Chapter 18: Probability 253
Relative Frequency..................................................................................................................
The frequency with which something happens is how often it happens, and that can be important
information. It’s probably good to know that I found money I had forgotten about in my coat
pocket four times. But there’s another piece of information that’s missing. I found money four
times, but was that four times last week? Four times last month? Four times last year? Four times
in my life? The answer to that makes the “four times” information more useful.
The relative frequency of an event is a number between zero and one that compares the number of
times something happens to the total number of things that happened. If I found money in my
pocket four times last week, that’s 4 days out of 7 days, or a relative frequency of^47. If it was four
times last year, the relative frequency is 3654.
DEFINITION
The relative frequency of an event is the ratio of the number of times it occurs to the
total number of events observed.
Suppose you watched TV one evening and kept track of the commercials that aired in the few
hours you watched. Here’s your record.
Commercials on Tuesday Night from 8 P.M. to 9 P.M.
Type of Commercial Number of Ads
Cars and trucks 3
Food and dr ink 2
Phones and tablets 3
Drugs and medicines 3
Security 2
Retail stores 5
Clothing and shoes 1
Other TV shows 6
The chart tells you that more of the commercials you viewed were for other TV shows than
anything else, but is 6 a lot? Six out of eight would be a lot, but you saw more than eight ads. Six
out of a hundred would not be a lot, but you didn’t sit through a hundred ads. How many did you
watch? Add up the column. Then you can compare each of the counts to the total to get the rela-
tive frequency.
