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Q010

4択Multiple choice応用Intermediate

期待値・確率思考・キャリア判断expected value · probabilistic thinking · career decisions

あなたは安定した大企業の職（年収10万ドル、ほぼ確実に継続）と、スタートアップのオファー（成功確率30％で年収換算25万ドル相当、失敗確率70％で年収換算6万ドル相当に転落）の間で迷っています。金銭的期待値だけで比較した場合の正しい評価と、それに加えて考慮すべき最も重要な点の組み合わせはどれですか。You are choosing between a stable corporate job ($100,000 per year, nearly certain) and a startup offer (30% chance of an outcome worth $250,000 per year, 70% chance of an outcome worth $60,000 per year). Which option correctly states the comparison by financial expected value alone and the most important additional factor to consider?

スタートアップの期待値は11万7千ドルで大企業を上回る。ただし自分のリスク許容度と最悪の場合に生活が維持できるかを併せて考える必要がある
スタートアップの期待値は25万ドルなので明らかに有利であり、迷う余地はない
大企業の期待値10万ドルの方が高いので、大企業を選ぶべきである
期待値は同じなので、直感で選べばよい

The startup's expected value is $117,000, higher than the corporate job, but you must also weigh your risk tolerance and whether you can sustain your life in the worst case
The startup's expected value is $250,000, so it is clearly superior and there is nothing to deliberate
The corporate job's expected value of $100,000 is higher, so you should take it
The expected values are equal, so just go with your gut

解答Answer

正解: ACorrect: A

0.3×25万＋0.7×6万＝11万7千ドルでスタートアップの期待値が高いですが、分散の大きさと最悪ケースへの耐性も判断に含めるべきです。0.3 × 250,000 + 0.7 × 60,000 = $117,000, so the startup has the higher expected value, but the variance and your ability to survive the worst case must also enter the decision.

解説Explanation

期待値とは、各結果の値にその確率を掛けて合計したものです。設問では 0.3×250,000＋0.7×60,000＝117,000ドルとなり、大企業の100,000ドルを上回ります。しかし期待値は「同じ賭けを何度も繰り返した場合の平均」を示すにすぎず、一度きりのキャリア判断では結果のばらつき（分散）と最悪ケースの深刻さが極めて重要です。7割の確率で6万ドルに落ちたとき、住宅ローンや家族の生活が維持できるかは人によって異なります。Annie Duke は『確率思考』で、良い判断とは期待値を計算した上で自分の状況に照らして賭けの大きさを決めることだと述べています。選択肢2は成功時の値のみを見た誤り、選択肢3は計算違い、選択肢4は計算を放棄しています。Expected value is the sum of each outcome multiplied by its probability. Here, 0.3 × 250,000 + 0.7 × 60,000 = $117,000, which exceeds the corporate job's $100,000. But expected value only tells you the average if the same bet were repeated many times; for a one-shot career decision, the spread of outcomes (variance) and the severity of the worst case matter enormously. Whether you can cover a mortgage and support a family on $60,000 in the 70% scenario varies by person. Annie Duke argues in Thinking in Bets that good decision-making means computing the expected value and then sizing the bet according to your own circumstances. Option 2 looks only at the upside, option 3 miscalculates, and option 4 abandons the calculation altogether.

課題Issue

現実には成功確率や成功時の金額を正確に見積もることはほぼ不可能で、数字は本人の楽観や希望に引きずられます。また、スキル獲得や人脈、やりがいといった非金銭的価値を期待値に織り込むのは難しく、金銭だけの比較は判断を歪めることがあります。In reality, success probabilities and payoffs cannot be estimated precisely, and the numbers are easily swayed by optimism. Non-financial value such as skills, networks, and fulfillment is hard to fold into an expected value, so a purely financial comparison can distort the decision.

改善案Improvement

確率は複数のシナリオ（悲観・中立・楽観）で幅を持たせて計算し、結論が変わる境界の確率を確認しましょう。最悪ケースに備えて生活費の6〜12か月分の緊急資金を確保し、失敗時の「次の一手」を事前に描いておくと、期待値の高い挑戦を取りやすくなります。Run the calculation across pessimistic, neutral, and optimistic scenarios and identify the probability at which your conclusion flips. Build an emergency fund covering six to twelve months of expenses and sketch your 'next move' in case of failure, which makes it easier to take positive-expected-value bets.