October 2014
Intermediate to advanced
288 pages
6h 27m
Japanese
Content preview from 実践 機械学習システム
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6.3
ナイーブベイズ分類器の紹介
119
表6-3
上の例では上手く分類できているようです。ただし、最後のツイート（結果が0で割られる）だけは
結果を出力できていません。これについてはどのように対応すべきでしょうか？
6.3.4
新出単語への対応
私たちは前述の通り確率を計算しましたが、実際は本当の確率を計算していません。私たちが計算
した確率は近似値であり、全体の一部からなるサンプルデータから求められたに過ぎません。私たち
は、訓練データ中に含まれるコーパスが、実際の確率についての全ての情報を物語っているという仮
定を暗に設定しました。しかし、その仮定は正しくありません。6つのツイートからなるコーパスだけ
では、まだ見ることのない他のツイートも含めた全ての情報を持っているとは到底考えられません。た
とえば、「text」という単語を含んだツイートは存在しますが、訓練データではその単語を含むツイート
はありませんでした。明らかに、私たちの仮定はとても大ざっぱな
ものであり、そのことを考慮すべき
なのです。このような問題に対して、実際に行われる対策が「1を足すスムージング」です。
「
1
を足すスムージング」は加算スムージング（
additive smoothing
）またはラプラス・スムー
ジング（
Laplace Smoothing
）と呼ばれます。ラプラス・スムージングはラプラシアン・ス
ムージング（
Laplacian smoothing
）とは関係ないことを注意しておきます（ラプラシアン・
スムージングはポリゴンメッ
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.