When a product has broad usage profiles how do you create one for a test protocol. Do you select the easiest? Obviously not because there is risk of a high failure rate from poor evaluation. Do you select the hardest use-case? You could but there is the risk you spend too much time and money making the product overly robust for most of your customers.
In these cases where test time is limited and only a single use case can be tested I create a “Composite” use-case. It captures many of the different types of stresses from the full list of use-cases but keeps an accumulated stress on the design that represents at least 98% of the customers.
Let’s say I have to create a use-case for test for a new vehicle. All driver types have been characterized for factors like driving duration, start stops, aggressive acceleration and braking.
- Use Case 1: We have the “Old lady” use-case. Does low mileage, accelerates and brakes very carefully. Sometimes doesn’t get used for days because she forgot where she parked it and takes the bus home.
- Use Case 2: “Punk Ass kid”, parents bought him the car. He has no respect for the vehicle, wants to impress his friends and his brain is completely pickled by hormones. This menace to society does burn-outs, prides himself on doubling the speed limit, has to slam on the brakes to not hit just about everything. Didn’t know you are supposed to do oil changes and will drive across three states on a Friday night just because his best friends cousin said he will buy them beer and he knows some college girls.
- Use Case 3: “Soccer Mom”, does some highway driving, does some city driving. 10k mile a year, sticks to the maintenance schedule mostly because the waiting room has windows into the garage and you can watch the “mechanics” work. “Take it easy there 50 Shades”
So what is the reliability test use-case that should be used?
I would be comfortable making reliability statements on what we can call “The mid life crisis” use-case. It’s a 50 year old dude who finally bought that Mustang he always wanted. He mostly drives it to work on the highway, does start/stop driving when he has to drop off and pick up the kids, meets up with his buddies on Saturday’s and does a burn-out when he leaves their house because he now has to put sunscreen on his head when he mows the yard.
That use-case probably has the range of stresses that cover over 98% of the vehicle owners but won’t drive design changes that make the product overly robust or late time to market.
A piece of advice: Read the room first if you plan to use this example when making a “use-case” pitch in a meeting.
-Adam
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