Designers use rules-of-thumb to make an unfamiliar design problem easier to solve. Rules-of-thumb can help in making design decisions, doing fast conversions, making estimations. The conversion rule-of-thumb -- "A pint's a pound the world around" -- helps convert a volume of water into its weight. During a shipwreck, a 50-50-50 rule-of-thumb can be invoked: "50% of people can survive in 50°F water for 50 minutes." Web designers hold that screen fonts should be no smaller than 14-point type for readability. Mechanical engineers, when using bearings to support and help an axle or wheel rotate freely, will design using pairs of bearings (not 1 or 3).
Learning By Design asks students to conduct tests of their own designs, and from these experiments generate their own design rules-of-thumb. An initial rule-of-thumb might read "Vent holes increase parachute fall time." A better rule-of-thumb might say, "Vent holes up to 15% of total canopy area make a parachute's descent more stable, without a loss of drag." Students share their "design advice" via posters for others to read and review for accuracy and consistency with other classmates findings. Design rules-of-thumb connect the concrete world of practical to intermediate abstractions that link key design features to product performance.
They can be iteratively written and rewritten, just like any design. The best ones are short, memorable and easily applied to different situations.
Sometimes one design rule-of-thumb can conflict with another. In describing how parachutes fall, students may need to reconcile one rule-of-thumb that says, "More canopy results in greater drag and longer drop times", while another states, "Heavy things fall faster than light things." An improved rule-of-thumb that includes both might say, "Given the same canopy and drag force during falling, a heavier parachute system will fall faster then a lighter chute system."