Never sure, only a score
- A paymentScore: 73out of 100Below the line of 80goes through
Picture your bank's computer looking at a card payment. It does not say yes or no. It says 73. The number is how much the payment looks like fraud, out of 100. Somebody at the bank drew a line: above 80, block it. Below, let it through. Every model you will meet works like this.
- False alarms12 a dayMisses10 a day
Made up and round. Line at 80, out of 1,000 payments with 20 frauds. Two mistakes are possible, and they are different. The model can block an honest payment. Call that a false alarm. Or it can let a fraud through. Call that a miss. Move the line up, and you get fewer false alarms and more misses. Move it down, the other way round.
- Score: 73Line: 80
The line is at 80. A payment scores 73. What happens to it?
It is blocked. 73 sounds like fraud.
Not yet. 73 is a score, not a verdict. The verdict comes from the line, and the line is at 80. Below it, the payment goes through.It goes through. 73 is below the line.
Right. The model only scores. The line decides, and 73 is under 80, so the payment is treated as honest, whether or not it was.The model asks a person.
Not yet. Nothing in the model asks anybody. It gives a number; the line turns the number into a yes or a no, every time, on its own. Move the line. Out of 1,000 payments a day, 20 are fraud. The figures are made up and round.
Honest customers blocked120 a dayFrauds let through1 a dayWhere the line is: 50
Line at 50. Only 1 fraud in 20 gets through, and 120 honest customers a day have their card refused.
Honest customers blocked70 a dayFrauds let through3 a dayWhere the line is: 60
Line at 60. 3 frauds through, 70 honest customers blocked.
Honest customers blocked30 a dayFrauds let through6 a dayWhere the line is: 70
Line at 70. 6 frauds through, 30 honest customers blocked.
Honest customers blocked12 a dayFrauds let through10 a dayWhere the line is: 80
Line at 80. 10 of the 20 frauds get through, and 12 honest customers are blocked.
Honest customers blocked3 a dayFrauds let through15 a dayWhere the line is: 90
Line at 90. 15 frauds in 20 get through. Only 3 honest customers a day are blocked. Nothing about the model changed between these frames; only where the line sits.
The bank moves the line from 80 down to 60. What happens?
Fewer frauds slip through, and far more honest customers get blocked.
Right. Misses fall from 10 to 3; false alarms rise from 12 to 70. The line trades one mistake for the other. It cannot shrink both.Nothing changes for honest customers. Only frauds are affected.
Not yet. Honest payments have scores too, and some score above 60. At a line of 60, 70 of them a day are blocked, against 12 at a line of 80.Both mistakes fall. A lower line catches more.
Not yet. A lower line catches more of everything, frauds and honest payments alike. Misses fall and false alarms climb, from 12 to 70.Move the control.
- A false alarmcosts the customerA misscosts the bank
Who pays for each mistake is not the same person. A false alarm costs the customer: a card refused at the till, a call to the bank, a shop walked out of. A miss costs the bank, or the person whose card was stolen. Wherever the line sits, somebody chose who takes which mistake.
The bank moves the line up to 90. Tap the bar that grew.
Tap the part of the picture that answers it.
Honest customers blocked3 a dayFrauds let through15 a dayFrauds let through
Right. A high line lets most payments past, frauds included. Misses went from 10 to 15 while honest customers blocked fell from 12 to 3.Honest customers blocked
Not yet. This bar shrank. A higher line blocks fewer payments of every kind, so fewer honest customers are stopped, and more frauds get past.- The model gives a scoreA person draws the lineOne mistake is chosen over the other
The same shape sits under every machine decision in this course. A model about who gets a loan, who gets bail, whose CV gets read: each gives a score, and a person draws the line. The maths picks the score. A person picks which mistake happens more, and to whom.
Set how many frauds there are in 1,000 payments, and where the line sits. The figures are made up and round.
Honest customers blocked70 a dayFrauds let through2 a dayFrauds in 1,000 payments: 10 · Where the line is: 60
10 frauds, line at 60. 70 honest customers blocked; 2 of the 10 frauds let through.
Honest customers blocked20 a dayFrauds let through4 a dayFrauds in 1,000 payments: 10 · Where the line is: 75
10 frauds, line at 75. 20 honest customers blocked; 4 of the 10 frauds let through.
Honest customers blocked3 a dayFrauds let through8 a dayFrauds in 1,000 payments: 10 · Where the line is: 90
10 frauds, line at 90. 3 honest customers blocked; 8 of the 10 frauds let through.
Honest customers blocked70 a dayFrauds let through3 a dayFrauds in 1,000 payments: 20 · Where the line is: 60
20 frauds, line at 60. 70 honest customers blocked; 3 of the 20 frauds let through.
Honest customers blocked20 a dayFrauds let through8 a dayFrauds in 1,000 payments: 20 · Where the line is: 75
20 frauds, line at 75. 20 honest customers blocked; 8 of the 20 frauds let through.
Honest customers blocked3 a dayFrauds let through15 a dayFrauds in 1,000 payments: 20 · Where the line is: 90
20 frauds, line at 90. 3 honest customers blocked; 15 of the 20 frauds let through.
Honest customers blocked70 a dayFrauds let through8 a dayFrauds in 1,000 payments: 50 · Where the line is: 60
50 frauds, line at 60. 70 honest customers blocked; 8 of the 50 frauds let through.
Honest customers blocked20 a dayFrauds let through20 a dayFrauds in 1,000 payments: 50 · Where the line is: 75
50 frauds, line at 75. 20 honest customers blocked; 20 of the 50 frauds let through.
Honest customers blocked3 a dayFrauds let through38 a dayFrauds in 1,000 payments: 50 · Where the line is: 90
50 frauds, line at 90. 3 honest customers blocked; 38 of the 50 frauds let through.
There are 50 frauds in the thousand, and the line is at 90. How many frauds get through?
3. The high line blocks almost everything.
Not yet. 3 is the honest customers blocked, not the frauds. A high line blocks very little, which is why so few honest customers are caught and so many frauds get through.About 38 of the 50. A high line lets most frauds past, and there are many to let past.
Right. At 90 the line lets three quarters of frauds through, whatever their number. Three quarters of 50 is about 38.None. Frauds score high, and the line is high.
Not yet. Frauds do not all score high; that is why the model is not perfect. At a line of 90 most of them sit below it and go through.Move both controls.
- Pupils scoredThe lineFlagged for help
A school buys a model that scores each pupil's chance of failing the year and flags those above a line for extra help. The head wants to miss nobody who needs it. Which way does the line move, and what is the cost?
Up. A higher line is stricter, so it catches more.
Not yet. A higher line flags fewer pupils, not more. Stricter here means fewer flags, and fewer flags means more pupils in trouble go unflagged.Down. More pupils who were fine get flagged too, and the extra help is spread thinner.
Right. Missing nobody means catching more, which means a lower line. A lower line also catches pupils who did not need it: the false alarms of this model.It does not matter where the line goes. The model decides who needs help.
Not yet. The model gives a score. The line decides who is flagged, and the head is the person drawing it. Moving it is what decides who gets flagged.
Lesson complete
A model gives a score, not a yes. Where the line is drawn trades one mistake for the other.