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Always ask 'out of which group?' before you divide.

In your head, jot if needed · no calculator why?

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Builds on: Probability Basics (not open yet)

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.

• Rate within a group par 14 s not tried yet

  worked example

  There are 260 leads. 40 are referrals, and 7 of the referral leads buy. What is the conversion rate among referrals?

  Answer: 17.5%

  1. "Among referrals" means divide by referrals: 7 ÷ 40 = 17.5%.
  2. Dividing by all 260 leads would answer a different question.

• Share of buyers from a group par 21 s not tried yet

  worked example

  Leads from trade shows: 105 buy, 105 don't. Leads from the website: 35 buy, 115 don't. What share of all buyers came from trade shows? Round to one decimal place if needed.

  Answer: 75%

  1. Now the group is "buyers": 105 ÷ (105 + 35) = 105 ÷ 140 = 75%.
  2. That's a different question from "what share of trade-show leads buy" (105 ÷ 210 = 50%).

• Flagged: how likely is it real? par 30 s not tried yet

  worked example

  Of 10,000 orders, 200 are fraudulent. A filter flags 90% of fraudulent orders and 2% of honest orders. If an order is flagged, what is the chance it's fraudulent? Round to the nearest whole percent.

  Answer: 48%

  1. Flagged fraud = 90% of 200 = 180; flagged honest = 2% of 9,800 = 196.
  2. Among the 376 flagged orders: 180 ÷ 376 ≈ 47.9%, about 48%. When the bad group is small, false alarms pile up.

• Which denominator? par 10 s not tried yet

  worked example

  You want the no-show rate among online bookings. What goes in the denominator?

  1. All bookings
  2. All online bookings
  3. All no-shows
  4. Online bookings that were no-shows

  Answer: All online bookings

  1. The group after "among" is the whole you divide by: all online bookings.
  2. Only the ones that were no-shows go on top.

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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