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26 - Lecture
Course: Statistical Methods (PSYC 201 A)
27 Documents
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University: Grove City College
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Z-Score cont.
Z score: 1.96 +- / 95% confidence (sometimes this is +- 1.645, but not at the same time)
5% error - seeking 95% confidence
+- 1.96 (two tailed); or +- 1.645 (one tailed)
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99% confidence: two-tailed 1% error z-score - 2.576 +-
One-tailed = 2.327
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What two z-scores define the lower 95% of the distribution? (or upper 5%)
~ +- 2.00 (more precise = +- 1.96
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What 2 z-scores cover the middle 99%?
+- 2.576 or ~ +- 1.6
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What is the probability that I will choose someone with an IQ > 145 (P(>145))
Z = X - mew (raw score - mean) / sigma/standard deviation
Z = 145 - 100 / 15 = 3.00
probability = 0.13% or 0.0013
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If you have an infinite number of scores, it is a population, not a sample
Textbook: Z = X-M / SD (sample) (Not right, it’s a "t" curve)
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Z - score associated with the 10th percentile?
Closer to -1. ~1.20 (z = - 1.28)
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What z score is associated with the 90th percentile?
(same question) z = 1.28
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What z score is associated with the 30th percentile?
Z = - 0.52
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… 70th?
Same, 0.52
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50th?
Median! Z = 0.00
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Percentiles: 100 part split
Decile: 10-part split (high school grades)
Quartile: divide the distribution into four parts
In percentiles:
We'll never refer to the 0th or 100th percentile
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Percentile ranks refer to people
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Percentiles refer to scores
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Figuring percentiles of small data sets:
What percentile is associated with a score of 46?
What percentile is associated with a score of 34?
What percentile is associated with a score of 27?
2/6 - Lecture
Monday, February 13, 2023
10:55 AM
PSYC 201 A Statistical Methods Page 1
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