Let us be systematic in setting this scenario up for analysis: We first describe an experiment, then the event, and then use the relevant formula:

• First, the experiment: select a participant who used the new acne cream (since this is what is "given").
• Next, the event: E = the event that the participant reported improved skin.
• N = Sample size = 1200 (the number of participants who used the cream)
• Frequency of E, fr(E) = number of times E occurs = number of participants reporting improved skin = 800.

Now we use the formula:

Estimated probability of E

=

fr(E) N

=

800 1200

= 2/3.

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