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Psychology 290 Lab z-tests & t-tests March 5 - 7, 2007

Psychology 290 Lab z-tests & t-tests March 5 - 7, 2007. z-test One sample t-test SPSS – Chapter 7. z-test. Used when the population standard deviation ( s ) is known. Used to assess whether the sample came from a particular population or not.

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Psychology 290 Lab z-tests & t-tests March 5 - 7, 2007

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  1. Psychology 290 Labz-tests & t-testsMarch 5 - 7, 2007 • z-test • One sample t-test • SPSS – Chapter 7

  2. z-test • Used when the population standard deviation (s) is known. • Used to assess whether the sample came from a particular population or not. • Critical values derived from the same z-table from when we calculated percentiles • Critical value depends simply on significance level and one-tail or two tail design.

  3. z – test equation

  4. z-test hypothesis testing • Alpha level typically set at 0.05 • Use z-table to determine the z value associated with the chosen significance level located “beyond z” • For a one-tail test, zcrit = + 1.64 • For a two-tail test, zcrit = + 1.96 • To reject the null, zobt must land in the region of rejection (either less than –zcrit or greater than + zcrit)

  5. One vs. Two Tail • One Tail Hypothesis Two Tail Hypothesis or 1.64 -1.96 1.96 -1.64

  6. One Sample t-test • Used when the population standard deviation is not known. • Use the sample standard deviation to calculate an estimate of s and the standard error. • Distributions is standardized but does not form a normal curve unless the sample size is 120 or more. • Critical values are determined by degrees of freedom and using Student’s t-table.

  7. t-test equation Degrees of freedom df = n - 1

  8. t-test hypothesis testing • Use degrees of freedom, significance level, and one or two tail design to determine tcritical • To reject the null hypothesis, | tobtained | must be greater than, or equal to, tcritical

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