Difference between revisions of "Measuring incidence rates"

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==Cohort studies measuring incidence rates==
  
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The computation of effects with incidence rates is similar to calculation of effects from incidence proportions (risk).  The incidence rate of disease in exposed (IRe) and unexposed (IRu) can be computed as follows:
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[[File:0676.Page3 formula3a IRe.gif-550x0.png|800px|frameless|left]]
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[[File:1830.Page3 formula3a IRu.gif-550x0.png|800px|frameless|left]]
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A rate difference can be computed: 
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The relative effect of the exposure on disease occurrence can be measured by computing the rate ratio minus 1.
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[[File:3757.Page3 formula4 relative effect.gif-550x0.png|800px|frameless|left]]
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The rate ratio is:
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[[File:5008.Page3 formula5 rate ratio.gif-550x0.png|800px|frameless|left]]
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==Example==
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Breast cancer cases and person-years of observation for women with tuberculosis repeatedly exposed to multiple x-ray fluoroscopies and unexposed women with tuberculosis
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{| class="wikitable" style="text-align:right; vertical-align:bottom;"
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|-
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! Radiation  exposure
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! Person-years
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! Breast cancer
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! Rate/10000 p-y
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! Rate ratio
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! Rate difference
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! Relative effect
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|-
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| Yes
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| 28010
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| 14
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| 14.6
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| 1.86
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| 6.7
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| 0.86
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|-
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| NO
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| 19017
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| 15
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| 7.9
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|
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|
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|
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|}
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Source: Boice & Monson [1]
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One can express the result by saying that the relative effect is 0.86 which would suggest an 86 % increased rate of breast cancer among exposed. One can also express the results by saying that the rate of breast cancer is 1.86 times higher in the exposed cohort than in the unexposed cohort.
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=References=
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1. Boice, J. D., and R. R. Monson. Breast cancer in women after repeated fluoroscopic examinations of the chest. Journal of the National Cancer Institute 1977 59: 823–832.
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<div style="display: inline-block; width: 25%; vertical-align: top; border: 1px solid #000; background-color: #d7effc; padding: 10px; margin: 5px;">
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'''FEM PAGE CONTRIBUTORS 2007'''
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;Editor
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:Masja Straetemans
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;Original Authors
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:Alain Moren
 +
:Jean Claude Desenclos
 +
:Marta Valenciano
 +
:Arnold Bosman
 +
;Contributors
 +
:Lisa Lazareck
 +
:Masja Straetemans
 +
</div>
  
 
[[Category:Cohort studies]]
 
[[Category:Cohort studies]]

Latest revision as of 21:50, 10 April 2023

Cohort studies measuring incidence rates

The computation of effects with incidence rates is similar to calculation of effects from incidence proportions (risk). The incidence rate of disease in exposed (IRe) and unexposed (IRu) can be computed as follows:

0676.Page3 formula3a IRe.gif-550x0.png


1830.Page3 formula3a IRu.gif-550x0.png


A rate difference can be computed:

The relative effect of the exposure on disease occurrence can be measured by computing the rate ratio minus 1.

3757.Page3 formula4 relative effect.gif-550x0.png


The rate ratio is:

5008.Page3 formula5 rate ratio.gif-550x0.png


Example

Breast cancer cases and person-years of observation for women with tuberculosis repeatedly exposed to multiple x-ray fluoroscopies and unexposed women with tuberculosis

Radiation exposure Person-years Breast cancer Rate/10000 p-y Rate ratio Rate difference Relative effect
Yes 28010 14 14.6 1.86 6.7 0.86
NO 19017 15 7.9

Source: Boice & Monson [1]

One can express the result by saying that the relative effect is 0.86 which would suggest an 86 % increased rate of breast cancer among exposed. One can also express the results by saying that the rate of breast cancer is 1.86 times higher in the exposed cohort than in the unexposed cohort.

References

1. Boice, J. D., and R. R. Monson. Breast cancer in women after repeated fluoroscopic examinations of the chest. Journal of the National Cancer Institute 1977 59: 823–832.

FEM PAGE CONTRIBUTORS 2007

Editor
Masja Straetemans
Original Authors
Alain Moren
Jean Claude Desenclos
Marta Valenciano
Arnold Bosman
Contributors
Lisa Lazareck
Masja Straetemans
Retrieved from "https://femwiki.org/index.php?title=Measuring_incidence_rates&oldid=1670"

Contributors