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Population and sample

The population is the whole group you want to know about; the sample is the part of it you actually measure to estimate the rest.

Select any words here to ask your Maestro about them.

Your store has 400 customers. You ask the first 40 in on Saturday, and 28 say they'd recommend it. What share of the 400 would?

Would you recommend the store? Each dot is a customer; the faint ones didn't answer. Example data.

What it's for: you can almost never measure everyone: ask every customer, check every part off the line. You measure a sample and estimate the population from it. What you calculate from the sample, like its mean or a percentage, is an estimate; you almost never see the population's own number.

How to read it: when a poll says "62% of Americans", the population is Americans and the sample is the people they asked. Before you believe it, ask who they asked and how many. A huge sample isn't a guarantee either: see big data.

Practice it in How many decimal places should a percentage have?, where 17 answers out of 40 have to speak for a whole team.

How it's used

How to draw a sample in Excel

The 400 customers in A2:A401

  1. marked: =RAND()In B2, next to the first customer; copy it down to B401.
  2. Select A1:B401, then Data > Sort, by column BShuffles all 400.
  3. The first 40 rowsYour sample.
  4. df.sample(40, random_state=1)The same in pandas; random_state gives you the same sample every time.
RAND changes every time Excel recalculates: before you sort, copy column B and paste it as values.

When to look closer

STDEV.S or STDEV.P?

Your workshop's grades, in B2:B6

  1. marked: Grades: 6, 7, 8, 9 and 10

You have the grades for the 5 students in your workshop, and that group is all you care about. STDEV.S or STDEV.P?

Ask your Maestro about Population and sample

It answers from the free course page closest to this word, the way it answers while you study.

Example

You: What does "Population and sample" mean, in plain words?

Your Maestro: I'll explain it with a case of yours: it sticks better than a definition. Which group at work do you only know part of?

Asking about: Answers from the page "How do I show each group's total next to every row in SQL?"

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Related terms

  • 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.

  • Standard deviation

    Standard deviation measures how far data lands from the average: with an average of 30 and an SD of 2, most values fall from 28 to 32. In Excel: =STDEV.S.

  • 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%.

  • Big data

    Big data is data so large, fast or varied that it doesn't fit on one computer, so it's stored and processed across many machines.

The whole glossary