90-50-10 rule

The 90-50-10 rule is a heuristic stating that the ideal conversion for a bonus should be 90% for the easy part, 50% for the medium, and 10% for the hard.

These numbers are intended to reflect how a bonus plays with the field at its target difficulty. A set may have unpredictably different conversion if played by teams that are too strong (or too weak).

90% easy conversion

Though different sources ping this to other values like 85%, the most important aspect is remembering that an easy part should converted by almost (but not exactly) 100% of teams. A bonus whose easy part that is converted by 100% of teams is bad because it is easier than others in the set - a set where every easy part is converted 100% is bad because there was no point in asking the easy parts at all.

There needs to be a possibility that a team will not know the information in an easy part. Players should always be required to know a concrete fact about the answer, with the floor raised at each higher difficulty. At novice difficulties "touch your ass" clues like "this is the color of the sky" can be forgiven but otherwise make conversion artificially high.

50% medium conversion

This is likely the hardest mark to successfully hit, making it one of the most important ways to control the perceived difficulty of a set. Most teams will convert almost all easy parts and almost no hard parts but will expect to get or be close to getting the majority of medium parts - if medium parts are consistently harder than usual then it will be noticed by almost everyone.

An important reference point is the powermark of a tossup. Most editors would expect less than half of players to convert a question in power - this is borne out by buzzpoint data. Therefore, a medium part cannot exclusively use power clues if it aims to achieve 50% conversion.

10% hard conversion

Hard part conversion is likely the least impactful on the overall performance of the field as it is primarily used to differentiate the top teams - however, even in those cases medium part conversion can be more significant.

Like a lead-in these parts should ideally be written with a concrete means by which a player could know the answer in mind. Like medium parts, it is necessary to make these easier than the median power clue.