When a product is new, there are a lot of failures, but these lessen as repairs and refinements are made. These early failures are often referred to as the infant mortality rate, though they have nothing to do with actual babies.

The number of failures eventually levels out and consists of those caused by random accidents and general wear and tear. This is called the incidental failure rate.

When products have been used over a long period, they begin to fail due to age. This is called the end of life rate.

When the early failure rate is charted, it is a downward sloping line showing the initial high number of failures dropping.

When the incidental failure rate is charted, it is a straight line, since the rate is neither increasing nor decreasing.

When the end of life failure rate is charted, it is an upward sloping line as more and more parts wear out and failures increase.

The combination of these three rates — a downward slope, a flat line, and an upward slope — form what looks like the cross section of a bathtub, hence its name: the bathtub curve.

Let’s say you buy a new car. You’ve had your car a month and there’s suddenly a problem with the transmission or the brakes or the steering. You take it to the dealer and they fix it for free because it’s covered by a warranty. Why is there a warranty? Because of the bathtub curve. Because when a product is new it tends to suffer some failures. Car manufacturers know this, and to preserve the goodwill of customers, they fix any initial failures at no charge.

Though your new car is covered by a warranty, it’s a hassle to have to get it worked on. Ditto for your new computer, refrigerator, washer and dryer, and other major purchases. When a new product breaks, the reviews can be vicious and sales can suffer. Therefore car makers (and computer makers, appliance makers, etc.) strive to increase quality so as to reduce the number of initial failures. Some take this more seriously than others, and some are better at it.

There is joy in buying new, but there can be wisdom in buying used. If you buy something that has made it past the initial failure phase (someone else went through the hassle of using the warranty to get those early breakdowns fixed), but is early in the incidental failure phase, you stand a good chance of it having a long, healthy life.

However, if you buy something that is too far into the incidental phase and too close to the end of life phase, you may question the wisdom of buying used.

Products don’t come with a bathtub curve so you could see at a glance if it looks like a wading pool, a bathtub, or a deep, narrow swimming pool. The best you can do is know the curve exits, look for reviews of all three phases, and draw your own.

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