Hi Anon 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. htm
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I would go for an apostrophe 's' for singular nouns or uncountable nouns, so I go for ' for convenience's sake ', as how we say "for goodness' sake " ; ' for ***'s sake'; ' for mum's sake ' ' for old times' sake '.