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REVIEW OF BASICS PART II Probability Distributions Confidence Intervals Statistical Significance

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Probability Distributions Probabilities are relative frequencies Probabilities vary between 0 and 1

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MEAN +1SD+2SD-1SD-2SD 68% 95% +3SD-3SD 99%

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z - SCORES z-score: standard score measuring in units of standard deviations standard normal distribution: normal distribution in z-score units

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Confidence Intervals Range of values thought to contain the population mean Go up and down from the sample mean Level of confidence determines how many standard deviations to go

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MEAN +1SD+2SD-1SD-2SD 68% 95% +3SD-3SD 99%

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STATISTICAL SIGNIFICANCE Inferential statistics are used to make inferences about populations based on samples These inferences are based on hypothetical probability distributions

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Sampling Distribution Hypothetical frequency distribution of all the sample means from a population Sampling variation Standard error: standard deviation of sampling distribution

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Testing the Null Null Hypothesis: no difference Assume the Null Hypothesis is true until proven other wise Reasonable doubt standard (p <.05) Reject or Fail to Reject the Null

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Confusion about p p is the probability of a Type I error: rejecting the Ho when the Ho is true What is the probability that you could get results this extreme if the Ho is true? NOT the probability that the Ho is true given these results

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Problems with NHST Binary decision Does not show effect size Null is probably never literally true

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Take-Home Points We can make probabilistic statements about a population It’s better than guessing

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