I was recalling the data visualisations done by u/zonination in 2015 on reddit which was building from government research by Sherman for the CIA. Pulling apart the 2019 survey data we can show the percentage probability that respondents assigned to various words. I have re-ranked them most to least probable.
Here the range on each bar is the inter quartile range - 50% of responses fall within this range. Some words are narrowly understood - about even means 50% with no range for most people. Some have a broad range meaning they are less clear - we believe and we doubt being to good examples with a 20% range.
Taking these and combining with the guidance from the 5e DMG that the DC of a 'about even' task should be DC15 we end up scaling all this to have a maximum at DC 30. This fits exactly to the bounded accuracy system.
What is very interesting to me is the steady trend of probability descriptions from 'certain' to 'even chance' and then a jump to 'probably not'. All is well until you hit 50-50, then once things go past that it immediately jumps to 1-in-3 and gets worse from there. Here are the ranges expressed as DC for 5e.
|Probability Word||High DC||Mid DC||Low DC|
|Almost No Chance||31||30||29|
|Chances are Slight||28||26||25|
|Better than Even||14||13||13|
|Very Good Chance||8||6||4|
I am not convinced this is more useful than our previous example so for comparison, here is that rescaled for 5e also.
|Descriptor||% span||d20 span||DC|
|Almost no chance / remote||1 to 5||1||30|
|Very unlikely / highly improbable||5 to 20||4||25|
|Unlikely / improbable (improbably)||20 to 45||9||18|
|Roughly even chance / roughly even odds||45 to 55||11||15|
|Likely / probable (probably)||55 to 80||16||7|
|Very likely / highly probable||80 to 95||19||3|
|Almost certain(ly) / nearly certain||95 to 99||20||1|