Introduction
to Probability

 Definition
 Probability is defined for a specific outcome in
a situation where several different outcomes are possible.
If the possible outcomes are denoted A, B, C, D,
etc., then the probability of A is defined as:
 Examples
 Tossing Coins: When you toss a balanced coin, the
outcome is either heads or tails. Thus, there are a total
of 2 possible outcomes. The probability of tossing a head
is
 Selecting Cards: There are 52 cards in an ordinary
deck of cards. Thus, there are a total of 52 possible outcomes.
 The probability of drawing a Heart (there are
13 hearts) is:
p(Heart) = 13/52 = 1/4 = .25 = 25%
 The probability of drawing an Ace (there are
4 aces) is:
p(Ace) = 4/52 = 1/13 = .0769 = 7.69%
 The probability of drawing a Green card (there
are 0 green cards) is:
p(Green) = 0/52 = .00 = 0%
 The probability of drawing a card (there are
52 cards) is:
p(Card) = 52/52 = 1.00 = 100%
 Definition
 Random Sampling: An independent random sample must
satisfy two requirements:
 Each individual in the population must have an equal
chance of being selected.
 If more than one individual is selected for the sample,
there must be constant probability for each and
every selection. (Sample with replacement)
