Ch. 6 - Probability
Sunday, November 11, 2012
PSYC 2530 Introductory Statistics
Chapter 6: Probability
Goal of inferential statistics is to begin with a sample and then answer a general
question about the population. We reach this goal in a two-stage process:
o 1) Identify the types of samples that would probably be obtained from a
o 2) Decide which population your sample likely came from
Probability always ranges between 0 and 1
Probability must have two components (called random sampling with replacement)
o Equal chance
o Constant probability
A random sample requires that each individual in the population has an equal
chance of being selected
An independent random sample requires that each individual has an equal chance
of being selected and that the probability of being selected stays constant from
one selection to the next if more than one individual is selected.
o ± 1 sd = 68.2%
o ± 2 sd = 95.4%
o ± 3 sd = 99.7%
Probability = proportion of a frequency distribution located under an area of the
Using the unit normal table (table B.1):
o 1) The body always corresponds to the larger part of the distribution whether
it is on the right-hand side or the left-hand side. Similarly, the tail is always
the smaller section whether it is on the right or the left.
o 2) Because the normal distribution is symmetrical, the proportions on the
right-hand side are exactly the same as the corresponding proportions on the
left-hand side. To find proportions for negative z-scores, you must look up the
corresponding proportions for the positive values of z.
o 3) Proportions are always positive.
o The table