NGHIEN CUTU- TRAODOI yv^ PhifoTng phap li^a chon tap thong so chuan doan (*) TS NGUYEN VAN DUNG H - la phan tin tdc nhdn dUdc ve he thdng td thong so' flit vlin fie Tren cd sd mo hinh chan dodn ta se xdc dinh dUdc mot sd tap hifu ban cdc hU hdng - sif cd X=[Xj] [j=\,2, ,m) va tap cdc thdng sdchan doan: S = {Si](i=\,2, ,n) Tinh trang ki thuat cua he thdng difdc phan dnh thdng qua tap cdc thong sd chan doan (Si) Viec kiem tra, xac dinh va phan tich toan bp cdc thdng sd la mpt viec lam bet sdc phdc tap, CO ddn den hien tUdng dU thifa thdng tin, ddng thdi nd se tang khdi tUdng dung cu do, tang dp phdc tap cua thiet bi bie'n ddi va xd 11 thdng tin, tang gia chan doan Do dd vide hdp li hod tap cac thdng sd chan doan cd y nghia ldn ve mat thifc te'ciing nhif li thuyet NghTa la chung ta can phai gidi ban so' thdng sd chan doan den mdc hdp li, nhUng vin dam bao chda difng day du thong tin ve tinh trang kl thuat cua he thdng, lam cd sd cho viec dif bao hanh trinh dif trif ciia he thdng khai thdc S n - la sd lUdng cac trieu chdng dac tnfng cho hif hdng X ' , Ta se cd: P{xj)=—{ne\\ cdc hU hdng co ddng xac sua't); 1 m n ni log •^'".^ir ".If Tren cd sd ket qua tinh toan dUdc ta se thie't lap ma tran xac sua't va thdng tin nhUBang 1.2 Giai quyet ve'n lie 2.1 Ccfsdli thuyet Viec lifa chpn tap thong sd cha'n doan hdp li cd the difdc tien hanh bang nhieu phUdng phap khac nhau: - PhUdng phap ke't hdp giifa ly thuye't thong tin va ly thuyet xac suat; - PhUdng phap lifa chpn theo each kiem tra tdi thieu; - Phu'dng phap dp dung ly thuyet thdng tin; - Phifdng phap danh gid thdng qua tieu chuan "trpng lu'dng" cua cac thdng sd; - PhUdng phap dp dung ly thuye't tap md 2.1.1 PhUOng phdp ket hop gida li thuye't thong tin vdi ll thuye't xdc suat Giifa cac hif hdng va cac thdng sd cha'n doan cua he thdng cd mdi lien he phdc tap De tien cho viec nghien cdu - xem xet cac quan he dd ta lap ma tran chan doan Vdi cac cot bieu dien tap cac huhdng sU cd (X) va cac hang bieu dien tap cac thong sd cbd'n doan (S.) Gia tri cua cac phan tif ma tran: a[(,y']se nhan gia tri la ne'u tham sdthu' Si cd phan dng vdi hU hdng X., bie'u hien bang sU vUdt qua gidi han cho phep, va nhan gia tri bang neu tham sd thd Si khong co phan dng vdi huhdng X (Bang 1.1) X s, X, X2 X3 1 S2 1 S3 1 S4 0 Bangl.l Ne'u gpi: P, - la xac xua't khdng dieu kien, nhU vay he thdng cac hu hong X se nam X , cdn he thdng cac thdng so'S se nam Si P(xj) - la xdc sua't hU hdng da xua't hien Xj hoac xac sua't he thdng X trang thai X P(si) - la xac sua't trieu chdng da ro rang Si hoac xac sud't he thdng S trang thdi S^ Bdng 1.2 Td ket qud bang 1.2 ta thay tin tdc nho nhat nhan dUdc td trang thai co xac sua't ldn nha't, dieu dd hoan toan phii