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Glacius
StatisticsConcept reference

Populations and samples

A target population is the group a study aims to describe, and its observed sample is the group from which the study collected data.

1 min read1 figureSources & further reading
On this page 7 sections
  1. Overview
  2. The target population is the group a study aims to describe
  3. A collection may also contain records outside the target
  4. Convenient data does not change the study’s stated goal
  5. Key takeaway
  6. Sources & further reading
  7. Concept connections

01The target population is the group a study aims to describe#

The target population is the group a study aims to describe. The observed sample is the group actually measured. Specify who belongs and any relevant time window.

Suppose the target is records a,b,c,d,ea,b,c,d,e, but only b,db,d are observed. All five remain in the target; the sample contains b,db,d.

The target population includes records a,b,c,d,e. An inner observed-sample box contains b and d. Records a,c,e are target members without observations in this sample. Areas are schematic and do not encode representativeness.The target population includes records a,b,c,d,e. An inner observed-sample box contains b and d. Records a,c,e are target members without observations in this sample. Areas are schematic and do not encode representativeness.
Figure 1The target population includes records a,b,c,d,e. An inner observed-sample box contains b and d. Records a,c,e are target members without observations in this sample. Areas are schematic and do not encode representativeness.
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Check your reasoning

Study goal: all in class. Observed: 6 surveyed. Population / sample?

  1. AAll / six.
  2. BSix / all.
  3. CSix / six.
Show answer and explanation
All / six.

Target all; observe six.

02A collection may also contain records outside the target#

A collection may also contain records outside the target. If the study uses measured target records, keep only rows that meet both conditions. Here a flag of 11 means yes and 00 means no.

Check your reasoning

Target: eligible. Sample: measured target. (eligible, measured; 1=yes): a(1,0), b(1,1), c(0,1). Population / sample?

  1. A{a,b} / {b,c}
  2. B{a,b} / {b}
  3. C{b} / {a,b}
Show answer and explanation
{a,b} / {b}

Eligible a,b; sample b.

03Convenient data does not change the study’s stated goal#

Convenient data does not change the study’s stated goal. Morning visitors can be a sample for a study of all visitors. Having their data does not establish that they represent everyone.

Check your reasoning

Study goal: all app learners. Observed: 9 volunteers. “Population=9.” Population / sample?

  1. ANine / all.
  2. BNine / nine.
  3. CAll / nine.
Show answer and explanation
All / nine.

Nine were observed.

Key takeaway

Use the study goal to identify the population and the actual observations to identify the sample. Available data alone does not establish representation.

  • Identify the target and observed groups.

Sources & further reading

  1. [1]

Reference this concept

Link to this page, a section, or an individual figure.

Glacius. “Populations and samples.” Math behind ML. /learn/s-population