Normal Distribution
The normal curve is symmetric and bell-shaped. We convert values to a z-score to use the tables: z = (x − μ) / σ, where μ is the mean and σ is the standard deviation. A z-score tells you how many standard deviations a value is from the mean. The Formulae and Tables booklet gives P(Z ≤ z) for positive z. Use: P(Z > z) = 1 − P(Z ≤ z); P(Z ≤ −z) = 1 − P(Z ≤ z); and P(a ≤ Z ≤ b) = P(Z ≤ b) − P(Z ≤ a). For 'top 25%' questions, look up 0.75 in the body of the tables to find z, then solve x = μ + zσ.
