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Biased samples, false causes, mismatched definitions, and misleading averages.

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skills · practice stats

From rounds of this course only: box, review and trial answers are left out. Once a skill has 40 tries, it compares your first 20 tries with your last 20.

• Who's missing from the sample? par 14 s not tried yet

  worked example

  To judge product quality, you read only the 500 five-star reviews. Is that sample representative?

  1. Yes — a big enough sample fixes it
  2. No — samples can never be trusted
  3. Yes — five-star reviewers know the product best
  4. No — unhappy reviewers are missing

  Answer: No — unhappy reviewers are missing

  1. Unhappy reviewers are missing, so the sample leans one way.
  2. Who gets into the sample matters more than how many: a huge sample that leaves a group out is still wrong.

• Cause or coincidence? par 15 s not tried yet

  worked example

  Staff with more training make more errors, but they are given the hardest jobs. Have we isolated the training's effect?

  1. No — training must cause the errors
  2. Yes — the difference is big enough to prove it
  3. Yes — the two moved together, so one caused the other
  4. No — they get harder jobs, so we can't tell what the training did

  Answer: No — they get harder jobs, so we can't tell what the training did

  1. They get harder jobs, so we can't tell what the training did.
  2. Two things moving together can come from a third cause, a reversed cause, or chance. You need a comparison that changes only one thing.

• Same name, same number? par 15 s not tried yet

  worked example

  One report counts orders placed; another counts orders delivered. Can you compare them as the same "orders" number?

  1. Yes — just average the two reports
  2. No — always use whichever is bigger
  3. No — they measure different things; reconcile the definitions first
  4. Yes — both are called orders

  Answer: No — they measure different things; reconcile the definitions first

  1. They measure different things: cancelled and pending orders separate the two.
  2. Before comparing, check what's counted, the period, the source, and the accounting basis.

• What's a typical value? par 12 s not tried yet

  worked example

  Job values: $80, $200, $280, $1,690, $1,790. Which number best describes a typical job?

  1. Either — the mean and median are about the same here
  2. The largest value
  3. The median — one or two big jobs pull the mean up
  4. The mean — it always describes the typical job best

  Answer: The median — one or two big jobs pull the mean up

  1. Median $280 versus mean $808: the big jobs drag the mean up, so the median better describes a typical job.

rest ladder

Win 3 of your last 4 rounds and the course rests. A win is 90% right, within 2× the round's par. Pass the review when it comes back and the next rest is longer.

1. 1 day
2. 3 days
3. 7 days
4. 14 days
5. 30 days
6. 60 days
7. mastered · every 90 days

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