[throway88989898]

Neatly phrased:

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

[naasking]

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

[Matumio]

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

[naasking]

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

[throway88989898]

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