Chn mu? Qun th Mu S dng d liu/ thng tin ca mt s t tip cn c ni v s ng m khng th tip cn ht Chn mu L ci chng ta mun bit L ci m chng ta quan st Suy ra 5. Chn mu? Mu ca mt qun th phi suy ra c nhng thng tin hu ch v qun th . Do vy, mu phi m bo c c nhng bin thin c bn gia cc c th nh qun th. Mt qun th cng khng ng nht Th xc sut mt mu kh c th m t qun cng ln S l mt sai lm nu suy c tnh ca mu thnh c tnh ca qun th. V Th s lng c th ca mu phi cng ln c th m t qun th tt Mt mu phi c s lng c th ln cho th suy c tnh ca mu thnh ca qun th 6. Chn mu? Chn mu l mt quy trnh la chn c th t qun th cho quan st, c th coi kt qu quan st mu thnh kt qu quan st qun th, mt mc chp nhn m xc nh c. Mu l i din ca mt qun th. Mc i din phi c xc nh/ o lng c. C hai cch chn mu: Chn mu khng ngu nhin Chn mu ngu nhin 7. Cc khi nim c bn Ton th: L tp hp l thuyt ca tt c cc c th, khng xc nh khng gian v thi gian Qun th: L tp hp l thuyt ca tt c c th theo mt c tnh, trong mt khong khng gian v thi gian xc nh. Mu/ qun th quan st c: L tp hp mt s lng c th, la chn t mt qun th trn mt c tnh mu quan tm. c tnh mu: L c s xc nh, la chn c th ca qun th vo mt mu, c s lng c th t hn, v d c th mt qun th ngi c th l c th ngi, h gia nh, lng/ xm Danh sch/ khung mu: L danh sch cc c th ca mt qun th, gip h tr k thut cho qu trnh chn mu C mu: L s lng c th c la chn t mt s lng xc nh/ khng xc nh c th ca qun th vo mt tp hp mu. Sc mnh mu: L mc suy din kt qu thng k trn mu thnh kt qu ca qun th 8. Phn loi cc phng php chn mu Chn mu Ngu nhin Ngu nhin n Theo cm H thng Phn tng Khng ngu nhin Ch tiuCh ch Thun tin Nm bng tuyt 9. Chn mu ngu nhin Chn mu ngu nhin n m bo mi c th ca qun th c la chn vi xc sut nh nhau vo mu Ghp cp mi c th vi mt s ngu nhin, cc c th c la chn theo s ngu nhin ca con s. Xc sut la chn = C mu Tng s c th ca qun th 10. Chn mu ngu nhin n Khong cch gia cc c th l ngu nhin, khng c quy lut no 11. u im Khng cn nhiu thng tin v qun th Tnh ga tr cao, xc nh c sai s thng k D dng phn tch d liu Hn ch Tn km Yu cu danh sch c th trong qun th Khng cn chuyn mn ca nghin cu vin Nguy c sai s ngu nhin 12. Chn mu h thng C th u tin c la chn ngu nhin trong qun th, cc c th tip theo c la chn theo mt khong cch xc nh so vi c th trc . Khong cch xc nh c gi l khong cch mu = Tng s c th ca qun th/ c mu 13. Chn mu h thng Khong cch gia cc c th bng nhau Mang tnh quy lut 14. u im Chi ph hp l, hay c s dng Tnh ga tr cao, xc nh c sai s thng k Cc c th d tip cn Hn ch Sai s chu k do bn thn danh sch c th cng c th c tnh chu k Yu cu danh sch c th ca qun th 15. Chn mu phn tng Chia qun th theo mt c tnh c th thnh cc nhm/ tng (strata) (V d: Chia qun th sinh