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APPENDIX V. METHOD OF CALCULATING OBSERVED TO EXPECTED RATIO AS A MEASURE OF STRENGTH OF ASSOCIATION
ii) total number of deaths where cause 1 is present
iii) total number of deaths where cause 2 is present
iv) total number of deaths from all causes
The expected frequency can be ascertained by multiplying the number of deaths in which cause 1 is present by the number of deaths in which cause 2 is present, and then dividing this product by the total number of deaths in that time period and demographic. This number represents the frequency with which the two causes would be expected to appear together if there was no association between the causes.
A ratio of observed to expected joint frequency can then be calculated.
Worked example (diabetes mellitus(E10-E14) and Ischaemic heart diseases(I20-I25)):
This example is using data from 1997-2001 (for all ages) coded to ICD-10
Total number of deaths with diabetes present = 49,012
Total number of deaths with Ischaemic heart diseases present = 190,524
Total number of deaths from all causes = 641,489
Expected number of deaths with both causes (assuming independence) = (49,012 x 190,524) / 641,489 = 14556.7
Ratio actual/expected = 24543/14556.7 = 1.69
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