Three ways your dashboard can be correct and still lie
Our dataset said the average loan was 2.3 million kroner. The number that actually mattered was 255,000. Both were correct. Only one of them was true. This is a writeup of three ways a dashboard can be arithmetically perfect and still lie, using real figures from an analysis of 1,000 Norwegian debt consolidation applications. If you build reporting for anyone, you have probably shipped at least one of these. 1. Summing a field that contains two different things A debt consolidation loan pays off your expensive credit card debt. It also, if you own property, rolls your existing mortgage into the same new loan. Same column in the database. Same loan_amount . Utterly different meaning. SELECT AVG ( loan_amount ) FROM applications ; -- 2,300,000 That query is right and the answer is useless. Of that 2.3 million, roughly 1.9 million is an existing mortgage being moved from one lender to another. The expensive debt, the part the customer actually has a problem with, averages 255,000 . So the headline figure overstates the thing you care about by a factor of nine. Nothing in the schema warns you. loan_amount is a number, AVG is a function, the result renders fine. The bug is that one column is holding two concepts and only a human who understands the domain will notice. -- what you actually wanted SELECT AVG ( unsecured_debt ) FROM applications ; -- 255,000 If a column can mean two things depending on another column, split it. Every time. 2. Reporting the mean when the distribution has a tail Income in this dataset runs from ordinary salaries up to about five million kroner. A handful of very high earners drag the mean upward: Mean income: ~635,000 Median income: 647,000 for homeowners, 550,000 for renters Look at what happens there. The mean sits between the two medians and describes neither group. Someone reading only the mean concludes the typical applicant earns 635,000. Nobody earns 635,000. It is an artefact. df . groupby ( ' housing ' )[ ' income ' ]. agg ([ ' mean