割合 and 比率: shares of a whole and ratios
Distinguish a share of a whole from a ratio between groups without inferring actual counts from a ratio alone.
A share of the whole
割合, read わりあい, means a proportion or share. 参加者の半分が学生です。 says that half the participants are students.
学生の割合は五十パーセントです。 also gives students a fifty-percent share. The total number of participants cannot be recovered from that proportion alone.
A ratio between amounts
比率, read ひりつ, means a ratio comparing amounts. 赤と青の比率は二対一です。 gives a red-to-blue ratio of two to one.
Red accounts for two parts and blue for one. Red occupies two of the three total parts, not twice the whole.
Units and actual counts
割合 and 比率 overlap in use. First identify the comparison base and expressions such as パーセント and 対.
比率は同じですが、人数は違います。 says that the ratios match but the numbers of people differ. Equal ratios do not guarantee equal counts.
Does a fifty-percent student share necessarily mean fifty students?
Show the answer and why
No. It means fifty students only when the total is one hundred. The total count is needed.