One of the most common and most expensive errors in individual investing is the conflation of product admiration with investment merit. The investor who loves a product naturally gravitates toward the company that makes it—the coffee drinker who buys the coffee chain, the technology enthusiast who buys the hardware manufacturer, the electric vehicle owner who buys the automaker. This impulse feels like informed investing; it is grounded in direct personal experience with the product, which seems like a form of due diligence. It is, in fact, a category error that substitutes familiarity and affinity for the analysis that investment decisions actually require.
The reason product quality does not translate reliably into investment quality is that what determines investment returns is not the absolute quality of a business but the relationship between that quality and the price at which the business can be purchased. A business that makes a genuinely excellent product, that has a large and loyal customer base, and that is widely admired as a consumer brand is typically a business that has already attracted substantial investor attention, commands a premium valuation, and therefore offers a prospective return that may be entirely inadequate despite—or because of—its evident quality. The best businesses are not always the best investments; frequently they are among the worst, precisely because their quality is so thoroughly understood and priced in.
The consumer bias in investment selection also leads to systematic concentration in particular sectors. The individual who selects investments based on products she personally uses will find her portfolio skewed toward consumer discretionary companies, technology platforms, and retail brands—the businesses whose products are most visible in daily life—and underrepresented in the industrial, financial, energy, and materials companies that constitute a significant fraction of the economy but generate few consumer-facing touchpoints. This is not inherently wrong, but it is a concentration that is driven by familiarity rather than by any analysis of where value is available.
There is a further problem with using personal product experience as investment due diligence: the investor's individual experience is a sample of one, and it may not be representative of the broader market. The investor who personally loves a particular product and assumes that her enthusiasm is representative of the market may be correct, but she has no systematic way to verify this. More importantly, even if the market broadly shares her enthusiasm, the question of whether this enthusiasm is adequately reflected in the price—or whether it is already in excess—requires analysis that personal product experience cannot provide.
The discipline of separating product assessment from investment assessment requires the investor to ask, after forming a view about a product's quality: what would this business need to earn, grow, and return to its shareholders in order to justify its current market price? This question is not answered by examining the product; it is answered by examining the income statement, the balance sheet, the competitive dynamics, and the long-run economics of the industry. The investor who consistently performs this analysis—regardless of how compelling the product experience is—will find that many businesses she admires as consumers are businesses she would pass on as investors, and that many of the most attractive investments are in companies she has never personally encountered as a customer.