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                <name>home/Rate</name>
                <summary>Epi Concepts Page</summary>
                <description>
                </description>

                <author>Garth Braithwaite</author>
                <company>Utah Department of Health/Software Technology Group</company>
                <versions>
                        <version><number>1.2</number><date>Oct 16, 2008</date><who>Maria</who>
                                <description>initial release (adapted from NM's XML page)</description></version>
                        <version><number>1.1</number><date>Aug 5, 2008</date><who>Lois</who>
                                <description>split out from count_rate page</description></version>
                        <version><number>1.0</number><date>Jun 13, 2008</date><who>Lois</who>
                                <description>initial release (adapted from Utah's html page)</description></version>
                </versions>
        </ibis:doc>


        <TITLE>Health Event Rates</TITLE>
        <CONTENT>
        <a name="TOP"></a>
                This page describes crude rates and age- and sex-specific rates for health events.  Click a bar below to expand or collapse its content.<br/><br/>

                <ibis:ExpandableContent titleLevel="2"><HIDE/>
                <TITLE>Crude Rates</TITLE>
                <CONTENT>
                <!--h1>Crude Rates</h1-->
        
                        <a href="home/Glossary.html#C">Counts</a> of health events are useful, 
                        but have limitations for those who need to compare populations of unequal size, 
                        for instance, a subpopulation versus an overall state population. Knowing the population 
                        sizes can help to interpret counts, but computing a <text class="Bold">
                        rate</text> will allow direct comparison between populations of unequal size that are 
                        otherwise similar (e.g., similar age composition, similar culturally). <br/><br/>
                        
                        According to the dictionary, a rate is, "a quantity, amount, or degree of something [numerator], 
                        measured per unit of something else [denominator]." In public health, the numerator is the number 
                        of people among whom an event occurred during a certain period of time, and the 
                        denominator is the total number of people in the population at risk for the same 
                        period of time. A rate has four components:
                        <div><ol>
                                <li>A specified time period. </li> 
                                <li>The numerator, the number of people in whom an event occurred during a given period of time, and </li>
                                <li>The denominator, the total number of people in the population at risk for the same period of time. 
                                This is also referred to as the "person-years at risk."</li> 
                                <li>A constant. The result of the fraction is usually multiplied by some factor of 10 (such as 100,000), 
                                so that the rate may be expressed as a whole number. </li></ol></div><br/>

                        <div class="Note">
                                <img src="../view/image/info_icon.gif" alt="info icon" width="15" height="15" title="Additional Information"/>
                                Many measures used in public health assessment specify a time period of one or more <text class="Bold">calendar years</text>. 
                                This is because many public health numerator datasets have calendar year production periods. But 
                                other time periods are also commonly used; for example calendar weeks in the instance of notifiable diseases. 
                                To calculate the "person-years at risk" for a time period that is less than one year, you need to 
                                multiply the population estimate by the portion of the year represented in the numerator. For instance, 
                                to calculate a crude rate for the number of cases of disease over a 10-week period, your denominator
                                would be the July 1 population estimate multiplied by 0.1923 (10 weeks/52 weeks).<br/>
                        </div><br/>

                        In general, a rate is called a "crude rate" if it has not been adjusted for the age, race, ethnicity, sex,
                        or other characteristic composition of a population.<br/><br/>
                        
                        Table 1 shows an example of crude rate calculations for heart disease by sex.<br/><br/>
                        
                        <h3>Table 1: Crude Death Rate due to Heart Disease by Sex, New Jersey, 2004</h3><br/>
                <table class="Info">
                        <tr>
<!--                            <th width="75"><p align="center"><b>Sex</b></p></th>
                                <th width="75"><p align="center"><b>Number of Deaths</b></p></th>
                                <th width="75"><p align="center"><b>Population Estimate</b></p></th>
                                <th width="75"><p align="center"><b>Crude Death Rate (Deaths per 100,000 Population)</b></p></th>       removed by MLB on 10/20/08 so column headers align center in IE-->
                                <th><p align="center" style="font-weight:bold"><b>Sex</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Number of Deaths</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Population Estimate</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Crude Death Rate (Deaths per 100,000 Population)</b></p></th>
                        </tr>
                        <tr>
                                <td>Male</td>
                                <td><p align="center">9,598</p></td>
                                <td><p align="center">4,235,853</p></td>
                                <td><p align="center">226.6</p></td>
                        </tr>
                        <tr>
                                <td>Female</td>
                                <td><p align="center">10,966</p></td>
                                <td><p align="center">4,463,026</p></td>
                                <td><p align="center">245.7</p></td>
                        </tr>
                </table><br/><br/>

                        Using the values, above, for males as an example...
                        
