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How is random sampling different from random assignment?

A study can choose people at random, assign them to groups at random, do both, or do neither. Those choices answer different problems: who the findings may describe, and how fairly the study compares conditions.

3–5 min

Questions you could explore

  • Which step helps with representativeness?
  • Which step helps separate a treatment effect from pre-existing group differences?

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Maths and statistics

When one long wait changes the average

An average wait can sound straightforward until one person waits much longer than everyone else. The mean and median offer two ways to summarize that same group. Their different responses to an unusually large value raise a practical question: what do we want an average to tell us?

5–7 min

When one long wait changes the average

Maths and statistics5–7 minAn average wait can sound straightforward until one person waits much longer than everyone else. The mean and median offer two ways to summarize that same group. Their different responses to an unusually large value raise a practical question: what do we want an average to tell us?

Why does the Gregorian calendar sometimes skip a leap year?

Maths and statistics2–3 minThe familiar 'every four years' rule has exceptions. The Gregorian calendar treated 1900 and 2000 differently because its century rules refine how calendar years track the solar year.

Maths and statistics

What does a poll’s margin of error leave out?

A poll's neat plus-or-minus figure can make its uncertainty look more complete than it is. Sampling variability is only one source of error; wording and who takes part can also affect the result.

4–5 min

What does a poll’s margin of error leave out?

Maths and statistics4–5 minA poll's neat plus-or-minus figure can make its uncertainty look more complete than it is. Sampling variability is only one source of error; wording and who takes part can also affect the result.

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