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Odds Ratio vs Relative Risk

Odds Ratio vs Relative Risk. Genetic studies of multifactorial diseases. Department of Medical Genetics Medical University of Warsaw. Probability – definition. Probability of an event P(A) denotes the frequency of it’s occurrence in a long (infinite) series of repeats.

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Odds Ratio vs Relative Risk

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  1. Odds Ratio vs Relative Risk Genetic studies of multifactorial diseases Department of Medical Genetics Medical University of Warsaw

  2. Probability – definition • Probability of an eventP(A) denotes the frequency of it’s occurrence in a long (infinite) series of repeats Probability0.25 (1/4) indicates that certain event is observed in 1 from 4 or 25% occurrences It’s represented by a number in the range from 0 (event never occurs) to 1 (event occurs every time)

  3. Odds • Odds - describe probability as a ratio (proportion) Odds take higher values than probability For example, if probability of getting ill is 1/5, the odds of getting ill is:

  4. Odds vs. probability • Frequency of blue balls: 4/10 = 2/5 (two out of five balls are blue) • Proportion of blue balls vs. yellow balls: 4:6 = 2:3 (2 to 3)

  5. Influence of the environment Polygenic diseases (multifactorial diseases) • Many genes involved • Multiple factors (including environmental ones) + genetically predetermined susceptibility to the action of these factors AD AR Polygenic

  6. atherosclerosis hypertension diabetes neoplasms neurodegenerative diseases immunological disorders (rheumatoid arthritis, multiple sclerosis etc.) … and many others Polygenic diseases(examples)

  7. Ethiology: single gene vs polygenic (multifactorial) New methods of analysis needed!

  8. GWAS atherosclerosis polymorphisms

  9. Genetic background of a disease – ideal study • Genotyping: everybody is genotyped and classified • Monitoring: occurrence of the disease is monitored for a long time • Comparison: frequency of the disease in two groups

  10. Genetic background of a disease – ideal study (an example) 26% (n=128) Genotype (+) 15% (n=75) Genotype (-)

  11. Comparing the risks - measures • Risk: % of affected individuals • Risk difference • Risk ratio (relative risk, RR) • Example - a disease occurred: - in 26% of individuals with given genotype- in 15% of the persons without the genotype Risk difference:26% - 15% = 11% Risk ratio (RR):26% /15% = 1.7

  12. Comparing the risks- RD vs RR • If a disease is rare,the risk difference may be small,but the risk ratio – large ! • Example: - risk among those with genotype: R=1.5%- risk among those without: R =0.3% Difference= 1.5% - 0.3% = 1.2% Ratio(RR)= 1.5% /0.3% = 5.0

  13. Comparing the risks- RD vs RR – which is better? • Difference – an absolute measure - Useful in estimating how many people are ill because of the genotype • Ratio – a relative measure - better for estimating the risk in a given patient - better for the purpose of basic studies

  14. Affected Healthy (a+b) a b Genotype(+) observation Affected Healthy c d (c+d) Genotype(-) Comparing the risks- associations in a table = a/(a+b) / c/(c+d) RR = Proportion of frequency of affected among those withthe genotypeto the frequency of affected among those without the genotype

  15. RR – relative risk Affected Yes No a b a Yes a + b RR = Genotype c d c c + d No

  16. Affected Healthy (a+b) a b Genotype(+) observation Affected Healthy c d (c+d) Genotype(-) Comparing the odds – odds ratio (a table) = [a/(a+b) / (1-a/(a+b))] / [c/(c+d) / (1-c/(c+d))] OR Proportion of the odds among those withthe genotypeto the odds among those without the genotype

  17. OR - odds ratio Affected a Yes No a + b a b a Yes 1 − a + b OR = Genotype c d c c + d No c 1 − c + d

  18. OR calculated for occurrence of disease among those with vs. without the genotype

  19. OR calculated for occurrence of genotype among those with vs. without the disease (cn.)

  20. Comparing the risks- OR vs RR – rare diseases • If a disease is rare(both among those with and without genotype), RR and OR areverysimilar ! • Example: • Frequency of RA among • - those withHLA-DR4 - 0.018, • - the rest -0.003 • RR = 0.018/0.003 = 6.0 • OR = 0.01833/0.00301 = 6.09 • „rare” means<10%

  21. Comparing the risks- OR vs RR – frequent diseases • If a disease is frequent, RR and OR aredifferent ! • Example: • - Risk in the presence of genotype =0.6 • - Risk in the absence of genotype = 0.1 : • RR = 0.6/0.1 = 6.0 • OR = 0.6/0.4 / 0.1/0.9 = 13.5 • „frequent” means>10%

  22. It isusuallyeasierto determine OR than RR !

  23. 7 493 496 4 OR vs RR – (practical considerations) Disease Genotype (+) Genotype Genotype (-) RR=(7/500)/(4/500) RR=7/4 the key factor: proportion 7/4 (considerable errorispossible)

  24. Disease + - 493 700 7 493 Genotype - + 496 400 496 4 RR=1,76 OR=1,75 RR=1,31 OR=1,75 The error can be minimized by enlarging the group of affected RR has changed but OR has not ! Disease + Disease + - 493 707 + Genotype + = 404 496

  25. RR is the preferred measure of strength of association between genotype and disease, esp. in a medical/diagnostic setting In practice it is easier to determine OR OR is a good approximation of RR,when the disease is rare (<10%) both in those with and without the genotype Conclusions

  26. What to remember? When to use and how to calculate RR? Whendiseasesarefrequent >10% When to use and how to calculate OR? Whendiseasesarerare <10%

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