FACT SHEET:
Understanding and Calculating Statistical Terms in Your Presentations

Extension and outreach educators frequently use statistics in their presentations and programming efforts to emphasize important points, make an argument for targeted programs, and present trends relevant to their program areas.  In recent years statements such as, AThe teen pregnancy and birth rates have declined throughout the 1990s@, have made headlines and confirmed that our prevention efforts are indeed effective. 

What is less clear is the exact meaning of some of these statistical terms to the general population that absorbs and ultimately needs to evaluate the statistics.  What does a 1998 teen birth rate of 35.6 in Pennsylvania mean?  How is this different from a 1998 teen birth ratio of .10 in Pennsylvania?  It is important for Pennsylvanian's to understand the meaning of these terms, how they are related, and ultimately how to evaluate the implications of changing rates and ratios.

•  Numbers: Researchers, policy makers, and educators often report raw numbers in order to present a simple overview of the frequency of an event.  For example, in 1998 there were 14,820 births to teen girls aged 15-19 in the state of Pennsylvania.  Keeping track of the number of teen births over a decade is useful in that it allows us to describe trends.  WIC program officials, for example,  use these data to estimate the population it will need to serve through its program.  Numbers alone, however, do not let us make comparisons across populations.  For example, Mississippi is a state with a smaller overall population when compared to Pennsylvania, and thus we would expect that the number of teen births in Mississippi would be smaller than in Pennsylvania.  Nevertheless, teen births could be a larger problem in Mississippi because a greater proportion of teen girls in their state experience a birth. 
   

• Ratios and Proportions: These terms have the same meaning and can be used interchangeably in your talks.  The teen birth ratio for Pennsylvania in 1998 is the number of teen births that year divided by the number of all births in Pennsylvania in the same year.  The 1998 teen birth ratio for Pennsylvania is calculated as follows:  
  

  14,820 births to PA females aged 15-19	=.10
  145,606 births to all PA females

The teen birth ratio, or the proportion of teen births in Pennsylvania in 1998 was 0.10.

Ratios and proportions are often converted to a percent by multiplying by 100:

.10 x 100 = 10 percent

Thus, when the teen birth ratio is converted into a percent, we find that 10% of all births in Pennsylvania in 1998 were to teen girls between the age of 15 and 19 years. What you gain by reporting ratios, proportions, and percents is the ability to compare these statistics across populations. Knowing that 10% of all births in Pennsylvania are to teen mothers also allows program and service providers to plan for the special needs of this population and of their children.

• Rates: The rate is a useful statistic to report because it informs us of the risk to a specific population of experiencing a specific event, and it controls for population size, so rates are comparable across populations. For example, the teen birth rate in Pennsylvania in 1998 is the number of births to PA teen females aged 15-19 in 1998 divided by the total number of teen females aged 15-19 in Pennsylvania in 1998, usually multiplied by 1,000:

In Pennsylvania in 1998, for every 1,000 teen females aged 15-19, there were 35.6 births. This is also called an age-specific rate since is it calculated only for a specific age group. Rates of events can be calculated for any specific age group or for the overall population. So, a teen birth rate of 35.6 in PA in 1998 is comparable to the US teen birth rate of 51.1 in 1998. Teen females in PA in 1998 were at a lower risk of experiencing a teen birth compared to the overall U.S. population of teen females in 1998. Age specific rates can also be calculated for specific sub-populations, such as racial groups, and can be compared. For example, in 1998 the teen birth rate in PA was 26 for Non-Hispanic White teens, 115 for Hispanic teens, and 99 for Non-Hispanic Black teens. Clearly racial and ethnic minority groups in PA are at greater risk of experiencing a teen birth. Knowing this can inform extension program efforts.