vin trng HY H Ni thnh 3 nhm/ tng: hc vin tin s, hc vin thc s v hc vin i hc). Cc c th trong mi tng ng nht, nhng khng ng nht gia cc tng. C th p dng chn mu ngu nhin n hay chn mu h thng mi tng (strata). S mu mi tng tham gia vo tng mu c th bng nhau (chn mu phn tng khng cn xng) hay t l vi s c th ca mi tng (chn mu phn tng cn xng) 16. Tng 1 Tng 2 N= 2000 N = 6000 Chn mu phn tng khng cn xng 1000 1000 17. Tng 1 Tng 2 N= 2000 N = 6000 Chn mu phn tng cn xng 1000 3000 18. u im m bo mi nhm u c tnh i din trong tng mu Mi nhm u c thng k v so snh Gim sai s h thng Hn ch Yu cu thng tin chnh xc v t l gia cc tng Chi ph c c danh sch mi tng 19. Chn mu theo cm/ chm Cm y l cm a l ni cc cc th sinh sng. u tin l chn cm, sau mi chn c th trong cm. Phng php hay c s dng khi khng c danh sch c th ca qun th 20. Cc loi chn mu theo cm/ chm Chn mu theo cm 2 bc: Chn cm chn c th trong cm Chn mu theo cm nhiu bc: V d: chn 7 vng sinh thi Vit Nam, chn 1 tnh mi vng sinh thi, chn 1 huyn 1 tnh, chn 1 x 1 huyn, 1 thn/ bn 1 x, chn 200 h gia nh 1 thn/ bn theo c mu. 21. u im Ch ph thp/ hy s dng Khng yu cu danh sch c th ca c qun th, ch yu cu danh sch cm, tiu cm, tiu tiu cm, v danh sch c th ca n v cm cui cng Xc nh c c im ca c cm v qun th u tin dng khi qun th qu ln, v d nh c mt quc gia. Hn ch Sai s ln hn so vi cc phng php ngu nhin khc 22. Phn loi cc phng php chn mu Chn mu Ngu nhin Ngu nhin n Theo cm H thng Phn tng Khng ngu nhin Ch tiuCh ch Thun tin Nm bng tuyt 23. Chn mu khng ngu nhin Chn mu thun tin C th c la chn vo mu mt cch thun tin, sn c v d tip cn. Ti sao dng: nhanh v chi ph thp Phi gii trnh v nh gi gi tr nu chn mu thun tin. 24. u im Chi ph rt thp Hay c s dng Khng cn danh sch c th trong qun th Hn ch bin thin v sai s khng o lng v kim sot c. Kh gii trnh vic suy lun kt qu thng k trn mu ra ca qun th. 25. Chn mu c ch ch (Judgment or Purposive Sampling) Chn c th c mt s c tnh mong mun vo mu vi ch ch ca nghin cu vin. 26. u im Chi ph va phi Hay c s dng Mu la chn m bo c mc tiu Hn ch Sai s! Kh gii trnh vic suy lun kt qu thng k trn mu ra ca qun th. 27. Chn mu theo ch tiu Mu m bo c tnh ca qun th, i din mt mc m nghin cu vin mong mun 28. u im Chi ph va phi Rt hay s dng Khng yu cu danh sch c th trong qun th C phn no tnh cht ca chn mu phn tng Hn ch Khng o lng v gii trnh c bin thin v sai s 29. Chn mu Nm bng tuyt (Snowball sampling) Chn mu bt u t mt c th, c chn mt cch ngu nhin hay khng ngu nhin. Cc c th tip theo c chn t c th ban u hay trc , ging nh tr chi nm bng tuyt, ai trng th c la chn. 30. u im Chi ph thp Hu ch trong mt s trng hp c th Dng xc nh nhng qun th him Hn ch Sai s do c th la