As far as I know, many (and possibly most) style guides say that the addition of an apostrophe at the end of words that are spoken with an S sound at the end (such as convenience and goodness) is formally correct. In other words:
- for goodness' sake
- for convenience' sake
However, I believe that many consider the apostrophe to be completely optional.
There is an interesting write-up on this topic here:
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For the sake of convenience, ...
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