Probability Questions
Basic Probability

Key Words:
Find the probability…
“1” randomly selected…

Formula(s):
# of times "A" happened
( )
Total # of observations
P A
=
OR
# of ways "A" can happened
( )
Total # of possible outcomes
P A
=
Addition Rule

Key Words:
Find the probability…
“1” randomly selected… “or”
…

Formula:
(
29
(
29
(
29
(
)
P AorB
P A
P B
P A
B
=
+

# of subjects in group A
# of subjects in group B
# of subjects belonging to both A & B
(
)
Total
Total
Total
P AorB
=
+

Conditional Probability

Key Words:
Find the probability…
“1” randomly selected… “given that”…

Formula:
(
29
(
29
(
29

P A
B
P A B
P B
a
=
of Subjects in Both Groups/Total
of Subjects in Group B/Total
Number
Number
=
*short cut:
Only look at the numbers in the row or column relating to the “given that”
condition, the total of that row or column is your denominator, then just get your
numerator.
Multiplication Rule
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 Spring '08
 McGuckian
 Probability, Probability theory, Key Words

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