PROBABILITY TREE DIAGRAMS

A tree diagram represents the outcomes from a multi-step experiment.
A sample tree diagram is shown below.

TWO-STEP PROBABILITY TREE DIAGRAMS
   

A multi-step experiment has more than one step.
For example, here are some two-step experiments:

The branches emanating from any point on a tree diagram must have probabilities that sum to  1 .
In the sample tree diagram above (initial configuration), we have:

To find the probability of any path, multiply the probabilities on the corresponding branches.
In the sample tree diagram above (initial configuration), there are six paths (going from top to bottom):

Notice that:   [beautiful math coming... please be patient] $(0.14 + 0.49 + 0.07 + 0.12 + 0.15 + 0.03) = 1$
Always do this check on your own probability tree diagrams!

EXAMPLES:
TWO FLIPS OF A FAIR COIN

Consider the two-step experiment ‘two flips of a fair coin’.
Let   H   denote Head, and   T   denote Tail.

Note that the sample space for this experiment is:

[beautiful math coming... please be patient] $\,S = \{\text{HH},\text{HT},\text{TH},\text{TT}\,\}\,$

The probability tree diagram is shown at right.
From the tree diagram, we can easily compute various probabilities:

[beautiful math coming... please be patient] $P(\text{both heads}) = P(\text{HH}) = (0.5)(0.5) = 0.25$
[beautiful math coming... please be patient] $P(\text{both tails}) = P(\text{TT}) = (0.5)(0.5) = 0.25$
[beautiful math coming... please be patient] $P(\text{no heads}) = P(\text{TT}) = 0.25$
[beautiful math coming... please be patient] $P(\text{at least one head})$
    $= P(\text{HH or HT or TH})$
    $= P(\text{HH}) + P(\text{HT}) + P(\text{TH})$
    $= 0.25 + 0.25 + 0.25 = 0.75$
(alternate method)
[beautiful math coming... please be patient] $P(\text{at least one head})$
    $= P(\text{not TT})$
    $= 1 - P(\text{TT})$
    $= 1 - 0.25 = 0.75$
[beautiful math coming... please be patient] $P(\text{exactly one head})$
    $= P(\text{HT or TH})$
    $= P(\text{HT}) + P(\text{TH})$
    $= 0.25 + 0.25 = 0.50$
TWO FLIPS OF A FAIR COIN
TWO FLIPS OF A FAIR COIN
BALLS IN AN URN, WITH REPLACEMENT

Suppose that an urn contains 2 black balls, 1 red ball, and 4 green balls.
Two balls are chosen, with replacement, from the urn.
That is, after the first ball is chosen and its color is recorded, it is returned to the urn.
Assume that all balls are equally likely to be chosen.

Note that the sample space for this experiment is:

[beautiful math coming... please be patient] $S = \{\text{BB},\text{BR},\text{BG},\text{RB},\text{RR},\text{RG},\text{GB},\text{GR},\text{GG}\}$

The probability tree diagram is shown at right.
From the tree diagram, we can easily compute various probabilities:

[beautiful math coming... please be patient] $\displaystyle P(\text{BB}) = \frac27\cdot\frac27 = \frac4{49}$
[beautiful math coming... please be patient] $\displaystyle P(\text{RR}) = \frac17\cdot\frac17 = \frac1{49}$
[beautiful math coming... please be patient] $\displaystyle P(\text{GG}) = \frac47\cdot\frac47 = \frac{16}{49}$
[beautiful math coming... please be patient] $P(\text{BB or RR})$
    $= P(\text{BB}) + P(\text{RR})$
    $= \frac4{49} + \frac1{49}$
    $= \frac5{49}$
[beautiful math coming... please be patient] $P(\text{at least one black ball is drawn})$
    $= P(\text{BB or BR or BG or RB or GB})$
    $= \frac4{49} + \frac2{49} + \frac8{49} + \frac2{49} + \frac8{49}$
    $= \frac{24}{49}$
[beautiful math coming... please be patient] $P(\text{at most one black ball is drawn})$
    $= P(\text{exactly one black ball or no black balls})$
    $= P(\text{all outcomes except BB})$
    $= 1 - \frac4{49}$
    $= \frac{45}{49}$
[beautiful math coming... please be patient] $P(\text{exactly one black ball is drawn})$
    $= P(\text{BR or BG or RB or GB})$
    $= \frac2{49} + \frac8{49} + \frac2{49} + \frac8{49}$
    $= \frac{20}{49}$
[beautiful math coming... please be patient] $P(\text{two different color balls})$
    $= P(\text{all outcomes except BB, RR, GG})$
    $= 1 - \frac4{49} - \frac1{49} - \frac{16}{49}$
    $= \frac{28}{49}$
BALLS IN AN URN, WITH REPLACEMENT
BALLS IN AN URN, WITH REPLACEMENT
BALLS IN AN URN, WITHOUT REPLACEMENT

Suppose that an urn contains 2 black balls, 1 red ball, and 4 green balls.
Two balls are chosen, without replacement, from the urn.
That is, the first ball is chosen and its color is recorded. It is not returned to the urn. Then, the second ball is chosen.
Assume that all balls are equally likely to be chosen.

Note that the sample space for this experiment is:

[beautiful math coming... please be patient] $S = \{\text{BB},\text{BR},\text{BG},\text{RB},\text{RG},\text{GB},\text{GR},\text{GG}\}$

There is no ‘RR’ in the sample space, since if a red ball is chosen on the first draw, then there are no red balls remaining in the urn.

The probability tree diagram is shown at right.
From the tree diagram, we can easily compute various probabilities:

[beautiful math coming... please be patient] $\displaystyle P(\text{BB}) = \frac27 \cdot \frac16 = \frac2{42} = \frac{1}{21}$
[beautiful math coming... please be patient] $P(\text{RR}) = 0$
[beautiful math coming... please be patient] $\displaystyle P(\text{GG}) = \frac47 \cdot \frac36 = \frac{12}{42} = \frac{2}{7}$
[beautiful math coming... please be patient] $P(\text{the same color is drawn twice})$
    $= P(\text{BB or GG})$
    $= \frac1{21} + \frac2{7} = \frac1{21} + \frac6{21}$
    $= \frac{7}{21} = \frac13$
[beautiful math coming... please be patient] $\displaystyle P(\text{BR}) = \frac27 \cdot \frac16 = \frac2{42} = \frac{1}{21}$
[beautiful math coming... please be patient] $\displaystyle P(\text{RB}) = \frac17 \cdot \frac26 = \frac2{42} = \frac{1}{21}$
[beautiful math coming... please be patient] $P(\text{a black ball and a red ball are drawn})$
    $= P(\text{BR or RB})$
    $= \frac1{21} + \frac1{21}$
    $= \frac{2}{21}$
BALLS IN AN URN, WITHOUT REPLACEMENT
BALLS IN AN URN, WITH REPLACEMENT
Master the ideas from this section
by practicing the exercise at the bottom of this page.

When you're done practicing, move on to:
Choosing Things: Does Order Matter?

 
 
On this exercise, you will not key in your answer.
However, you can check to see if your answer is correct.
If a problem gives probabilities in decimal form, then give your answers in decimal form.
If a problem gives probabilities in fraction form, then give your answers as fractions in simplest form.
(MAX is 6; there are 6 different problem types)