# ＃回に＃度: probability or sequence?

In a game manual, there is a description of a rare item that sometimes appears instead of a regular one. The rule determining when this rare item appears is this:

16回に１度、Ｘのかわりに出現。

In an earlier question concerning the same sentence I got quite a lot of answers with translation. Every answer translates the condition as

Once every 16 times

However I am still not sure whether this means:

• There is a probability of 1/16 that a rare item appears.
• The rare item appears every 16th time.

Which one is correct? And, how to express the second one in Japanese?

16回に1度

means literally "once per 16 times", but I would understand it as "on average once every sixteen times" (which means a probability of 1/16, i.e. a ratio of 1:15).

"Every 16th time" could be translated

16回ごとに

Here ごと is 毎【ごと】, e.g.

ご利用10回毎に2,000Pプレゼント!
Receive 2,000 points with every tenth use!

(Of course I cannot say how your game is programmed, and every sixteenth time would imply a probability of 1/16, but not the other way around.)