Probability
Probability measures how likely something is, from 0 (never) to 1 (certain). Favorable outcomes ÷ possible outcomes: rolling a 6 on a die is 1/6.
Select any words here to ask your Maestro about them.
You roll a die 20 times and get a 6 five times: 25%, more than you expected. If you keep going to 2,000 rolls, what do you think happens to the share of sixes?
How to calculate it: if every outcome is equally likely, divide the outcomes you care about by all the possible outcomes. With real data, you estimate it with the relative frequency: if 30 of 200 customers came back, the chance one comes back is about 15%.
How to read it: 0 is impossible, 1 is certain, 0.5 is a coin flip. You can write it as a fraction (1/6), a decimal (0.17) or a percentage (17%).
With few tries, what you see can land far from the probability, as in the figure. That's why a percentage from a handful of cases deserves caution.
Practice it in If the system catches 90% of frauds, what does an alert mean?.
How it's used
How to calculate it
Favorable outcomes ÷ possible outcomes
- Rolling a 5 or 6: 2 ÷ 6 = 0.33, or 33%Works when every outcome is equally likely.
- marked: A customer coming back, if 30 of 200 did: 30 ÷ 200 = 0.15, or 15%With data, you estimate the probability with the relative frequency.
- Not coming back: 1 − 0.15 = 0.85, or 85%The chance something doesn't happen is 1 minus the chance it does.
When to look closer
Two likely things together are less likely
Two suppliers, each on time 90% of the time
- Supplier A on time: 0.9
- Supplier B on time: 0.9
You need both to show up on time. How often do you think that happens?
Ask your Maestro about Probability
It answers from the free course page closest to this word, the way it answers while you study.
You: What does "Probability" mean, in plain words?
Your Maestro: I'll explain it with a case of yours: it sticks better than a definition. What probability do you use at work without calling it that?
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Related terms
- Percentage
A percentage says how many parts out of every 100 something is. You get it by dividing the part by the total and multiplying by 100: 45 out of 180 customers is 45 ÷ 180 × 100 = 25%.
- Frequency table
A frequency table counts how often each value shows up (absolute frequency). Relative frequency is that count over the total: 5 of 10 = 50%.
- Mean (average)
To find the mean (the average), add up all the values and divide by how many there are: 15, 7, 8, 13 and 7 add up to 50, and 50 ÷ 5 = 10.