Chuong 3 Phan lép Naive Bayes
3.5 Thua(t toiin phan loai viln ban Naive Bayes
véri mot 1ua)t quyet d)mh. La mot 1ua)t chung de chon nhiéu nhat c:ie gia thuyet co kha néng x:iy ra, dieu nay dupe biet den nhu la no.r/miim a posteriori hay 1uat quyet dinh MAP. Classifier tuong ring la chuc uang phfin lip dupe xiic du) ih nhu sau:
Mot chi y rang gin dinh doc 1a°p co the“ dan den mot so két qu:i khong mong muon trong tinh to:in sau xiic suat. Trong mot so truéing hpp khi co mot phu thu(oc giua su quan s:it, xiic sufit ké trén co the“ mau thuan voi x:ie suat tie'n de thir hai do moi xiic suat luon nho hon hoa) c bang mot.
Mac du ning su that co the up d rig rong rai, gi:i dinh doc lap thuéing khong chinh xiic, cue naive Bayes classifier co vai thuoc tinh 1:inn cho no him ich trong thuc hénh. Dac biet thuc hanh, s;r tiich riéng ciia 1éip co dieu kien phan loai da)c trung co nghia la moi phan loai co the dupe me tinh doc lap nhu la mot phan phoi mot chiéu. To:in bo classifier la manh du de’ bit qua cac thieu sot nghiem trong ciia no song nhiing mo hinh xiic suat naive.
3.5 Thua(t toiin phan loai viln ban NaiveBayes Bayes
Ki thu(at phan hoach ciia Naive Bayes dua trén cci scr dinh li Bayes va d)ac biet phu hpp cho cue triténg hpp phén loai co kich thucic dau vao la 16n. Mac du Naive Bayes khii don gum nhung no co kha nding phan loai tot hon rat ohiéu phuong Thuat toiin Bayes va King d;ing
phiip phfin hoach phuc tap khiic. Véri moi loai van bun, thu(at to:in Naive Bayes tinh cho moi
t:ii lieu d› sé dupe gén cho loai vén bun mo co xiic suat hau nghi(em cao nhat nén dupe biéu dien bang cong thuc:
Tém lai phfl n loqi viln bun sir dqng thu"at toén Naive Bayes cé the dien dqt m9t céch ngfin gpn nhu sau:
Voi moi vén ban D (document), nguoi ta
P( C, | D) =
" tinh cho moi loai mot xiic
ciia Naive Bayes xiic suat ciia 'i ti tron Hi
cue tit dong ther ciing doc lap véri vi. tri ciia cue tit song t:ii lieu. Xiic suat P(D|C,) dupe tinh to:in tit tfin sufit xuat hi(en ciia cue tit don wk (word) song tai 1ie(u D
Nhu vay bieu thuc (1) co the dupe viét lai nhu sau: P( C, | D) = P( C, ) P( D) l< j<1 P(w, | C,)
Gin tri. léin nhat ctia x:ie sutit P(C, | D) dupe dna ra boi ngucri tain cong t:ie phan loai. Gin tri. my dupe gpi la ngufing hay ranh roi giita cue léip van ban rné chiing co the chua tai lieu D.
Vi di¿: PhSn lopi thu’ di§n t0’ bring Naive Bayes classifier
Day la mot vi dp ve“ tain viéc naive Bayesian de“ phan loai cue tai 1ie(u phan loai van de. Xem xét cue v% de ciia phiin loai c:ie tai lieu theo noi dung ciia ho, vi dq vao thu r:ie va khong phiii la thu rite song cue thu dien tit. Hay tuéing tuong ning cue tai ' 1’ hoc ciia cue tai lieu co the tain mo hinh nhu la bo cue tit dupe viét nhu:
Xii ly nhu v°ay da don gién c:ie y tuiing, hon niia bang ciich gin sit ning x:ie suiit ctia mot tit song mot tai lieu la do( c lap véri chiéu diii ctia mot Hi 1ie(u hoac ta“t ca cue tai lieu cling mot chie“u dai.
Sau do, xiic suat ciia mot tai lieu D, cho mot ldp hoc C, la
Bay gicr, theo dinh nghia: Thuat toiin Bayes va ting d;ing
Nén co:
GU dinh rang thoi diem chi co hat léip hoc, 5 v:i = 5 (vi du nhu thu r:ie va khong ph:ii la thu rite).
qua Bayesian trén, co the vie't:
Do dé›:
Vi vay co the“ vie't:
Bring ciich sit d;ing cue két Thu(at toiin Bayes v:i ring
Tre“n thirc té xiic suat p(5 D) co the dupc tinh de déng tir log Ip (S D) / p
( 5 | D)) dna trén nh(an dmh (5 D) + p (= S D) —— 1.
Cuoi cung, ciic Hi 1ie(u co the dirpc phfin loai nhu sau:
,nguoc lai no khong phai la thu rac. Thu)at toiin Bayes va ring