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英検準1級・CEFR B2平均とばらつきの読み方

平均だけじゃ、分からない

What the Average Doesn't Tell You

Two classes take the same exam. Class A's scores average 75. Class B's scores also average 75. On paper, the classes look identical. They are not.

In Class A, almost every student scored somewhere between 70 and 80 — a tight, predictable cluster. In Class B, half the students scored above 95, and half scored below 55; almost nobody actually landed near 75 at all. The average is the same number in both cases, but it describes two completely different classrooms.

An average collapses a whole spread of individual results into a single figure, and in doing so, it necessarily throws away information about how spread out those results actually were. A teacher deciding how to plan the next lesson needs to know which classroom she is walking into — one where nearly everyone is roughly ready to move on, or one that is really two groups in disguise, one that needs a challenge and one that needs to review the basics.

The same trap shows up outside the classroom. Two neighborhoods can report the exact same average household income, yet in one, most families earn a similar, comfortable amount, while in the other, a small number of very high earners sit alongside many families struggling to get by. A city planner who only reads the average risks building services for a neighborhood that, in the second case, barely exists.

Whenever a decision rests on an average, one follow-up question is worth asking before anything else: how much did the individual numbers actually vary around it?

254 words

Vocabulary Check

  1. "An average collapses a whole spread of individual results into a single figure" — here, "collapses" is closest to —

    1. compresses into one simplified value
    2. physically falls down
    3. increases dramatically
    4. becomes more detailed
    答えを見る

    正解：A．compresses into one simplified value

  2. "...a tight, predictable cluster" describes scores that are —

    1. spread far apart
    2. grouped closely together
    3. impossible to measure
    4. always increasing
    答えを見る

    正解：B．grouped closely together

  3. "...almost nobody actually landed near 75" means almost nobody —

    1. scored close to 75
    2. attended the exam
    3. studied for 75 minutes
    4. passed the exam
    答えを見る

    正解：A．scored close to 75

  4. "...really two groups in disguise" means the class is actually —

    1. two separate classrooms physically
    2. two distinct groups hidden behind one shared average
    3. wearing costumes
    4. graded by two different teachers
    答えを見る

    正解：B．two distinct groups hidden behind one shared average

  5. "Whenever a decision rests on an average..." — here, "rests on" is closest to —

    1. is delayed by
    2. is based on
    3. is canceled because of
    4. is unrelated to
    答えを見る

    正解：B．is based on

  6. "...nearly everyone is roughly ready to move on" — here, "roughly" is closest to —

    1. harshly
    2. approximately
    3. never
    4. exactly
    答えを見る

    正解：B．approximately

  7. "...it necessarily throws away information" means the average —

    1. stores information for later use
    2. loses or discards certain information
    3. exaggerates the information
    4. checks the information twice
    答えを見る

    正解：B．loses or discards certain information

Comprehension

  1. Why do Class A and Class B "look identical" on paper but are actually different?

    1. They took different exams.
    2. They share the same average, but their scores are distributed very differently.
    3. Class B has more students than Class A.
    4. The teacher graded them using different scales.
    答えを見る

    正解：B．They share the same average, but their scores are distributed very differently.

    第2段落：平均は同じ75点だが、Aは70〜80点に集中、Bは95点台と55点未満に二極化しており、実態がまったく異なる。

  2. What does the passage mean by an average "throwing away information"?

    1. The average is calculated incorrectly.
    2. The average hides how spread out or varied the individual results actually were.
    3. The average deletes some students' scores.
    4. The average always underestimates performance.
    答えを見る

    正解：B．The average hides how spread out or varied the individual results actually were.

    第3段落の説明：個々の結果のばらつきという情報が、1つの数字に圧縮される過程で失われる、ということ。

  3. According to the new example, what risk does a city planner face if they only look at a neighborhood's average income?

    1. They might overestimate how many people live there.
    2. They might plan services for a "typical" family that barely exists in a neighborhood actually split between very high and very low earners.
    3. They might accidentally lower everyone's taxes.
    4. They might confuse two different neighborhoods with each other.
    答えを見る

    正解：B．They might plan services for a "typical" family that barely exists in a neighborhood actually split between very high and very low earners.

    追加した段落の要点。平均所得が同じでも、実態が「多くが同水準」か「高所得層と苦しい家庭に分かれている」かで、必要な行政サービスは全く違う、という応用例。

  4. Why does a teacher need more than just the average score, according to the passage?

    1. Because averages are always calculated incorrectly.
    2. Because the same average can hide either a uniform class or a class split into two very different groups.
    3. Because students dislike being graded on averages.
    4. Because school rules require more detailed data.
    答えを見る

    正解：B．Because the same average can hide either a uniform class or a class split into two very different groups.

    第3段落：同じ平均でも「ほぼ全員が同水準」の場合と「二極化している」場合があり、対応（次の授業計画）が変わってくる。

  5. According to the passage, what question should follow any decision based on an average?

    1. Who calculated the average?
    2. How much did the individual numbers actually vary around it?
    3. Is the average higher than last year?
    4. How many students are in the class?
    答えを見る

    正解：B．How much did the individual numbers actually vary around it?

    最終段落が示す実践的な問い。ばらつき（分散・スプレッド）を確認せよという結論。

Summary Writing

Summarize the passage in 60–70 words.

モデル要約と解説を見る

Two classes share the same average exam score of 75, but their results look nothing alike: one class clusters tightly around 75, while the other splits into a high-scoring half and a low-scoring half, with almost no one near 75. Because an average hides how spread out the real numbers are, a decision based on it alone can mislead. The writer suggests always asking how much the numbers actually varied.

70 words

なぜこの要約になるか

  • 元の英文は5段落＝「具体例①（2つのクラス）→説明（平均が捨てる情報）→具体例②（地域の平均所得）→応用（先生の判断）→結論的な問い」という展開。70語では具体例の数字（70〜80点、95点/55点）をそのまま残しつつ、2つ目の具体例と先生の判断の部分は要約から外している。
  • "collapses...into a single figure"という比喩表現は、要約では"hides"というより平易な動詞に言い換えている。難しい語をそのまま抜き出す必要はなく、意味が同じなら簡単な語に置き換えてよい。
  • 結びの問い（"how much did the numbers vary?"）は、この英文全体の実践的な結論なので、要約の最後にもそのまま残す。

Your Turn

A school announces that the "average commute time" for its students is 25 minutes, and decides that this is short enough that no changes are needed. What question would you ask before accepting this decision? Write your opinion in English, using 120–150 words.

「平均だけでは分からないこと」を軸に、具体的に何を確認すべきかを述べる。理由は1〜2つ。

使える型

  • Before accepting this, I would ask...
  • This is important because...
  • For instance, ...
  • Therefore, ...

考えの道具箱

この英文が扱っているのは、既刊『行事局、数字に、聞いてみた。』第1章の「平均のワナ」と同じ考え方です。平均だけを見ても実態は分からない——同じ平均でも、みんなが同じくらいの点数をつけた場合と、点数が真っ二つに分かれた場合とでは、中身がまったくちがう、というルールをそのまま英語のクラス比較に応用しています。

共通テストの英語でもグラフや数値を読み取らせる設問が増えています。「平均」という数字を見たときほど、「ばらつきはどうか」を自分に問い直す癖をつけてください。

「平均のワナ」のページを見る →

Read the Original

The Decision Deck cover

この考え方が生まれた物語を、英語のまま読む。

The Decision Deck

Did you compare the numbers?

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