Understand Tableau Concepts
How Tableau thinks
Every view is a query. Dimensions define partitions; measures aggregate inside them. Know discrete versus continuous, row-level versus aggregate calculations, and generated fields such as Measure Names, Measure Values, Number of Records, and Latitude/Longitude. Measure Names and Measure Values appear when multiple measures share an axis. Also know aliases, groups, sets, bins, hierarchies, and how parameters differ from fields: parameters are workbook-level constants, while calculated fields are computed per row or per partition.
The order of operations is the highest-value concept. A reliable working order is: extract filters, data source filters, context filters, dimension filters, FIXED LOD expressions, INCLUDE and EXCLUDE LOD expressions, measure filters, table calculations, then reference lines and trend lines. Totals and forecasts sit in later stages. This explains why the same filter changes different calculations differently.
Traps: assuming filter order is intuitive; forgetting that context filters change FIXED results; trying to feed a table calculation into an LOD expression, which is computed earlier; expecting a dimension filter to restrict a FIXED calculation; and confusing aggregate filters with row-level filters. Also remember that a parameter is not a field and does not appear in the data pane like a column.
Worked rule: when a number looks wrong, ask at which stage it is computed. Row-level calculation, aggregate, LOD, table calculation, and reference line are different layers. Test by removing filters one at a time, then by adding a context filter to see whether an LOD changes. That single diagnostic resolves most calculation mysteries.