chn khng hon ton c lp Kh gii trnh vic suy din kt qu thng k trn mu ra ca qun th. 31. Tnh ton c mu Thng tin/ d liu cn thit Loi nghin cu Cc thut ton thng k s dng c tnh ca c th - Th hin qua bin s (khc bit c bn gia bin lin tc bin phn hng). Kt qu cc nghin cu trc c a vo lm c s tnh ton tin cy (95%, 90%) Ga tr p chp nhn kt qu kim nh (0,05/ 0,001) Mc bin thin ca kt qu mu so vi kt qu thc ca qun th ( chnh xc tng i) Sc mnh ca mu ni ln mc kt qu mu i din cho kt qu ca qun th 32. Tnh ton c mu Tnh ton c mu n gin: Da vo cng thc c bn, Khng tnh n thit k nghin cu, Khng tnh n cc thut ton thng k s dng trong thit k Tnh ton n gin, bng my calculator thng thng Thng khng chnh xc, ch c tnh cht gii trnh mc chn mu v tnh c mu trong cng hay bo co nghin cu. 33. Tnh ton c mu Tnh ton c mu nng cao: Da vo mt s phn mm tnh ton c mu tt (V d: Sampling Size for Health Studies ca WHO) cp mt s thit k nghin cu c bn + thut ton thng k hay s dng cho thit k + bin s/ d liu lin qua C cng thc tnh c mu cho gii trnh, phi chp mn hnh, lu dng jpg Chnh xc hn tnh ton c mu n gin Cha ton din so vi tnh ton c mu chuyn nghip 34. Giao din chn cc thit k nghin cu trong phn mn tnh c mu nng cao 35. Giao din tnh ton c mu tnh ra c mu Nhp gi tr mong mun cho mu Cng thc my tnh tnh c mu cho thit k 36. Tnh ton c mu chuyn nghip Da vo phn mm tnh ton c mu chuyn nghip, trong gi SAS, hoc chuyn nghip vi n-query. Phn mm rt t (c 1 triu USD cho SAS, 1050 USD cho nquery). Hc vin c nhu cu hc thm ng dng ca phn mm nquery c th lin h vi gio vin Nghin cu vin nhp thit k nghin cu, thut ton thng k s dng v cc d liu lin quan n bin s vo phn mm tnh ra c mu. Nghin cu vin c th phn tch nhy mi lin quan gia Sc mnh mu v c mu. Nghin cu vin phi c kin thc chuyn su v thit k nghin cu v cc thut ton thng k c th s dng c phn mm cng nh phin gii kt qu. 37. Loi thit k Bc 1: Nhp d liu v thit k nghin cu, tc ng v thut ton thng k phn tch 4 bc s dng phn mn nquery tnh ton c mu chuyn nghip Loi d liu u ra S nhm mu Loi phn tch thng k C th ha nhp liu thut ton thng k 38. Nhp gi tr c th ca bin s v mong mun c tnh mu Bc 2: Xc nh gi tr c th ca bin s v mong mun c tnh mu 4 bc s dng phn mn nquery tnh ton c mu chuyn nghip Cc tnh ra c mu 39. Nhp gi tr c th ca bin s v mong mun c tnh mu Bc 3: Xc nh sc mnh mu t cc c mu v ngc li 4 bc s dng phn mn nquery tnh ton c mu chuyn nghip Cc kt qu 40. Bc 4: Phn tch nhy ca mi quan h Sc mnh mu C mu 4 bc s dng phn mn nquery tnh ton c mu chuyn nghip Biu ca phn tch nhy 41. Tnh ton c mu c bn 3 cng thc c bn (phn in m l cng thc gc): - T trung bnh n = (ZS/E) 2= (Z(d-c)/E) 2 = N(ZS/E) 2/(N-1) =(ZS/E) 2 - T t l n = Z2 p(1-p)/ E2 = Z2 