Ratio Metric
Averaging daily conversion rates gives a different answer from dividing monthly totals. Only one of them answers the question people asked.
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Stream Windowing
A window is a group selected by event time, not a moment when something runs. Which shape you pick decides whether the numbers can be added up at all.
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Semi-Additive Fact
A balance can be summed across accounts but not across days. Semi-additive facts need a point in time or an average along the dimension where sums are nonsense.
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Non-Additive Fact
Ratios and distinct counts cannot be added up, and averaging them is usually wrong too. Store the additive parts, aggregate those, and compute the ratio last.
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Split-Apply-Combine
Learn how groupby splits rows, applies a function, and combines the results, why text columns raise, and how missing keys vanish from a summary.