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Probability
Psychology B5 - Statistics
Statistical Notation
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Odds are expressed as a fraction or ratio.
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Chance is expressed as a percentage.
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Probability is expressed as a decimal.
Probability
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Probability is equal to the relative frequency (rel. f) of an event occurring.
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Probability is a positive decimal between 0 and 1.
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0 = Never
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1 = Always
We answer probability questions in terms of proportions.
Statistical Notation
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The symbol for probability is p
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p (X > 4); Possible scores between 1 and 10.
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p (X < 4); Possible scores between 1 and 10.
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What is the probability that a coin will come up heads?
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p = .50
What is the probability that you will draw an Ace from a deck of cards?
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p = .08
Sampling and Probability
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Random, Scientific, Probability Sampling
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Sampling with Replacement
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After selecting an object or subject from a population it is returned to the population
before the next selection.
Independent Events
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Every person in the identified population has an equal chance of being selected for the
sample.
That one event occurs does not affect the occurrence of another event.
Compound (Dependent) Events
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That one event occurs does affect the occurrence of another event.
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Probability
Psychology B5 - Statistics
Obtaining Probability
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Use the same steps for finding the proportions using z-scores and the z-table.
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For every z-score you can find the corresponding proportion above or below that z-score.
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For every proportion you can find the corresponding z-score.
Probability and the Normal Distribution
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The proportion of the area under the normal curve for scores in any part of the distribution equals the
probability of those scores.
Intelligence Quotient
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μ = 100
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𝛔 𝐱 = 15
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What is the probability of randomly selecting someone from the population who qualifies for
MENSA (IQ > 130)?
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p (X > 130) =
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Probability
Psychology B5 - Statistics
Probability and the Normal Distribution
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What is the probability of selecting a z-score less than z = 1.62?
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p (z < 1.62) =
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What is the z-score that separates the top 5% from the rest of the distribution?
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z=
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What is the probability that a score will occur between two other scores?
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μ = 55
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𝛔𝐱 = 5
p (53 < X < 57)
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