バイアス
バイアス(偏り)とは「結果を真の値から偶然ではなく系統的に乖離させるもの」である。
観測値には「バイアス=系統誤差」と「バラつき=偶然誤差」という2種類の誤差が存在する。「バイアス=系統誤差」は「データの正確度=真度と妥当性」に関わり、「バラつき=偶然誤差」は「データの精度=再現性と信頼性」に関わる。
「バイアス=系統誤差」はデータをバラつかせるのではなく一方方向に平均値をずらすので、結果から「バイアス=系統誤差」に気づくのは難しいし気づけたとしても影響を計りにくい。しかし、「バイアス=系統誤差」の原因が分かれば測定や解析を改良して後から補正することができる。
一方で「バラつき=偶然誤差」は系統誤差とは異なって後から補正することはできないものの、標本数を多くするなどして統計学的処理をすることで比較的容易に影響を低減することができる。「バラつき=偶然誤差」は真の値から正負の方向に同じ程度の確からしさで働くので、同様の測定を何回も繰り返して行って平均を取れば偶然誤差の影響は打ち消し合って小さくなるはずだし、標本数が増えればバラつきは打ち消し合って小さくなると考えられるからである。
情報バイアス
研究対象者から必要な情報を得る際に発生するバイアス(偏り)である。
例えば、後ろ向き研究である症例対象研究では被験者の記憶が不正確となり情報バイアスの一種である想起バイアス(思い出しバイアス)が生じる。
他には、治療効果などの情報を測定する際に発生する測定バイアスも情報バイアスの一種である。
新薬の有効性を検証するために標準治療薬と新薬の治療効果の差を調べる際に、観察者がどの被験者に標準治療薬が投薬されてどの被験者に新薬が投薬されているのかを把握してしまうと、観察者が新薬の有効性を証明したいばかりに新薬を投薬されている被験者の治療効果を過大評価して標準治療薬を投薬されている被験者の治療効果を過小評価してしまうかもしれない。これが測定バイアスである。
測定バイアスを防ぐために盲検化が行われる。
盲検化
情報バイアスは基本的に解析(統計的手法)では調整できないので、研究デザインの段階でダミー(dummy)による盲検化(blinding)といった方法などで制御する必要がある。
盲検化には被験者が知らせない「一重盲検化」と被験者・測定者が知らない「二重盲検化」と被験者・測定者・結果評価者が知らない「三重盲検化」と被験者・測定者・結果評価者・データ解析者が知らない「四重盲検化」がある。
ダミー(dummy)
ダミー(dummy)とは盲検化の一手法であり治療をしているように見せかけることである。ダミー(dummy)には以下のような例がある。
選択バイアス
選択バイアスは研究対象者を選ぶ際に発生するバイアス(偏り)である。
母集団全体から抽出(サンプリング)された研究対象者が母集団を代表できていないときに生じる。
交絡バイアス
交絡バイアスは交絡因子の存在によって生じるバイアスである。
交絡因子は下記の3つの条件を満たす。
医学生・医師だからモテるのか?
交絡バイアスを分かりやすい例で確認してみる。
「医学生である」ことを要因として「モテる」ことを結果とする。
筆者自身が「医学生である」から「モテる」と感じたことはないが、世間一般的には「医学生である」から「モテる」という図式が成り立っているとされているような気がする。
ではどうして「医学生である」から「モテる」のだろうか。
理由の一つとして、「医学生である」ことは世間一般的には「お金持ち(親のお金)」と思われるから「モテる」ということが挙げられる。
お金目当てということである。筆者は全ての人がお金目当てだとは全く思っていないが、分かりやすいようにそうだとして考える。
これは「医師である」から「モテる」ことにも当てはまる。「医師である」ことは世間一般的には「お金持ち(自分のお金)」と思われるから「モテる」のである。
医学生と医師が「モテる」という現象が同じ「お金目当て」によって引き起こされることが分かったが、医学生は「親のお金でお金持ち」になっているのに対して医師は「自分のお金でお金持ち」になっているという違いがあることに注意する。
ここで医学生が「親のお金でお金持ち」になっていることは交絡因子に該当して、医師が「自分のお金でお金持ち」になっていることは中間因子に該当する。
中間因子は「要因→中間因子→結果」という流れになる必要があり、医師の場合では「医師である→医師であるから高収入でお金持ちとなる→お金持ちなのでモテる」という流れができる。
これが医学生の場合だと「医学生である→医学生であるからお金持ちとなる→お金持ちなのでモテる」という流れになってしまい当てはまらなくなる。
医学生は別に働いていないので収入もない。それなので「医学生であるからお金持ちとなる」というのはおかしくて、医学部には裕福な家庭出身の人が多いから「親のお金でお金持ちとなる人が医学生に多い」のである。
「親のお金でお金持ち」は、
という3つの条件を満たすので交絡因子である。
交絡バイアスを研究デザインで制御する方法
集団の限定(restriction)
交絡因子が明らかな場合に交絡因子が有る集団のみで研究するか交絡因子が無い集団のみで研究することである。
