万まん分ぶんの一いち

expression, noun

one ten-thousandth

Used in mathematical, statistical, or figurative contexts to express a very small fraction or probability.

• この薬くすりの副ふく作さ用ようが出でる確かく率りつは万まん分ぶんの一いちです。

  The probability of experiencing side effects from this medicine is one in ten thousand.