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PROBABILITY. CONDITIONAL PROBABILITY. Random variable. A variable defined on a sample space. Fx: The value of a card. Interpretation: A variable that takes different values with different probabilities. (Some) Characteristics of a random variable X: Mean value E(X).
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Random variable • A variable defined on a sample space. Fx: The value of a card. • Interpretation: A variable that takes different values with different probabilities. • (Some) Characteristics of a random variable X: • Mean value E(X). • Variation V(X) = E((X-E(x))^2). Cumulative mass function/distribution function of a random variable: F(x) = P(X ≤ x) Graphical representation by histogram.
Fractile/QQ-plots How can we access information about the distribution of a sample? By comparing fractiles – the QQ-plot in SPSS. The proposed distribution is good, if the fractiles follow a linear pattern. • Notice: • The variability of the points decreases when the sample size increases. • The variation of the points around the line is largest in the ends. Hence, pay more attention to the points in the middle of the plot. • Detrended QQ-plots • The points should be spread evenly above/below the horizontal line. • There should be no pattern.
Examples of QQ-plots QQ-plot QQ-plot QQ-plot Normal distribution Skew distribution Uniform distribution Detrended QQ-plot Detrended QQ-plot Detrended QQ-plot