研究への参入・除外基準を交絡因子の有無で決めることで、研究に交絡因子が影響を及ばさないようにするために行う。
例えば、飲酒と肺がんの関係を調べる研究で喫煙が交絡因子となっている場合を考える。
このような場合に集団の限定(restriction)を行って、喫煙する人のみ(喫煙群のみ)の中で飲酒と肺がんの関係を調べたり喫煙しない人のみ(非喫煙群のみ)の中で飲酒と肺がんの関係を調べる。
マッチング(matching)
交絡因子が明らかな場合に比較する群間で交絡因子の偏りがなくなるようにすることである。
例えば、高血圧と虚血性疾患の発生の関係を調べる研究で年齢が交絡因子となっている場合を考える。
このような場合にマッチング(matching)を行って、症例群にx歳の人を選ぶなら対象群にもx歳の人を選ぶというように交絡因子でペアリングすることで症例群と対象群の交絡因子の偏りを均質化してから高血圧と虚血性心疾患の発生の関係を調べる。
無作為化(ランダム化)=無作為割付
複数の群に被験者を無作為(ランダム)に振り分けることであらゆる背景因子が均等に群に割り付けられて、それぞれの群の性質が均質化されることである。
集団の限定やマッチングでは考慮した予め分かっている交絡因子しかコントロールできないのに対して、無作為化では考慮していない未知の交絡因子にも対応できる。また、選択バイアスも無作為化によって対応できる。
バイアスに関する医師国家試験問題
105H1:ある集団で飲酒と肺癌の関係を調査した際、因果関係があるようにみえた。しかし、この集団では飲酒者の大半は喫煙者であり、非飲酒者の大半は非喫煙者であった。喫煙者の中でも非喫煙者の中でも、飲酒と肺癌に関連は認められなかった。
この調査において「喫煙」はどれにあたるか。
出典:第105回医師国家試験問題
- 誤差
- 交絡因子
- プラセボ効果
- 選択バイアス
- 測定バイアス
解答：2.交絡因子
解説：このケースでは「要因=飲酒・結果＝肺癌・交絡因子＝喫煙」という関係になっている。
115E17:臨床研究におけるバイアスと交絡について誤っているのはどれか。
出典:第115回医師国家試験問題
- 情報バイアスは対象者から情報を得る際に生じる。
- 選択バイアスは対象者の選択方法から生じる。
- 交絡因子は研究デザインにより調整できる。
- 交絡因子は原因と結果の両方に関連する。
- 情報バイアスは統計的手法で調整できる。
解答：5.情報バイアスは統計的手法で調整できる。
解説：1.←情報バイアスは研究対象者から必要な情報を得る際に発生するバイアス(偏り)である。
2.←選択バイアスは研究対象者を選ぶ際に発生するバイアス(偏り)である。
3.←交絡因子によって交絡バイアスが生じる。「バイアス=系統誤差」は原因が分かれば測定や解析を改良して補正することができるので、交絡バイアスの原因である交絡因子が分かれば研究デザインで補正することは可能である。
4.←交絡因子は要因と関連があり結果に因果的な影響を与えるので、原因と結果の両方に関連すると言える。
5.← 情報バイアスは対象者から情報を得る際に生じるので、情報を得た後に行う統計の際に調整することはできない。
バイアスに関する歯科医師国家試験問題
114B37:疫学調査において第3の因子が曝露と疾病発生の両者に関係することにより、曝露因子と疾病発生量との関係が歪められる現象はどれか。1つ選べ。
出典:第114回歯科医師国家試験問題
- 交絡
- 偽陽性
- 因果の逆転
- 情報バイアス
- 選択バイアス
解答:1.交絡
バイアスに関する保健師国家試験問題
第97回保健師国家試験午後問題22:気管支喘息の有病率の地域比較調査を行ったところ、A地区では問診で判定 し、B地区では呼吸機能検査で判定していたことが分かった。
疫学調査法におけるこのような問題点を何というか。
出典:第97回保健師国家試験問題
- 交絡
- 偶然誤差
- 情報の偏り
- 選択の偏り
解答：3.情報の偏り
解説:A地区では問診で気管支喘息の有病率を判定しているのに対してB地区では呼吸機能検査で気管支喘息の有病率を判定している。このように測定方法が異なると情報バイアス(情報の偏り)の一種である測定バイアスが生じる。
第96回保健師国家試験午後問題18:市全体のクラミジア感染率を推計するため、市内のある大学の学生から希望者を募り、抗体検査を実施した。
この方法で最も大きなバイアスはどれか。
出典:第96回保健師国家試験問題
- 選択バイアス
- 交絡バイアス
- リコールバイアス
- インフォメーションバイアス
解答:1.選択バイアス
解説：市内のある大学のクラミジア感染率を推計するためなら市内のある大学の学生から希望者を募って抗体検査を行えば良い。しかし、市全体のクラミジア感染率を推計するために市内のある大学の学生から希望者を募るとその時点で選択バイアスが生じる。その大学の学生の集団が市全体を代表できるとは限らないからである。
リコールバイアスは想起バイアス(思い出しバイアス)と同義であり、インフォメーションバイアスは情報バイアスと同義である。
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