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1<?xml version="1.0" encoding="ISO-8859-1"?>
2
3<HTML_CONTENT xmlns:ibis="http://www.ibisph.org">
4
5        <ibis:doc>
6                <name>home/Rate</name>
7                <summary>Epi Concepts Page</summary>
8                <description>
9                </description>
10
11                <author>Garth Braithwaite</author>
12                <company>Utah Department of Health/Software Technology Group</company>
13                <versions>
14                        <version><number>1.2</number><date>Oct 16, 2008</date><who>Maria</who>
15                                <description>initial release (adapted from NM's XML page)</description></version>
16                        <version><number>1.1</number><date>Aug 5, 2008</date><who>Lois</who>
17                                <description>split out from count_rate page</description></version>
18                        <version><number>1.0</number><date>Jun 13, 2008</date><who>Lois</who>
19                                <description>initial release (adapted from Utah's html page)</description></version>
20                </versions>
21        </ibis:doc>
22
23
24        <TITLE>Health Event Rates</TITLE>
25        <CONTENT>
26        <a name="TOP"></a>
27                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/>
28
29                <ibis:ExpandableContent titleLevel="2"><HIDE/>
30                <TITLE>Crude Rates</TITLE>
31                <CONTENT>
32                <!--h1>Crude Rates</h1-->
33       
34                        <a href="home/Glossary.html#C">Counts</a> of health events are useful,
35                        but have limitations for those who need to compare populations of unequal size,
36                        for instance, a subpopulation versus an overall state population. Knowing the population
37                        sizes can help to interpret counts, but computing a <text class="Bold">
38                        rate</text> will allow direct comparison between populations of unequal size that are
39                        otherwise similar (e.g., similar age composition, similar culturally). <br/><br/>
40                       
41                        According to the dictionary, a rate is, "a quantity, amount, or degree of something [numerator],
42                        measured per unit of something else [denominator]." In public health, the numerator is the number
43                        of people among whom an event occurred during a certain period of time, and the
44                        denominator is the total number of people in the population at risk for the same
45                        period of time. A rate has four components:
46                        <div><ol>
47                                <li>A specified time period. </li> 
48                                <li>The numerator, the number of people in whom an event occurred during a given period of time, and </li>
49                                <li>The denominator, the total number of people in the population at risk for the same period of time.
50                                This is also referred to as the "person-years at risk."</li> 
51                                <li>A constant. The result of the fraction is usually multiplied by some factor of 10 (such as 100,000),
52                                so that the rate may be expressed as a whole number. </li></ol></div><br/>
53
54                        <div class="Note">
55                                <img src="../view/image/info_icon.gif" alt="info icon" width="15" height="15" title="Additional Information"/>
56                                Many measures used in public health assessment specify a time period of one or more <text class="Bold">calendar years</text>.
57                                This is because many public health numerator datasets have calendar year production periods. But
58                                other time periods are also commonly used; for example calendar weeks in the instance of notifiable diseases.
59                                To calculate the "person-years at risk" for a time period that is less than one year, you need to
60                                multiply the population estimate by the portion of the year represented in the numerator. For instance,
61                                to calculate a crude rate for the number of cases of disease over a 10-week period, your denominator
62                                would be the July 1 population estimate multiplied by 0.1923 (10 weeks/52 weeks).<br/>
63                        </div><br/>
64
65                        In general, a rate is called a "crude rate" if it has not been adjusted for the age, race, ethnicity, sex,
66                        or other characteristic composition of a population.<br/><br/>
67                       
68                        Table 1 shows an example of crude rate calculations for heart disease by sex.<br/><br/>
69                       
70                        <h3>Table 1: Crude Death Rate due to Heart Disease by Sex, New Jersey, 2004</h3><br/>
71                <table class="Info">
72                        <tr>
73<!--                            <th width="75"><p align="center"><b>Sex</b></p></th>
74                                <th width="75"><p align="center"><b>Number of Deaths</b></p></th>
75                                <th width="75"><p align="center"><b>Population Estimate</b></p></th>
76                                <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-->
77                                <th><p align="center" style="font-weight:bold"><b>Sex</b></p></th>
78                                <th><p align="center" style="font-weight:bold"><b>Number of Deaths</b></p></th>
79                                <th><p align="center" style="font-weight:bold"><b>Population Estimate</b></p></th>
80                                <th><p align="center" style="font-weight:bold"><b>Crude Death Rate (Deaths per 100,000 Population)</b></p></th>