Np(1-p)/ E2 (N-1)+ Z2 pq - T phn v n = pc(100-pc) Z2/ E2 - Z cho mc tin cy 95% = 1.96, 99% = 2.58 - Cng thc trn c th m rng ra - Qun th c s lng mu xc nh v khng xc nh bng cch nhn hoc khng vi h s N/(N-1), - T l d bo xc nh v khng xc nh bng cch nhn v khng vi h s p*(1-p) - Qun th xc bin thin hay khng bin thin bng cch nhn v khng nhn vi bnh phng lch chun, bnh phng 1/6 (hiu gi tr ln nht gi tr thp nht) 42. Cng ty A quyt nh tuyn sinh vin mi tt nghip. Gim c cn quyt nh mc lng cnh tranh. T kinh nghim ca cc cng ty khc cho thy, lng sinh vin mi ra trng giao ng t 3,8 4,2 triu, mc trung bnh l 3,9 triu. lch chun vo mc 0,3 triu. Cng ty mun lm mt nghin cu nh, la chn mt s sinh vin lm mu, v mun suy kt qu mc lng trung bnh ca nhm sinh vin trong mu suy ra mc lng trung bnh ca tt c sinh vin mi tt nghip. tin cy: 95%, v gi tr x ch trong khong +/- 50 nghn. Hy tnh c mu cho nghin cu ca cng ty? N = (1,96 x 0,3/ (2 x 0,05)) 2 = 34,57 V d 1 43. V d 2 VTV chun b pht sng mt chng trnh gio dc sc khe bui sng. Kinh nghip t VOV sc khe cho thy, chng trnh gio dc sc khe ch thu ht c 2 trn 10 ngi xem truyn hnh ti thi im bui sng. VTV mun lm mt nghin cu nh xc nh c t l ngi xem chng trnh ca h nu pht sng. H mun chn mt s ngi xem truyn hnh phng vn. H mong mun kt qu thng k trn mu phn nh c kt qu thc s mc x dch +/- 5% v vi tin cy 99%. C mu m VTV cn kho st l bao nhiu? N = 2,58 2 x 0,2 x (1-0,2) / (2 x 0,05) 2 = 106,5 44. V d 3 Xc nh c mu cn thit iu tra ph n ti mt x X bit v vic khm thai 3 ln trc sinh l cn thit. Khong tin cy l 95% vi sai s 5%. Mt nghin cu tin hnh cch y 2 nm cho thy 40% ph n c hi ni bit khm thai 3 ln trc sinh l cn thit. N = 0,4 x (1-0,4) x (1,96/ (2 x 0,05))2 = 92,19 45. Chn mu t qun th BN1 BN11 BN21 BN31 BN41 BN51 BN61 BN71 BN81 BN91 BN2 BN12 BN22 BN32 BN42 BN52 BN62 BN72 BN82 BN92 BN3 BN13 BN23 BN33 BN43 BN53 BN63 BN73 BN83 BN93 BN4 BN14 BN24 BN34 BN44 BN54 BN64 BN74 BN84 BN94 BN5 BN15 BN25 BN35 BN45 BN55 BN65 BN75 BN85 BN95 BN6 BN16 BN26 BN36 BN46 BN56 BN66 BN76 BN86 BN96 BN7 BN17 BN27 BN37 BN47 BN57 BN67 BN77 BN87 BN97 BN8 BN18 BN28 BN38 BN48 BN58 BN68 BN78 BN88 BN98 BN9 BN19 BN29 BN39 BN49 BN59 BN69 BN79 BN89 BN99 BN10 BN20 BN30 BN40 BN50 BN60 BN70 BN90 BN90 BN100 C 100 bnh nhn t BN1 n BN100 46. Bi tp chn mu t qun th Hy chn 30 bnh nhn mt cch ngu nhin t 100 bnh nhn bng 3 cch Cch 1: Ct 100 mnh giy, ghi tn 100 bnh nhn, cho vo 1 ci r, ri nhm mt, th tay vo r, ly ra 30 t Cch 2: In danh sch 100 bnh nhn ra, m bng s ngu nhin, cho BN1 mt s ngu nhin bt k trong bng, 99 bnh nhn tip theo cng nhn c mt s ngu nhin tip theo t s ngu nhin ca BN1 Cch 3: Dng hm rand() trong Excel cho