Chuong 3 Phan lép Naive Bayes
3.2 C:ie mii hinh x:ie suat Naive Bayes
Tom lai, cue ’ hinh xiic suat cho mot classifier la mot ’ hinh co dieu ciia cue ke't quit hay c‹ic /cip
Van de la néu so c:ie da)c trung ii la l6n hay khi mot dac trung co the chiém mot c:ie gin tri., sau do dua vao mot mo hinh trén cue bing xiic suat la khong the 1ém dupe. Do vay, chiing ta cong thuc hoa lai c:ie mo hinh de de xu 1y.
Bang ciich sit d;ing dinh ly Bayes, co dupe:
Trong thuc hanh, chi elm quan trim téri tir ' ciia
phan ' khi ma
phu thu(oc vao C v‹i cue gin tri. ciia cue da)c trung ciia I , da cho, nén u so la hung thuc su.
Tit so tirong duong voi mo hinh xiic suat co the dupe vie't lai nhu sau, sit d;ing dinh nghia ciia x:ie suat co dieu ki(en:
do do co the dupe the hieu nhu:
Dieu my co ngliia la ducri su doc lap gia dinh o trén, cue dieu kien phiin phoi tre“n cue 1dp hoc biéu C co the dupe the“ hieu:
o day Z 1:i mot nhfin to xiic d)mh ½ xich phq thuoc vao I , Fz, .., F , chang han mot hung so néu cac gin tri. ciia cac bie'n da)c trung déu dupe biet.
c:ie thu(at " ciia r thaw “ hinh naive Bayes tuong ring co G - 1) + tin thaw so. Trong thuc Ie, thuéng k —— 2 (phfin loai nhi phiin) va r = 1 (cac bie'n Bernoulli nhu la c:ie da)c trung) dupe pho biéu, va nhu va)y
tong ' luon cue thaw so
“ hinh naive Bayes la 2ii + 1, cr day ii la so c:ie da)c trung nhi phén sit d;;ng cho cue du do:in.
3.3 Ii6c luirng thaw sit
Tat cii cue thaw la, 1dp hoc uu tién va cue da)c trung phén phoi xiic suiit) co the’ dupe gan dting véri c:ie tiin so tién quan tit viec thiet lap dao tao. Day la cue diinh gia maximum likehood khii uang co the’ xay ra. Cut da)c trung khong riéng biet elm phiii dupe rfii rac dau tién. Su roi rac co the khong giiim siit (cue réng bu(oc lua chon da)c biet) hoac giiim siit (réng buoc hu6ng dan bcii thong tin song dli lieu dao tao).
Thu)at torn Bayes v:i ting dj;ng
Néu mot 1dp hoc va gin tri. da)c trung khong bao gio xay ra cling véri nhau trong
' sé dupe
phii hiiy tat ca cue thong tin trong cue xac sufit khi chting dupe nhan ’rig. Vi vay, mong muon ke't hpp mot mau nho chinh siia trong ta’t c:i c:ie