Yardsticks

In 1951, CIA analysts unanimously agreed a Soviet invasion of Yugoslavia was a “serious possibility.” When Sherman Kent asked his colleagues what odds they’d had in mind, answers ranged from 20% to 80%: we’re not very good at thinking in numbers ¹.

The problem runs both ways. Vague words fail to map to numbers; that’s Kent’s case. But numbers also fail to map to intuition. A billion seconds means nothing to most people; “one career” means something to everyone. The original yardstick was Henry I’s arm: the distance from his nose to his outstretched thumb ². The story is probably legend, but it lasted centuries because a body is easier to picture than a number. The “yardsticks” below do the same thing; swap raw figures for a vacation, a Porsche, a pregnancy.

Money

• £1 ≈ 1 minute of median UK work (~£35k/year)
• $100k/year: $50/hr, $0.83/min (2,000 working hours)
• A coffee a day (£4): ~£1,500/year
• $10 tax on every webpage: US deficit (~$10 trillion)
• Imagining 10x your wealth is hard; imagining everything costing a tenth is easy. A Porsche 911 at $10,000.
• Bill Gates (~$50/second): picking $100 off the floor is a bad trade

Odds

• Full house in poker: 1 in 693
• Four-of-a-kind in poker: 1 in 4,164
• Struck by lightning: 1 in 576,000
• Winning a 6/49 lottery: 1 in 14,000,000
• Shark attack death: 1 in 300,000,000
• Same birthday in a room of 23: ~50% (most people guess far higher)
• Meteor hits your house: 1 in 182,000,000,000

Time

• A vacation (12 days): 1,000,000 seconds
• Working year: 260 days
• 10,000 hours (the mastery cliché): ~5 working years
• A career (30 years): 1,000,000,000 seconds
• Human lifespan: ~79 years
• Earth remains habitable: ~1.1 billion years
• Age of the Earth: ~4.5 billion years
• Age of the Universe: ~13.8 billion years

Speed

• Bullet travels 4cm: 1ms
• Cheetah runs 200m: 11s

Latency at Human Scale

If a CPU cycle were one heartbeat (0.5s):

• L1 cache: 1 heartbeat (0.5s)
• Branch mispredict: a yawn (5s)
• L2 cache: a long yawn (7s)
• Mutex lock/unlock: making instant coffee (25s)
• Main memory: brushing teeth (100s)
• Compress 1KB (Zippy): a TV episode (50m)
• Send 2KB over 1 Gbps: lunch to dinner (5.5h)
• SSD random read: a weekend (1.7d)
• Read 1MB from memory: a long weekend (2.9d)
• Datacenter round trip: a short holiday (5.8d)
• Read 1MB from SSD: two weeks off (11.6d)
• Disk seek: a university semester (16.5w)
• Read 1MB from disk: a pregnancy (7.8m)
• Packet CA → Netherlands → CA: a bachelor’s degree (4.8y)

Distance

• Sprint in 10 seconds: 100m
• Down the street: 1km
• London to Edinburgh: ~650km
• Earth to Moon: ~384,000km — about 10 laps of the equator

Data

• A page of text: ~2KB
• The whole of Wikipedia: ~22GB — fits on a cheap USB stick
• Human genome: ~1.5GB — smaller than a film download

Energy

• A banana: ~100 calories; also ~0.1 μSv of radiation (the “banana equivalent dose”)
• A Mars bar: roughly one hour of walking

Volume

• 1 cubic metre: a large wardrobe
• A litre of water: 1kg (the metric system’s party trick)

Quantities

• A school year group: ~300
• Ryanair 737: 189 seats
• Wembley Stadium: 90,000
• 1 second in 12 days: one part per million
• People in the UK: 67 million
• People on Earth: 8 billion

Correlation (r)

How much does knowing X help you guess Y? Square r to find out: r = 0.68 for height and weight means r² ≈ 0.46, so if you know someone’s height, you are almost halfway to their weight. Below 0.3 the relationship is weak; above 0.8, each variable nearly determines the other.

• 0.13: Income and happiness (within a country) ³
• 0.43: SAT total and college GPA ⁴
• 0.50: Parent-child height ⁵
• 0.68: Height and weight ⁶
• 0.76: Identical twins’ IQ, raised apart ⁷
• 0.86: Identical twins’ IQ, raised together ⁸
• 0.96: Self-reported vs. measured height ⁹

Reliability ¹⁰

Deaths

• One plane crash a week: ~10,000 deaths a year
• Deaths per day: 148,000
• Births per day: 296,400