mi bnh nhn mt s ngu nhin, tro bnh nhn bng sort, la chn 30 bnh nhn t trn xung di Hy chn 30 bnh nhn h thng t 100 bnh nhn 47. Tnh ton c mu nng cao - Hc vin mang theo my tnh thc hnh - Hc vin copy phn mm Sample Size Determination in Health Studies 48. Bi tp 1.1.1 Trung tm y t d phng huyn X mun xc nh t l hin nhim ca bnh lao trong nhm tr em di 5 tui trn a bn. Mu nghin cu cn bao nhiu tr khi kt qu ca mu ch bin thin di 10% so vi gi tr tht ca qun th, vi 95% tin cy. Ngoi ra, t l hin nhim c bit khng vt qu 10%. Kt qu: 139 49. Bi tp 1.1.2 Vin v sinh dch t trung ng mun xc nh t l tr em trong ton quc c tim phng ng theo quy nh. Mu nghin cu cn bao nhiu tr nu mun kt qu ca mu nghin cu x dch 5% so vi kt qu thc ca qun th, vi tin cy l 95% Kt qu: 385 50. Bi tp 1.3.1 T l thnh cng ca mt can thip tim mch ngoi khoa X c bo co trong y vn l 70%. Mt phng php mi Y c xut mang li hiu qu tng t. Mt c s iu tr khng c nng lc ngoi khoa mun p dng phng php iu tr mi Y. Trc h cn phi nghin cu bao nhiu bnh nhn kim nh gi thuyt phng php mi cng c t l thnh cng l 70% vi p = 0,05. Nghin cu mun c mt sc mnh 90% rng gi tr x dch +/- 5%. Kt qu: 912 51. Bi tp 2.1.2 Mt nghin cu vi 50 cng nhn ang tham gia mt d n thy li cho thy 40% s h b nhim sn mng. Mt nghin cu tng t cng vi 50 cng nhn nhng khng lm cho d n thy li no, t l nhim sn mng ch l 32%. Mt nhm dch t hc mun tin hnh mt nghin cu ln hn c c kt qu khc bit gia hai nhm mc p=0,05, vi mc tin cy 95% th h phi la chn bao nhiu cng nhn mi nhm cho nghin cu. Kt qu: 704 52. Bi tp 2.2.2 mt nghin cu, mt nhm dch t hc so snh 50 bnh nhn mc mt bnh thn kinh vi 50 bnh nhn khng mc bnh lm nhm chng. 30 ngi thuc nhm bnh nhn (60%) v 25 ngi thuc nhm chng (50%) i bin, nh bt c. Gi nh t l ngi lm ngh i bin, nh bt c trong qun th tng ng, v nhm dch t hc mun lm mt nghin cu ln hn c hai nhm bnh v chng, tnh ra c s khc bit gia hai nhm vi mc tin cy 90% v p = 0,05, th mu h phi chn mi nhm l bao nhiu? Kt qu: 519 53. Bi tp 3.1.1 Ti th trn X, bnh t ang l mt vn sc khe cng ng. Khong 30% ngi dn tin rng h ang phi s dng ngun nc nhim ly bnh. Mt s nghin cu trong y vn cho thy ngun nc nhim c nguy c lm ngi dn nhim bnh cao gp 2 ln v vi mc tin cy 95%, x dch trong 25%. Nhm nghin cu n th trn X phi chn bao nhiu mu bnh nhn v nhm chng xc nh xem ngun nc ni y c phi l nguyn nhn dn n s ly lan ca bnh t hay khng? Kt qu: 408 54. Bi tp 4.1.1 Mt nhm nghin cu dch t hc ln k hoch nghin cu xc nh xc sut bnh phi X c lin quan n cht Y nhim trong khng kh. Mu nghin cu hai nhm c v khng tip xc vi cht Y l bao nhiu? nu nghin cu mun xc nh nguy c tng i ti 50% gi tr thc ca qun th v vi tin cy 95%. T trong y vn, bnh phi