When is a pair of primary and secondary cases said to be in discordance?

A pair of primary and secondary cases is said to be in discordance when they are incorrectly ordered. In this context, discordance typically refers to a situation where the expected relationship or classification between two cases is not met. When cases are incorrectly ordered, it means that the primary case does not correspond accurately to the outcome or status of the secondary case, which creates a discrepancy. This can lead to confusion in analysis and interpretation, especially in classification tasks.

The other options do not accurately reflect the definition of discordance. Identical outcomes would mean that both cases are in agreement, while agreement itself implies that the cases are in alignment. Cases belonging to different sets do not inherently indicate a discordant relationship; they may simply be part of different groups without any implication of order or classification status. Thus, the recognition of incorrect ordering as a cause of discordance is crucial in ensuring that data analysis remains valid and coherent.