Populations and Samples

1
Populations and Samples

• and Parameters and Statistics

2
Populations and Parameters

• A population is any large collection of objects
  or individuals, such as people, students, or
  trees about which information is desired.
• A parameter is any summary number, like an
  average or percentage, that describes the entire
  population.

3
Parameters

• Examples include population mean ?, the
  population variance ?2 and population proportion
  p.
• 99.999999999999. of the time, we dont (...or
  cant) know the real value of a population
  parameter.
• Best we can do is estimate the parameter!

4
Samples and Statistics

• A sample is a representative group drawn from the
  population.
• A statistic is any summary number, like an
  average or percentage, that describes the sample.

5
Statistics

• Examples include the sample mean ?, and the
  sample variance s2, and the sample proportion
  (p-hat)
• Because samples are manageable in size, we can
  determine the value of statistics.
• We use the known statistic to estimate the
  unknown parameter.

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Example

• An unknown percentage of Stat 250-like students
  at PSU have a tattoo.
• Use class as a representative (??) sample of the
  population of all Stat 250-like PSU students.
• Spring 98 12.0 had tattoos
• Fall 98 10.9 had tattoos
• Spring 99 7.7 had tattoos