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CHUONG 9: I CUONG V SAI SO HE THO NG, GA Y NHIE U VA C HO I TRONG

MUC TIEU:1. Liet ke c cac sai so he thong (bias) va xac nh ung cac giai phap han che chung trong nghien cu sc khoe.2. Liet ke c cac bien so gay nhieu (confounding variables) va xac nh ung cac giai phap kiem soat tac ong cua chung trong nghi

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CHUONG 9: I CUONG V SAI SO HE THO NG, GA Y NHIE U VA C HO I TRONG

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    1. CHUONG 9: ?I CUONG V? SAI SO HE THONG, GAY NHIEU VA C HOI TRONG CAC NGHIEN CU SC KHOE PGs. Ts. L Hong Ninh

    2. MUC TIEU: 1. Liet ke c cac sai so he thong (bias) va xac nh ung cac giai phap han che chung trong nghien cu sc khoe. 2. Liet ke c cac bien so gay nhieu (confounding variables) va xac nh ung cac giai phap kiem soat tac ong cua chung trong nghien cu sc khoe. 3. anh gia c vai tro cua c hoi (chance) trong cac nghien cu sc khoe.

    3. I. CAC TNH HUONG: Van e 1 : Thuoc la va ung th phoi. Trong nghien cu ve moi quan he gia thuoc la va ung th phoi cho thay hut thuoc la co nguy c ung th phoi cao gap 20 lan so vi khong hut thuoc. Neu trong nghien cu nay, c cau tuoi 2 nhom hut thuoc va khong hut thuoc khac biet nhau, th du nh nhom hut thuoc co ty le ngi ln tuoi cao hn han so vi nhom khong hut thuoc th moi quan he tm thay qua nghien cu co the do tac ong bi yeu to tuoi. V cang ln tuoi th nguy c mac ung th cao, ch khong han la do thuoc la. Hn na, ngi cang ln tuoi th thi gian hut thuoc cang dai nen co the moi lien he tm thay qua nghien cu la do s ket hp gia tuoi va thuoc la len ung th phoi ch khong phai do tac ong rieng biet cua thuoc la len benh ung th phoi. Trong nghien cu nay Tuoi la mot bien so gay nhieu (confounding variables) moi quan he gia bien so oc lap (independent variable): thuoc la va bien so phu thuoc (dependent variable): ung th phoi. Trong trng hp nay, Tuoi co the lam phong ai moi quan he that gia thuoc la va ung th phoi.

    5. Van e 2: Hoat ong the lc va nhoi mau c tim Trong nghien cu nay, nhom co hoat ong the lc cao gom nhieu ngi tre tuoi hn nhom t hoat ong the lc, ma nhng ngi tre tuoi co nguy c nhoi mau c tim (NMCT) thap hn ngi cao tuoi. Do o, ket qua tm thay qua nghien cu khong chac la tac ong cua the lc khong anh hng en nguy c NMCT, co the thc s la chung co tac ong ngan nga NMCT nhng do nhom hoat ong the lc gom nhieu ngi tre tuoi hn nhom t hoat ong the lc nen khong tm thay vai tro thc s cua hoat ong the lc. Nh vay, tuoi a lam nhieu moi quan he gia hoat ong the lc va NMCT. Ngoai ra, neu trong nhom hoat ong the lc co nhieu phai nam hn nhom t hoat ong the lc, ma phai nam th co nguy c b NMCT cao hn phai n. Do o, khong tm thay tac ong thc s cua hoat ong the lc len viec nga NMCT, co the la do phai n nhom khong hoat ong the lc nhieu hn nhom co hoat ong the lc. Nh vay, co the la nhom t hay khong hoat ong the lc co ty le ngi va tre tuoi, va n cao hn nhom tang hay co hoat ong the lc, th chnh s ket hp tng tac gia tuoi va phai lam nha nghien cu khong tm thay tac ong that s cua hoat ong the lc trong viec ngan nga benh NMCT.

    7. Lap luan tng t, thuoc la cung co the la bien so lam nhieu moi quan he that s gia hoat ong the lc va NMCT. V nhng ngi hut thuoc la thng la nhng ngi t hoat ong the lc; va hut thuoc la th de b NMCT hn nhng ngi khong hut.

