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大阪大学
鈴木讓
行動計量学会第46回大会
2018年9月6日
分枝限定法で
モデル選択の計算量を低減する
ロードマップ
1. BNの構造学習
2. BNの構造学習の計算を分枝限定法で削減 (既存)
3. Gaussian BNの場合の分枝限定法の適用方法 (提案)
4. 実験
5. 結論と課題
ベイジアンネットワーク (BN)
分布の因数分解を、有向非巡回グラフで表現したもの
マルコフ同値
P=3変数の場合、11種類のBN
それぞれに事前確率をかけたものを比較
データから、事後確率最大のBNを見出す
確率のようなもの
条件付きスコアを求める
各構造のスコアを比較する
Xが先頭の構造だけで16個
Yが先頭、Zが先頭の場合も比較 各構造の事前確率は、等しいとする
(別の)条件付きスコアを求める
各構造のスコアを(別の方法で)比較する
全部で6個
p変数の場合、pの階乗個の比較
最短経路問題でとける
のすべてを計算しないで、
を計算して、
最適な構造を見出したい
計算量を低減する
動的計画法の適用 (Silander-Myllymaki, 2006)
分枝限定法の適用 (Suzuki, ICML-96)
の計算を削除できる
最適な構造が見出される
ことは、保証される
事後確率最大でも、情報量基準最小でもよい
この条件が成立すると、それ以上深い探索は不要
(Suzuki, ICML-96, UAI-17)
➖(尤度)は、非負
Gaussian BN (今回の検討事項)
多変量正規分布をあらわすBN
依存している
変数の個数
分枝限定法の
カットルール
の構成
ー(尤度)は、負になりうる
提案するカットルール
(残り)すべての変数に依存するとき、
尤度が最大になる
実験1 : 人工データ
分岐限定法を使用しないときとくらべ、
1/20から1/10の計算量1/10から1/5の時間
実験2: 実データ (Hitters, BeastCancer)
まとめ
Gaussian BNの構造学習の計算量の削減
• 計算ノード数で1/10-1/20, 計算時間で1/5-1/10に削減
(変数が20-25程度の場合)
• 連続BNで、はじめて分枝限定法を適用
• 一般性があるので、波及効果が大きい
今後の課題
Nを固定したときの計算量がpの多項式になることの証明
(離散BNでは証明が完成してる)
(来週からの国際会議で発表)

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分枝限定法でモデル選択の計算量を低減する