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Silhouette scores: useful signal and common misinterpretations
The silhouette coefficient compares how tightly a row sits in its assigned cluster with how close it is to the nearest other cluster. Values range from -1 to 1. The average is useful, but it can hide small, unstable or p
Simpson's paradox in channel conversion rates
An overall conversion ranking can reverse the ranking within every segment when channels contain different segment mixes. Inspect counts and denominators before concluding that one channel performs better for comparable
Single versus bidirectional filtering in Power BI
Filter direction determines which way a relationship propagates a filter. In a typical star schema, a dimension filters its related fact table. Turning on both directions allows filters to travel back through that relati
SMOTE inside cross-validation without leakage
SMOTE creates minority-class samples by interpolating between minority neighbours. If the neighbour search sees validation rows before a cross-validation split, information from the evaluation fold influences the synthet
Spark transformations and actions with an execution plan
Spark transformations describe a new dataset and are generally lazy; an action requests a result or write and triggers execution. The physical work depends on the optimized plan, partitioning and data. Counting API calls
Speech transcription evaluation beyond one accuracy number
Word error rate is useful, but one corpus number can conceal poor performance for a language, accent, noise condition or critical identifier. Evaluate transcription in the context of the task it feeds.
SQL dates: choose inclusive and exclusive boundaries correctly
For adjacent reporting periods, a dependable convention is to include the start and exclude the next period's start: timestamp >= start AND timestamp < nextstart. This is called a half-open interval.
SQL DISTINCT: when it fixes a count and when it hides a bug
DISTINCT removes repeated combinations of the selected expressions. It does not know whether two rows describe the same customer, the same event or two legitimate purchases with equal amounts. That business identity must
SQL funnel conversion when events arrive out of order
Funnel conversion needs a defined sequence, population and time window. Counting users who generated each event type independently does not prove that they completed those events in the required order. Sorting by warehou
SQL gaps and islands for consecutive activity days
The gaps-and-islands pattern groups adjacent observations into continuous runs. For daily activity, an island is a sequence of consecutive active dates for the same customer; a gap begins when the next active date is mor
SQL NULL values: why missing is not zero
In SQL, NULL marks an absent or unknown value. Zero is a known numeric value. Treating them as interchangeable changes the meaning of an analysis, even when the query continues to run.
SQL percentage change when the previous value is zero
Percentage change is (current - previous) / previous × 100. When the previous value is zero, the ordinary relative-change formula is undefined. A SQL query should preserve that distinction instead of replacing it with a
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