## A Guide to Statistics

The following is a brief description of two key statistics (percent and rate) used on this website and a word of caution about their use.

**Percent**

Percent means per 100. For example, 15% means 15 out of 100; 75% means 75 out of 100.

**Percent **= (Number in sub-group) ÷ (Number in whole group) x 100

**Example: Percent of babies born at low birth weight (LBW), 2002**

Percent = (Number LBW) ÷ (Total number of births) x 100

= 6,623 ÷ 73,250 x 100

= 9% of babies born in 2002 weighed less than 2,500 grams (5.5 pounds)

**Rate**

The easiest way to understand a rate is to think of a percent as a rate per 100. (Many indicators are presented as rates per 100,000 or 1,000.) In the example above, 9% of babies born at low birth weight could also be expressed as “9 babies per 100” were born at low birth weight.

**Rate **= (Number in sub-group) ÷ (Number in whole group) x MULTIPLIER

**Example: Rate of youth (ages 10-17) arrested for violent crimes per 100,000 youth (ages 10-17)**

Rate = (Number of youth ages 10-17 arrested) ÷ (Number of youth ages 10-17) x 100,000

= 3,037 ÷ 567,678 x 100,000

**= **535 per 100,000 youth ages 10-17 were arrested for violent crimes in 1998

** **

**Using Percentages or Rates with Small Numbers of Incidents**

Caution is necessary when using percentages and rates with small numbers of incidents. If the item to be measured has less than 5 occurrences (*e.g., *infant mortality in a given jurisdiction for a given year), then a percentage or rate should not be produced. One or both of the following methods can be employed to create a more stable percentage or rate:

- Multi-year averaging, which involves using a longer time period to produce the rate (
*e.g.*, using 3 or 5 years’ worth of data); or - Enlarging the geographic area (
*e.g.*, using a region containing several jurisdictions).

Both of these methods increase the number of observed events and the stability and reliability of percentages or rates calculated.