Buying a lottery ticket loses you money ex ante (in expectation), but if you win, it was the right decision ex post.
[2] The ex-ante (and ex-post) reasoning in economic topics was introduced mainly by Swedish economist Gunnar Myrdal in his 1927–39 work on monetary theory, who described it in this way: An important distinction exists between prospective and retrospective methods of calculating economic quantities such as incomes, savings, and investments; and [...] a corresponding distinction of great theoretical importance must be drawn between two alternative methods of defining these quantities.
Gunnar Myrdal further explained that ex ante disparity and ex post balance are made consistent through price changes, which result from the behavior of economic agents, which is based on ex ante anticipations: For these anticipations determine the behaviour of the economic subjects and consequently those changes in the whole price system which during a period actually occur as a result of the actions of individuals.
Other terms – as e.g. "income", "revenue", "return", "expenses", "savings", "investments" – imply, however, a time period for which they are reckoned.
However, the reference to ex ante and ex post analysis has become so usual in modern macroeconomics that the position of John Maynard Keynes to not include it in his work was currently considered as an oddity, if not a mistake.