                <div><ol>
                                <li>The specified time period is 2004. </li> 
                                <li>The numerator, or the number of events, was 9,598. </li> 
                                <li>The denominator, or the estimated population at risk, was the July 1, 2004 population 
                                estimate of 4,235,853.</li> 
                                <li>The constant was 100,000. </li> </ol> </div> <br/>
                                
                                The calculation for the crude death rate due to heart disease among males for 2004 looks like 
                                this: <br/>
                                
                                <img src="../view/image/formula_crude_rate3.gif" alt="info icon" width="371" height="104" title="Formula for Crude Rate"/>

                <br/>
                                
                <p><hr/></p>
                <br/>   
                                
                        <h3>FAQs for Crude Rates:</h3><br/>
                        <h3>Combining Years</h3>
                                <text class="Bold">Q:</text> I am looking at death rates for a five-year period. What 
                                should I use for a population denominator?<br/>
                                <text class="Bold">A:</text> If you are combining numerator values over the five years 
                                by summing them, then use the sum of the population counts over the same period. If you 
                                are combining numerator values by taking an average, then take an average of the 
                                population counts for the same time period and geographic area. Alternatively, 
                                you could also use an average over the five years in the numerator, and a "mid-point" 
                                population estimate, that is, a population estimate for the mid-point, or middle, year
                                in the denominator.<br/><br/>
                                
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                <a name="Spec"></a>
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                <TITLE>Age- and Sex-specific Rates</TITLE>
                <CONTENT>
        <!--h1>Age- and Sex-specific Rates</h1-->
                        An age-specific rate is calculated by dividing the total number of health events for the specific 
                        age-group of interest by the total population in that age group. In Table 2, the age- and sex-specific 
                        rates for suicide are shown. The example demonstrates that the greatest <text class="Bold">number</text> 
                        of suicides occur among adolescents and young adults, whereas the highest <text class="Bold">rate</text> 
                        occurs among elderly men.<br/><br/>
                        The calculation for an age-specific rate is the same as for a crude rate.
                        
                        <br/><br/>              

                        <h3>Table 2: Suicide Mortality Rates by Age and Sex, New Jersey, 2004</h3>
                        <br/>
                <table class="Info">
                        <tr>
                                <th></th>                                                               
                                <th colspan="3"><p align="center" style="font-weight:bold"><b>Male</b></p></th>
                                <th colspan="3"><p align="center" style="font-weight:bold"><b>Female</b></p></th>
                        </tr>
                        <tr>
                                <th><p align="center" style="font-weight:bold"><b>Age Group</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Suicide Deaths</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Population</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Age- and Sex-Specific Rate per 100,000 Population</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Suicide Deaths</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Population</b></p></th>
                                <th><p align="center" style="font-weight:bold"><b>Age- and Sex-Specific Rate per 100,000 Population</b></p></th> 
                        </tr> 
                        <tr>
                                <td>&lt;15</td>
                                <td><p align="right">5</p></td>
                                <td><p align="right">897,553</p></td>
                                <td><p align="right">*</p></td>
                                <td><p align="right">1</p></td>
                                <td><p align="right">855,569</p></td>
                                <td><p align="right">*</p></td>
                        </tr>
                        <tr>
                                <td>15-44</td>
                                <td><p align="right">250</p></td>
                                <td><p align="right">1,821,036</p></td>
                                <td><p align="right">13.7</p></td>
                                <td><p align="right">56</p></td>
                                <td><p align="right">1,792,745</p></td>
                                <td><p align="right">3.1</p></td>
                        </tr>
                        <tr>
                                <td>45-64</td>
                                <td><p align="right">155</p></td>
                                <td><p align="right">1,038,488</p></td>
                                <td><p align="right">14.9</p></td>
                                <td><p align="right">37</p></td>
                                <td><p align="right">1,112,479</p></td>
                                <td><p align="right">3.3</p></td>
                        </tr>
                        <tr>
                                <td>65+</td>
                                <td><p align="right">73</p></td>
                                <td><p align="right">456,880</p></td>
                                <td><p align="right">16.0</p></td>
                                <td><p align="right">21</p></td>
                                <td><p align="right">666,485</p></td>
                                <td><p align="right">3.2</p></td>
                        </tr>
                </table>
                <table class="Info">
                        <tr>
                                <td>* Number is too small to calculate a reliable rate.</td>
                        </tr>
                </table><br/><br/>
                