X ang phi bin mc 20% trong nhm ngi khng phi nhim vi cht Y. Kt qu: 44 55. Bi tp 4.2.1 Hai phng php iu tr mt bnh ung th c nh gi trong mt nghin cu thun tp th nghim lm sng. Ngi bnh c la chn ngu nhin cho mi phng php iu tr v c theo di tnh trng ti pht bnh trong 5 nm. Phng php iu tr A l mt phng php mi gim mt na nguy c ti pht trong 5 nm u. 35% ngi bnh c iu tr bng phng php iu tr B ti pht trong 5 nm u. Mu bnh nhn mi nhm iu tr A v B l bao nhiu nu nhm nghin cu mun c mc tin cy 90% c th loi tr gi thuyt Ho (Nguy c tng i =1). Nu khng th kim nh c chp nhn tin cy p=0,05. Kt qu: 131 56. Bi tp 6.1.1 Mu bnh nhn l bao nhiu nu nhm nghin cu mun xc nh t l mc mi ca mt bnh x dch trong khong 10% gi tr thc ca qun th, mc tin cy 95%. Kt qu: 385 57. Bi tp 6.2.2 Trn c s s liu 5 nm ca BV X, theo di mt s lng nh bnh nhn mc bnh X, t l mc mi bnh ny c bo co l 40%. Nhm dch t hc cn nghin cu trn bao nhiu bnh nhn kim nh xem t l mc mi bnh X trn cng ng c ng l 40% vi p=0,05 hay khng? Nghin cu c mong mun c mt sc mnh 80% xc nh t l mi thc hng nm l 50%. Kt qu: 146 58. Bi tp 6.3.2 bit v tc ng lu di ca ting n ti sc khe, nhm nghin cu A ln k hoch theo di dc, so snh hai nhm cng nhn lm vic trong mi trng c ting n v t ting n. Y vn c nu t l suy gim thnh lc hng nm ca cng nhn lm trong mi trng n o l 25%. Mu nghin cu phi l bao nhiu mi nhm kim nh loi b Ho (gia hai nhm khng c s khc bit). Ga thuyt Ha l t l gim thnh lc hng nm cng nhn lm vic trong mi trng t ting n l 10%. mnh ca kim nh l 80%, p=0,05 Kt qu: 23 59. Bi tp 7.2.2 Y vn c mt bi ghi nam gii cao huyt p c cn nng trung bnh l 70kg. Tuy nhin, ngi ta cho rng, nam gii Vit Nam cao huyt p khng nng nh vy v mun lm mt nghin cu kim tra iu . C mu phi l bao nhiu nu mun kim nh vi gi tr p=0,05 v sc mnh ca mu l 90%, gi tr cn nng chp nhn sai lch mc 5 kg, s bin thin/ lch chun l 20 kg. Kt qu: 169 60. Bi tp 8.1.1 Y vn c ni t l tr em nhim giun a cc nc pht trin l 30%. Trung tm y t huyn A c 4000 tr di 5 tui, mun lm mt nghin cu, chn mt s tr kim tra phn. H mun kt qu nghin cu trn mu nghin cu ca h ch chnh lch 5% so vi c qun th 4000, mc tin cy 95%. S tr m Trung tm chn t 4000 chu phi l bao nhiu? Kt qu: 977 61. Bi tp 8.2.1 3 huyn min ni A, B, C c s h gia nh l 2000, 3000 v 5000. Mt on nh gi nhanh ca B NN & PTNN cho thy t l h gia nh s dng nc sch hp v sinh l 10% 1 x huyn A, 15% 1 x huyn B v 20% 1 x huyn C. S Y t nghi ng kt qu ny v mun lm mt nghin cu xc nh t l thc s h gia nh s dng nc sch 3 huyn l bao nhiu. H c mt t tin tip cn h gia nh kho st, mun chn mu v mun kt qu thng k trn mu ch x dch 2 % so vi ton b qun th 10000 h 3 huyn, tin cy 95% cho cc kt qu thng k. Kt qu: 1158