hdp vdi li thuye't thong tin Thong sd S^ la thong sd cd lUdng thong tin be nha't, nghia la thong sd Sj khdng du thdng tin cho mpt hu hdng cu the nao Do thong sd S3 se bi loai khdi tap thdng sd chan doan, cd nhu vay cudi cilng ta se chpn dUdc bp cac thdng sd cha'n doan hdp li gdm k thong sd (k < n) dac trtfng cho tinh trang ki thuat cua toan bp ddi ttfdng chan doan 2.1.2 PhUOng phdp lifa chon theo cdch kiem tra toi thieu Dp ldn thong tin cua mpt tridu chdng Si nao dd ma tran chan doan se dtfdc cdc dinh thong qua bieu thdc sau: Jsi = m n Trong do: m.- so cac so cd hang thd i; n - sd cac so' hang thd i Gia sd ta cd ma tran cha'n doan va gia tri thdng tin cua cac thdng so'nhubang 1.3 So sanh gia tri thong tin cua cac thong so' vda xac dinh (Jsi) de chpn thong sd cd gia tri thong tin ldn nha't, dtfa vao bp thong sd cha'n dodn hdp li Khi so sanh cd the tdn tai nhieu thdng sd cd gia tri thdng tin ldn nha't, nhtfng ta chi chpn lay mpt thdng so' Thdng sd dtfdc chpn phai dam bao de nhat va dac tnfng nhat Sau chpn xong thdng sd thd nha't de chpn dtfdc thong sd tie'p theo thi ca'u true cua bang 1.3 se phai thay ddi nhtf sau: bang dtfdc chia hai phan, phan dau la nhffng hu hong cd phan dng vdi thdng sd vda chpn va phan thd hai la nhffng htf hdng khdng cd phan dng vdi thdng sd vda chpn Gid tri thong tin cua cac thdng sd ttt Bdng 1.3 ve sau se dtfdc xac dinh thong sd qua bieu thdc sau: (*) Hoc vidn Ky thuat Quan sif TAP CHI Cd KHI Vl|T NAM • So 146 - Thang nam 2009 NGHIEN Ciiu - T R A O DOI Trong do: m.- la sd cdc so cua hang thd i thudc nhdm L cua bang n - la sd cac sd cua hang thd i thupc nhdm Li cua bdng L - la sd nhdm Uong bang TUdng tif ta tid'p mc lam nhtf vay cho de'n cdc thdng sd cdn lai bang mang gid tri thdng tin bang Khi ta chpn dtfdc mpt tap cac thdng sd cha'n doan (k) dac tnfng cho tinh trang ky thudt cua toan bp he thdng (k < n) Vidu: x X3 X4 s, 0 Sz 0 1 S3 0 S4 1 Xa X4 X, x Jsi 1 0 0 1 S3 1 S4 1 s« s, Chpn thSng s^ S3 (L=2) x, X2 X3 X4 Jsi S3 1 0 S4 0 0 S, 1 Ke't qua cudi cung ta chpn dtfdc hai thdng sd S, va Sj de dUa vao bp thong sd chan dodn hdp ly 2.1.3 PhUcfng phdp dng dung ly thuyet thong tin Tren cd sd ma tran chan doan dtfdc thid't lap nhd xet quan he Id gic giffa cac thdng sdchan doan vdi cac htf hdng sif cd nhtf cdc phtfdng phap tren, d cdt cudi cilng ta bieu thi gid tri thdng tin ve trang thai cua he tho'ng Il(Si) chda mdi thdng sd Si gid tri thdng tin dtfdc xac dinh nhtf sau: Theo II thuye't thdng tin dp khdng xdc dinh cua he thdng dtfdc bieu didn thdng qua ham Entrdpi cua tap tham sd trang thdi va nd dUdc xdc dinh bang bieu thdc sau: H{XJ)=-XP,.log, p, Trong dd: p - la xac suat cua dd'i tUdng X dng vdi trang thai thdi Da'u (-) dd' H(Xj) ludn {+) vi pi