    8. II. CAC AC TRNG CUA YEU TO GAY NHIEU (CONFOUNDER) 1. Yeu to (bien so) gay nhieu va co lien quan en bien so oc lap va co bien so phu thuoc nhng 2 moi lien quan nay oc lap nhau. 2. Yeu to gay nhieu t no ton tai san trong moi quan he gia bien so oc lap va bien so phu thuoc bi v no la thuoc tnh (ac trng) co san tren oi tng nghien cu, ch khong do ngi nghien cu tao ra. 3. Yeu to gay nhieu co the lam phong ai, thu nho, lech chieu moi quan he gia bien so oc lap va bien so phu thuoc. 4. Tac ong lam nhieu la do co s phan bo khong bang nhau cua cac ac trng (bien so gay nhieu: tuoi, phai, hut thuoc,...) gia cac nhom nghien cu. nen nguyen tac loai bo tac ong lam nhieu la lam cho s phan bo cua chung bang nhau gia cac nhom.

    9. III. KIEM SOAT TAC HAI LAM NHIEU TRONG NGHIEN CU : Tac ong lam nhiem co the kiem soat trong thiet ke hay phan tch d lieu nghien cu. A. Trong thiet ke nghien cu : Trong thiet ke cac nghien cu phan tch co 3 bien phap c ban : S khu tru oi tng (Restriction), bat cap (matching) va s ngau nhien hoa (Randomization). Rieng s ngau nhien hoa ch co the ap dung cho cac nghien cu can thiep: th nghiem lam sang co nhom chng, th nghiem thc a ... 1. S ngau nhien hoa : La ky thuat e phan phoi oi tng vao cac nhom th nghiem: ngau nhien n, xep bloc. Vi c mau u ln, cac ky thuat phan phoi ngau nhien tren se giup lam quan bnh hoa cac ac trng (bien so gay nhieu) vao cac nhom nghien cu.

    10. 2. S khu tru oi tng: Nguyen tac kiem soat tac ong lam nhieu, la lam chung (bien so gay nhieu) giong nhau gia cac nhom, khi o se u tieu chuan e so sanh. Bien phap khu tru oi tng nham ch nhan vao nghien cu cac oi tng cung mang mot ac trng nao o ma thoi. Th du, neu phai tnh co the lam nhieu moi quan he gia bien so oc lap va bien so phu thuoc th , nha nghien cu co the ch chon nhng oi tng hoac la an ong hoac la an ba. neu tuoi la mot bien so gay nhieu th ngi nghien cu co the ch chon vao nghien cu cac oi tng khu tru mot lp tuoi nao o ma thoi. 3. S bat cap: Bat cap la ky thuat lam giong nhau (cac bien so gay nhieu) tng oi cac oi tng a vao cac nhom nghien cu. Do o, cac ac trng va ca mc o cua ac trng lam nhieu a c lam can bang ngay t luc chon oi tng vao cac nhom nghien cu. Th du, neu tuoi, phai la 2 bien so lam nhieu moi quan he gia hoat ong the lc va NMCT, neu ta tien hanh nghien cu bang mot nghien cu benh-chng th c mot ngi mac benh NMCT (nhom benh), phai nam, co o tuoi t 55-60 th ta chon mot ngi khong b NMCT (nhom chng) phai nam, co o tuoi 55-60.

    11. oi vi nghien cu benh chng khi thiet ke bat cap th viec trnh bay cung bang bang 2-2 nhng tr so moi o mang y ngha khac nhau: a : So cap ma ca benh va khong benh eu tiep xuc. b : So cap ma ca benh co tiep xuc ca khong benh khong tiep xuc. c : So cap ma ca chng (khong benh) th co tiep xuc ca benh th khong tiep xuc. d : So cap ma ca benh va chng eu khong co tiep xuc.

    13. B. Trong phan tch: Tac ong lam nhieu co the kiem soat bang 2 ky thuat trong phan tch: ky thuat phan tang va ky thuat phan tch a bien. 1. Ky thuat phan tch phan tang: Ky thuat nay lam ong nhat cac bien so la nhieu theo tng tang gia cac nhom, nh vay tac ong lam nhieu se c loai bo. e thc hien c ky thuat phan tch, nha nghien cu can phan tang trc theo cac bac gia tr cua bien so gay nhieu. Sau o, tnh cac so o lien quan gia bien so oc lap va phu thuoc trong tng tang va so o lien quan chung. Chi tiet ky thuat phan tang c trnh bay trong chu e dch te hoc phan tch.

