Always Buy Two (or More)?

Introduction

When I go to the store to buy something, I usually buy two of whatever it is, whether it is a jar of mayonnaise, a gallon of milk, or a pair of socks. My main motivations are to avoid running out of the item and, over the long run, to cut shopping time in half. Besides, if you like an item, by the time you want a second one, often that particular brand, style, or model is often no longer available. (I'm having that problem just now about socks.) However, in the case of apparel that naturally comes in pairs, such as shoes, socks, and gloves, theremay be an added bonus to multiple purchasing in terms of achieving greater use. Here we model the situation with common failure models and attempt to quantify that benefit.