Repertoire Buying Behaviour

Repertoire Buying Behaviour is the empirical reality that consumers in almost all categories do not buy exclusively from one brand. Instead, they maintain a 'repertoire' of acceptable brands and switch between them over time. This 'polygamous loyalty' is the norm, not the exception. The Dirichlet model mathematically predicts this behavior, showing that even the most 'loyal' customers of a brand also purchase from competitors. For marketers, this means the goal isn't to create 'brand love' or exclusive loyalty—which is statistically rare—but to ensure your brand is part of the consumer's mental repertoire and is physically available at the point of purchase. Growth is driven by increasing the number of people who include you in their repertoire, rather than trying to increase the 'share of wallet' of existing buyers.

The concept of Repertoire Buying Behaviour was pioneered by Andrew Ehrenberg and his colleagues at the London Business School, eventually forming the cornerstone of the Ehrenberg-Bass Institute for Marketing Science. While early marketing theory (and many modern textbooks) assumed that consumers were either 'loyal' or 'switchers,' Ehrenberg’s research in the 1950s and 60s revealed a different pattern. The definitive formalization came with the development of the NBD-Dirichlet model of brand choice, most notably detailed in the landmark paper 'The Dirichlet: A Comprehensive Model of Buying Behaviour' published in the Journal of the Royal Statistical Society (Goodhardt, Ehrenberg, & Chatfield, 1984). This work proved that brand choice is a stochastic (probabilistic) process, not a result of deep-seated psychological commitment.

The mechanism behind repertoire buying is rooted in both cognitive psychology and statistical probability. Humans are cognitive misers; we don't want to spend precious mental energy evaluating every single option every time we buy toothpaste or cloud software. Instead, we develop a 'repertoire'—a subset of brands we've tried and found acceptable.

Mathematically, this is explained by the Dirichlet model. The model assumes that: 1. Consumers have different long-run 'propensities' to buy from the category (some buy often, some rarely). 2. Consumers have different long-run 'probabilities' of choosing a specific brand within that category.

Crucially, these choices are independent of the previous purchase. If you bought Brand A last time, your probability of buying Brand B next time remains constant based on your long-term repertoire. This leads to 'polygamous loyalty.' A consumer might buy Brand A 60% of the time, Brand B 30% of the time, and Brand C 10% of the time. They aren't 'switching' in the sense of leaving Brand A; they are simply fulfilling their repertoire.

Psychologically, this is driven by 'Mental Availability.' A brand enters the repertoire through distinctive assets and consistent memory building. Once in the repertoire, the specific choice at the shelf or screen is often determined by 'Physical Availability'—is the brand there, and is it easy to buy? The 'Sole Loyalist' (someone who buys only one brand) is almost always a 'Light Buyer' of the category. They look loyal only because they've only made one purchase in the data window. As soon as they buy more frequently, they inevitably become repertoire buyers.