Ninety per cent sure
How many minutes in a week? Give a low and a high you are 90% sure hold it. A range that holds the truth scores; a narrow one scores more.
How it plays
Every quantity can be worked out from definitions — minutes in a week, inches in a mile — but you are asked for a range, not a number: the habit of saying how sure you are.
A range that misses scores nothing; one that holds the truth scores more the narrower it is. Across many questions, about nine in ten of your ranges should hold.
Keys
Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.
What it trains
This practises giving honest ranges for quantities you can reason about. It claims nothing beyond that habit.
Learn
How to get better at ninety per cent sure: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.
Your approach to each question
- Break the quantity into its definitions: weeks into days, days into hours, hours into minutes; miles into feet into inches.
- Multiply it out to a best estimate, rounding only where you have to. Note which factor you are least sure of.
- Set the range around your estimate: tight if every factor was exact, wider by about as much as your shakiest factor could be off.
- Type the low and the high. The range must hold the truth to score at all, and scores more the narrower it is — so never pad it “just in case” beyond what your doubt really is.
Techniques and tricks
Know the building blocks
A day is 1,440 minutes and 86,400 seconds; a week is 10,080 minutes; an hour is 3,600 seconds; a year of 365.25 days is 8,766 hours; four years with a leap day are 1,461 days.
e.g. Minutes in 3 weeks: 3 × 10,080 = 30,240
The unit facts
1 mile = 5,280 ft = 63,360 inches. 1 pound = 453.59 g. 1 MiB = 1,024 × 1,024 = 1,048,576 bytes. 1 acre = 43,560 ft², about 4,047 m². 1 US gallon = 231 in³, about 3,785 ml.
e.g. Grams in 5 pounds: 5 × 453.59 ≈ 2,268
How the score works
A range that holds the truth scores 1 minus a quarter of its width as a share of the true value; it counts as right if it holds the truth and is no wider than twice it. A miss scores nothing.
e.g. Truth 30,240: 29,000 to 31,500 scores about 0.98; 20,000 to 40,000 about 0.83
Widen by your weakest factor
If one factor is a guess — say you remember a pound as “about 450 g” — make the range wide enough to cover that guess being a few per cent off, and no wider.
e.g. 5 pounds with “450 g”: 2,250 is the estimate; 2,150 to 2,350 covers a 4% error
Check the size first
Count the digits before you type. The worst slips are factor slips — a missing × 60 or × 24 — that put you out by ten times or more, and no width of range rescues those.
As the levels climb
- There are three levels and the ladder stops there. The first asks for everyday durations: hours in days, minutes in weeks, seconds in hours.
- The second adds units and calendars: inches in miles, grams in pounds, days in years with leap years, bytes in MiB. Here the risk is a fact you half remember, so widen for that factor only.
- The third adds big products: seconds in days, square metres in acres, millilitres in US gallons, and the hours someone has lived by a birthday. Each is a chain of three or four factors; multiply carefully and keep the range tight.
Traps to avoid
- Too narrow on a shaky fact: a range of a few units around a guess you were not sure of. If the factor could be off, the range must cover it, or you score nothing.
- Too wide to count: a range more than twice the true value holds the truth but is not counted right, and scores little.
- MiB worked with 1,000 × 1,000. A mebibyte is 1,024 × 1,024 bytes, nearly 5% more — enough to fall outside a tight range.
- Days in years without the leap days. The question says “with leap years”: one extra day in every four years.
- Birthdays worked with 365 days a year. The game uses 365.25, the average with leap years: a 30th birthday is 30 × 8,766 = 262,980 hours.
How to practise
- Set Feedback to review “At each judgement”, then count in the review how many of your ranges held. About nine in ten is the target; all of them means you can afford to go narrower.
- Learn the building blocks and unit facts by heart. Every quantity here is exact, so knowing the factors turns guesses into narrow, safe ranges.
- Turn on Time pressure at “Normal” once the factors are known. Thirty seconds is enough to multiply out, not enough to dither.
- After a miss, compare your estimate with the working line: if your estimate was right and the range missed, it was too narrow; if the estimate was wrong, find the factor.
Levels
It is untimed unless you turn the clock on. There are 3 levels with the default settings, and the counter stops at the real top. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.
All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.
More probability
All games →The reversal
A treatment can win in every group and still lose overall. Read results by group and in total, and work out which comparison to trust.Dice, coins and cards
The chance of a 7 with two dice, of at least one six in four rolls, of two aces in a row — counted out exactly, and given as a fraction.Base rates
A test catches 90% of cases and wrongly flags 9% of the rest; 1% have it. You test positive — what is the chance it is real? Count it out.Expected value
What a bet is worth on average: a paid spin, a fair price, a raffle ticket, an insurance policy, a wheel's house edge, and which of two bets is better.How sure are you?
A quick claim — 17 × 23 = 391, 1 mile is more than 1.5 km — and seven taps for how sure you are it is true. Scored so honesty pays.