81                        </tr>
82                        <tr>
83                                <td>Male</td>
84                                <td><p align="center">9,598</p></td>
85                                <td><p align="center">4,235,853</p></td>
86                                <td><p align="center">226.6</p></td>
87                        </tr>
88                        <tr>
89                                <td>Female</td>
90                                <td><p align="center">10,966</p></td>
91                                <td><p align="center">4,463,026</p></td>
92                                <td><p align="center">245.7</p></td>
93                        </tr>
94                </table><br/><br/>
95
96                        Using the values, above, for males as an example...
97                       
98                <div><ol>
99                                <li>The specified time period is 2004. </li> 
100                                <li>The numerator, or the number of events, was 9,598. </li> 
101                                <li>The denominator, or the estimated population at risk, was the July 1, 2004 population
102                                estimate of 4,235,853.</li> 
103                                <li>The constant was 100,000. </li> </ol> </div> <br/>
104                               
105                                The calculation for the crude death rate due to heart disease among males for 2004 looks like
106                                this: <br/>
107                               
108                                <img src="../view/image/formula_crude_rate3.gif" alt="info icon" width="371" height="104" title="Formula for Crude Rate"/>
109
110                <br/>
111                               
112                <p><hr/></p>
113                <br/>   
114                               
115                        <h3>FAQs for Crude Rates:</h3><br/>
116                        <h3>Combining Years</h3>
117                                <text class="Bold">Q:</text> I am looking at death rates for a five-year period. What
118                                should I use for a population denominator?<br/>
119                                <text class="Bold">A:</text> If you are combining numerator values over the five years
120                                by summing them, then use the sum of the population counts over the same period. If you
121                                are combining numerator values by taking an average, then take an average of the
122                                population counts for the same time period and geographic area. Alternatively,
123                                you could also use an average over the five years in the numerator, and a "mid-point"
124                                population estimate, that is, a population estimate for the mid-point, or middle, year
125                                in the denominator.<br/><br/>
126                               
127                <p align="right"><a href="#TOP">Back to Top</a></p>                             
128                </CONTENT>
129                </ibis:ExpandableContent>
130
131                <a name="Spec"></a>
132                <ibis:ExpandableContent titleLevel="2"><HIDE/>
133                <TITLE>Age- and Sex-specific Rates</TITLE>
134                <CONTENT>
135        <!--h1>Age- and Sex-specific Rates</h1-->
136                        An age-specific rate is calculated by dividing the total number of health events for the specific
137                        age-group of interest by the total population in that age group. In Table 2, the age- and sex-specific
138                        rates for suicide are shown. The example demonstrates that the greatest <text class="Bold">number</text> 
139                        of suicides occur among adolescents and young adults, whereas the highest <text class="Bold">rate</text> 
140                        occurs among elderly men.<br/><br/>
141                        The calculation for an age-specific rate is the same as for a crude rate.
142                       
143                        <br/><br/>             
144
145                        <h3>Table 2: Suicide Mortality Rates by Age and Sex, New Jersey, 2004</h3>
146                        <br/>
147                <table class="Info">
148                        <tr>
149                                <th></th>                                                               
150                                <th colspan="3"><p align="center" style="font-weight:bold"><b>Male</b></p></th>
151                                <th colspan="3"><p align="center" style="font-weight:bold"><b>Female</b></p></th>
152                        </tr>
153                        <tr>
154                                <th><p align="center" style="font-weight:bold"><b>Age Group</b></p></th>
155                                <th><p align="center" style="font-weight:bold"><b>Suicide Deaths</b></p></th>
156                                <th><p align="center" style="font-weight:bold"><b>Population</b></p></th>
157                                <th><p align="center" style="font-weight:bold"><b>Age- and Sex-Specific Rate per 100,000 Population</b></p></th>
158                                <th><p align="center" style="font-weight:bold"><b>Suicide Deaths</b></p></th>
159                                <th><p align="center" style="font-weight:bold"><b>Population</b></p></th>
160                                <th><p align="center" style="font-weight:bold"><b>Age- and Sex-Specific Rate per 100,000 Population</b></p></th> 
161                        </tr> 
162                        <tr>
163                                <td>&lt;15</td>
164                                <td><p align="right">5</p></td>
165                                <td><p align="right">897,553</p></td>
166                                <td><p align="right">*</p></td>
167                                <td><p align="right">1</p></td>
168                                <td><p align="right">855,569</p></td>
169                                <td><p align="right">*</p></td>
170                        </tr>
171                        <tr>
172                                <td>15-44</td>
173                                <td><p align="right">250</p></td>
174                                <td><p align="right">1,821,036</p></td>
175                                <td><p align="right">13.7</p></td>
176                                <td><p align="right">56</p></td>
177                                <td><p align="right">1,792,745</p></td>
178                                <td><p align="right">3.1</p></td>
179                        </tr>
