Chapter 6

I. Introduction to Probability

A. Role of probability in inference

B. Definition - probability as relative frequency

number of outcomes classified as A
p (A) = ----------------------------------------------
total number of possible outcomes

C. Probability and frequency distributions

D. Random sampling

II. Probability and the normal distribution

A. Characteristics of normal distributions

5.Converting proportions to frequency: if proportion = f/N, then f = proportion * N

B. Sample problems using the unit normal table

III. Percentiles and percentile ranks

A. Definitions:
1.Percentile rank = the percentage of individuals in a distribution that fall at or below a given score (i.e., a %).
2. Percentile = the score associated with a particular percentile rank (i.e., an X).

B.The area of the unit normal curve below (to the left) of a z-score gives the percentile rank for the score (X) associated with the z when the proportion is multiplied by 100.

C.Converting a z-score for a given proportion back to a score (X) gives the percentile for the percentile rank of z. e.g., X = m + zs

C.Quartiles (Q1, Q2, & Q3) - divide a distribution up into four equal parts (quarters). The z-scores for the quartiles in a normal distribution are: Q1 = -.67, Q2 (median) = 0, Q3 = .67.

1. The semi-interquartile range for a normal distribution is given by: 0.67s