                <div class="Note">
                        <img src="../view/image/info_icon.gif" alt="info icon" width="15" height="15" title="Additional Information"/>
                        Looking at rates within groups is also called "stratification." In Table 2, the population has been stratified
                        by age and sex. The data in Table 2 also show how useful stratification can be. Not only are the suicide 
                        death rates much higher among men, the rate of suicide increases with age for men, but not for women.<br/>
                </div><br/><br/>
                                
                        <!--div class="Note">
                                <img src="../view/image/info_icon.gif" alt="info icon" width="15" height="15" title="Additional Information"/>
                                The <a href="home/Glossary.html#C">crude mortality rate</a> for a population depends on the mortality 
                                rate in each age group as well as on the proportion of people in each age group. For instance,
                                the age-specific rate for most causes of death will be higher for older age groups. As a result, 
                                crude death rates tend to be higher in populations with a larger proportion of older persons, 
                                and lower in populations with a larger proportion of younger persons.<br/>
                        </div><br/><br/><br/-->
                        
                        Age-specific rates are valuable for comparing rates across age groups, and crude rates provide a
                        useful summary measure to compare similar populations of different sizes, but the word "similar"
                        is a key concept. It can be misleading to compare crude rates across populations that have relevant
                        differences, such as different cultural traditions, or age, race/ethnicity, or sex composition. <br/><br/>
                        
                        One difference that is commonly controlled for statistically is age composition of the population. 
                        The <a href="home/Glossary.html#C">crude mortality rate</a> for a population depends on the mortality 
                        rate in each age group as well as on the proportion of people in each age group. For instance,
                        the age-specific rate for most causes of death will be higher for older age groups. As a result, 
                        crude death rates tend to be higher in populations with a larger proportion of older persons, 
                        and lower in populations with a larger proportion of younger persons. <br/><br/>
                        
                        An <a href="home/Glossary.html#A">age-adjusted rate</a> is a summary measure that may be used to 
                        compare mortality or disease risk in two populations with different age compositions.
                </CONTENT>
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                <ibis:ExpandableContent titleLevel="2"><SHOW/>
                <TITLE>Deciding Which Measure to Use</TITLE>
                <CONTENT>
                        <!--h1>Deciding Which Measure to Use</h1-->
                        The measure that best informs the question you are trying to answer is the one to use. 
                        This is a guideline, not a hard and fast rule, but generally: <br/><br/>
                        <table class="Info">                            
                                <tr>                    
                                        <th width="100" style="font-weight:bold">If the question is:</th>               
                                        <th width="100" style="font-weight:bold">Then use:</th> 
                                </tr>                   
                                <tr>                    
                                        <td>MAGNITUDE: How big is the problem? </td>
                                        <td>Number of events (count)</td>
                                </tr>
                                <tr>                    
                                        <td>PROBABILITY: What is the underlying risk in a population? </td>
                                        <td>Crude rate and confidence interval</td>
                                </tr>
                                <tr>                    
                                        <td>DISPARITY: Is there a difference in risk after controlling for age?</td>
                                        <td>Age-adjusted rate and confidence interval</td>
                                </tr>
                        </table>
                <br/>   
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                <h3>Go to the page on <a href="home/AARate.html">age-adjusted rates for health events.</a></h3>
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