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Neatly phrased:

Trends which appear in slices of data may disappear or reverse when the groups are combined.

Or perhaps even more succinctly: slicing data can introduce bias.

This is less accurate, because not slicing data can also lead to bias.

Except the original statement didn't make any claim about "not slicing", so neither does mine.

Not slicing is nevertheless slicing. The trivial selection. Rush's song "If you choose not to decide you still have made a choice"