April 2020
Intermediate to advanced
816 pages
18h 32m
Chinese
Content preview from 精通機器學習
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.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.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.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.
180
|
第六章：決策樹
CART
演算法的運作方式幾乎與之前一樣
，
不過它不是藉由將不純度最小化來拆開訓練
組
，
而是藉由將
MSE
最小化來拆開訓練組
。
公式
6-4
是這個演算法試圖最小化的代價
函數
。
公式
6-4 CART
回歸代價函數
如同分類任務
，
決策樹在處理回歸任務時很容易過擬
。
在不做任何正則化的情況下
（
也
就是使用內定的超參數
），
你會得到圖
6-6
左圖的預測
，
這些預測顯然很糟糕地過擬訓
練組
。
你只要設定
min_samples_leaf=10
就可以產生合理許多的模型
，
如圖
6-6
的右圖
所示
。
無限制
圖
6-6
將決策樹回歸器正則化
不穩定性
希望現在你已經確信決策樹有很多優點了
，
它們容易瞭解和解讀
、
容易使用
、
用途廣泛
，
且功能強大
。
但是它們也有一些限制
。
首先
，
你可能已經發現
，
決策樹喜歡直角的決策邊
界
（
每一個劃分處都與一個軸垂直
），
因此它們對訓練組的旋轉很敏感
。
舉例來說
，
圖
6-7
是個簡單的可線性拆分資料組
，
在左圖中
，
決策樹可以輕鬆地劃分它
，
但是在右圖中
，
將資料旋轉
45
°
之後
，
決策邊界看起來無謂的複雜
。
雖然這兩個決策樹都完美地擬合訓練
不穩定性
|
181
組
，
但右邊的模型極可能無法良好地類推
。
限制這個問題的方法之一是使用主成分分析
（
見第
8
章
），
它通常可以更妥善地旋轉訓練資料
。
圖
6-7
對訓練組的旋轉很敏感
更常見的問題是 ...
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Read now
Unlock full access
More than 5,000 organizations count on O’Reilly
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
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.