I'm exploring the relationship between buying $5 coffees at Starbucks and being extremely wealthy. Since common popular media has told me that I, too, could be a millionaire if I just stopped buying my coffee from coffee shops, I figure it is worthwhile to explore the validity of the claim.

A friend who happens to know a few millionaires says "I know a few millionaires and they don't buy their coffee from coffee shops. They make it at home! So, it must be the case that making coffee at home (and using many other penny-pinching tactics) causes people to be extremely wealthy."

Which common inferential error we discussed in class best describes the error that my friend is making?

Question

I'm exploring the relationship between buying $5 coffees at Starbucks and being extremely wealthy.  Since common popular media has told me that I, too, could be a millionaire if I just stopped buying my coffee from coffee shops, I figure it is worthwhile to explore the validity of the claim.

A friend who happens to know a few millionaires says "I know a few millionaires and they don't buy their coffee from coffee shops.  They make it at home!  So, it must be the case that making coffee at home (and using many other penny-pinching tactics) causes people to be extremely wealthy."

Which common inferential error we discussed in class best describes the error that my friend is making?

BlackTom AI · Accepted Answer

The question asks us to identify the inferential error in the friend’s reasoning about wealth and coffee-buying habits.
Option: 'Selecting on the DV' — this choice points to a classic bias in which you condition your analysis on the dependent variable, effectively sampling based on the outcome you’re trying to explain.
To see why this fits, consider what the friend did: they looked at a group of millionaires and noticed they don’t buy coffee from shops, then concluded that making coffee at home somehow causes extreme wealth. By selecting only millionaires (the outcome of interest, i.e., a high wealth level) and then inferring a causal factor (home-made coffee), they’ve conditioned on wealth itself. This can create a spurious pattern because the sample is not representative of the broader population of all people, and it ignores alternative explanations (e.g., wealth enabling expensive at-home setups, preferences among different socioeconomic groups, or third variables that influence both wealth and coffee habits).
Other potential errors you might think of include correlation vs. causation, post hoc reasoning, or ecological fallacy. However, in this particular scenario the key flaw is that the sample is drawn based on the dependent variable (wealth), which biases any inference about what causes wealth—the essence of selecting on the DV.
In short, the friend’s reasoning is an instance of selecting on the DV, because they conditioned their analysis on a subset defined by the outcome of interest (wealth) and then inferred a causal claim about coffee habits.

FA25_QTM_110_1 Quiz #3: FPoCI and Selecting on the DV

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