    14. 2. Phan tch a bien: Ky thuat phan tang tng oi de thc hien, no cho phep anh gia moi tng tac cua cac bien so gay nhieu len moi quan he gia bien so oc lap va phu thuoc. Tuy nhien, nhc iem c ban cua phng phap nay la chung khong giup anh gia cung mot luc nhieu bien so lam nhieu. That vay, vi bien so gay nhieu cha nhieu lam, th du nh 3 bien, th du phai tnh (2 cap: nam, n), tuoi chia 4 lp (45-49, 50-54, 55-59, 60-4); tnh trang hut thuoc (3 cap o: cha bao gi hut, a bo hut, hien ang hut) th so lp a len en 2 x 4 x 3 = 24 lp. Khi o c mau tnh toan ban au tng ng vi bien so nguyen phat (primary variables) se khong u e phan tch (tr so trong cac o se nho hn 5). Ky thuat phan tch a bien khac phuc nhc iem cua phan tch phan tang: kiem soat c cung luc nhieu bien so cung luc va ngay ca khi phan tch phan tang that bai do khong u so. Viec ng dung ky thuat va la chon dang hoi quy nao la van e kha phc tap, oi khi phai tham van chuyen gia. ay chung ta ch xet dang a hoi qui dang ng thang nh sau: Y = a + b1X1 + b2X2 + ... + bnXn

    15. Trong o : n : so bien so oc lap. X1 . . . Xn : la bo tr so cua cac bien so oc lap. b1 . . . bn : he so cua cac bien so oc lap. Vi cac phan mem ng dung hien co tai Viet Nam nh (SPSS/PC+, STATA) chung ta se de dang xac nh c phng trnh hoi qui nay. Trong mot so nghien cu, ma bien so phu thuoc (he qua ang quan tam nghien cu) la dang bien so nh phan: song/chet; khoi/ khong khoi; benh/ khong benh th ta dung mot dang hoi qui ac biet la hoi qui logistic. Phan tch hoi qui nay cho phep anh gia moi quan he gia bien so he qua (nh phan) va bien so phu thuoc sau khi kiem soat cung mot luc nhieu bien so gay nhieu. Dang hoi qui nay nh sau: Ky thuat phan tch can ke hoi qui a bien va hoi qui logistic c trnh bay trong chu e Dch te hoc phan tch.

    16. SAI SO HE THONG (BIAS) TRONG CAC NGHIEN CU SC KHOE.

    17. I. AI CNG: Sai lam mang tnh he thong dan en c lng sai moi quan he gia bien so oc lap va bien so phu thuoc. Xuat nguon: Chon la oi tng. The cach thu nhap thong tin. Ly giai thong tin. Tng thuat. anh gia: rat kho nh lng. Kiem soat : thiet ke nghien cu. II. CAC LOAI SAI SO HE THONG: Sai so la chon. Sai so thong tin.

    18. A. Sai so la chon: Xay ra khi chon oi tng. Chuan mc (tieu chuan) chon la. ? Lien can ti tiep xuc. ? Lien can ti he qua. Xay ra trong nghien cu: ? Benh - chng. ? oan he hoi cu. Th du: - Thuoc nga thai ? thuyen tac tnh mach? Phu n ang dung thuoc nga thai nhap vien tang. Bac s chan oan thuyen tac tnh mach tang. - Kch thch to n (Oestrogen) ? ung th co t cung. Phu n co dung Oestrogen ? tang. So sanh nhom benh-chng? Khong tham gia, khong hng ng vao nghien cu. - Ty le cac cau tra li.

    19. B. Sai so quan sat (thong tin, o lng): Do s khac biet mot cach co he thong trong thu nhap thong tin, so lieu ve tiep xuc va benh tat. 1. Sai so nh lai: (Recall bias) ong c nh lai: - Tien s tiep xuc. - Benh tat. Nghien cu benh - chng. oan he hoi cu.

    20. 2. Sai so phong van: Do chu y thai qua trong ghi nhan va ly giai thong tin. Trong nghien cu benh-chng, oan he tien cu hieu biet benh tat ? tiep xuc (?)? Trong nghien cu oan he hoi cu: hieu biet tiep xuc ? he qua tang. 3. Sai so xep loai: Xep loai sai ve benh tat & tiep xuc. - Xep loai sai ngau nhien : . JICK,H: thuoc diet tinh trung ? sinh con khuyet tat. . Nghe nghiep ? benh tat. - Xep loai sai khong ngau nhien. Khi co s tiep xep loai sai khac nhau ro ret gia cac nhom.

    21. III. KIEM SOAT SAI SO HE THONG: Thiet ke +++ 1. Chon dan so nghien cu: - Th du: chon nhom chng trong benh vien. Sai so phong van. Sai so nh lai. Sai so chon la. - Nghien cu oan he, th nghiem lam sang. Chon dan so on nh. Chon dan so nguy c.

    22. 2. Phng phap thu thap so lieu: Thiet ke dung cu ac thu thu nhap thong tin. Hng dan s dung dung cu thu thap thong tin. Bo cau hoi. Lam mu. 3. Nguon thong tin: Lay t ho s co san: t sai so. Khong bao phu. Dung nhieu nguon ho s. T bao cao (nghien cu can thiep).

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