180                        <tr>
181                                <td>45-64</td>
182                                <td><p align="right">155</p></td>
183                                <td><p align="right">1,038,488</p></td>
184                                <td><p align="right">14.9</p></td>
185                                <td><p align="right">37</p></td>
186                                <td><p align="right">1,112,479</p></td>
187                                <td><p align="right">3.3</p></td>
188                        </tr>
189                        <tr>
190                                <td>65+</td>
191                                <td><p align="right">73</p></td>
192                                <td><p align="right">456,880</p></td>
193                                <td><p align="right">16.0</p></td>
194                                <td><p align="right">21</p></td>
195                                <td><p align="right">666,485</p></td>
196                                <td><p align="right">3.2</p></td>
197                        </tr>
198                </table>
199                <table class="Info">
200                        <tr>
201                                <td>* Number is too small to calculate a reliable rate.</td>
202                        </tr>
203                </table><br/><br/>
204               
205                <div class="Note">
206                        <img src="../view/image/info_icon.gif" alt="info icon" width="15" height="15" title="Additional Information"/>
207                        Looking at rates within groups is also called "stratification." In Table 2, the population has been stratified
208                        by age and sex. The data in Table 2 also show how useful stratification can be. Not only are the suicide
209                        death rates much higher among men, the rate of suicide increases with age for men, but not for women.<br/>
210                </div><br/><br/>
211                               
212                        <!--div class="Note">
213                                <img src="../view/image/info_icon.gif" alt="info icon" width="15" height="15" title="Additional Information"/>
214                                The <a href="home/Glossary.html#C">crude mortality rate</a> for a population depends on the mortality
215                                rate in each age group as well as on the proportion of people in each age group. For instance,
216                                the age-specific rate for most causes of death will be higher for older age groups. As a result,
217                                crude death rates tend to be higher in populations with a larger proportion of older persons,
218                                and lower in populations with a larger proportion of younger persons.<br/>
219                        </div><br/><br/><br/-->
220                       
221                        Age-specific rates are valuable for comparing rates across age groups, and crude rates provide a
222                        useful summary measure to compare similar populations of different sizes, but the word "similar"
223                        is a key concept. It can be misleading to compare crude rates across populations that have relevant
224                        differences, such as different cultural traditions, or age, race/ethnicity, or sex composition. <br/><br/>
225                       
226                        One difference that is commonly controlled for statistically is age composition of the population.
227                        The <a href="home/Glossary.html#C">crude mortality rate</a> for a population depends on the mortality
228                        rate in each age group as well as on the proportion of people in each age group. For instance,
229                        the age-specific rate for most causes of death will be higher for older age groups. As a result,
230                        crude death rates tend to be higher in populations with a larger proportion of older persons,
231                        and lower in populations with a larger proportion of younger persons. <br/><br/>
232                       
233                        An <a href="home/Glossary.html#A">age-adjusted rate</a> is a summary measure that may be used to
234                        compare mortality or disease risk in two populations with different age compositions.
235                </CONTENT>
236                </ibis:ExpandableContent>
237       
238                <ibis:ExpandableContent titleLevel="2"><SHOW/>
239                <TITLE>Deciding Which Measure to Use</TITLE>
240                <CONTENT>
241                        <!--h1>Deciding Which Measure to Use</h1-->
242                        The measure that best informs the question you are trying to answer is the one to use.
243                        This is a guideline, not a hard and fast rule, but generally: <br/><br/>
244                        <table class="Info">                           
245                                <tr>                   
246                                        <th width="100" style="font-weight:bold">If the question is:</th>               
247                                        <th width="100" style="font-weight:bold">Then use:</th> 
248                                </tr>                   
249                                <tr>                   
250                                        <td>MAGNITUDE: How big is the problem? </td>
251                                        <td>Number of events (count)</td>
252                                </tr>
253                                <tr>                   
254                                        <td>PROBABILITY: What is the underlying risk in a population? </td>
255                                        <td>Crude rate and confidence interval</td>
256                                </tr>
257                                <tr>                   
258                                        <td>DISPARITY: Is there a difference in risk after controlling for age?</td>
259                                        <td>Age-adjusted rate and confidence interval</td>
260                                </tr>
261                        </table>
262                <br/>   
263                <p align="right"><a href="#TOP">Back to Top</a></p>                             
264                        <br/>
265                </CONTENT>
266                </ibis:ExpandableContent>
267
268                <h3>Go to the page on <a href="home/AARate.html">age-adjusted rates for health events.</a></h3>
269                <br/>                   
270        </CONTENT>
271</HTML_CONTENT>
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