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Table 2 Example of confounding in a hypothetical cohort study of intrauterine device use and salpingitis [8]

From: A critical appraisal of epidemiological studies comes from basic knowledge: a reader's guide to assess potential for biases

    Salpingitis
    Yes No Total Proportion with salpingintis
All women (n = 2,000) Use of IUD Yes 45 955 1,000 4·5 %
   No 15 985 1,000 1·5 %
   Crude RR = 4.5 % 1.5 % = 3.0 ( 95 %  CI  1.7 5.4 ) MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacqqGdbWqcqqGYbGCcqqG1bqDcqqGKbazcqqGLbqzcqqGGaaicqqGsbGucqqGsbGucqGH9aqpdaWcaaqaaiabisda0iabc6caUiabiwda1iabcwcaLaqaaiabigdaXiabc6caUiabiwda1iabcwcaLaaacqGH9aqpcqaIZaWmcqGGUaGlcqaIWaamcqqGGaaicqGGOaakcqaI5aqocqaI1aqncqGGLaqjcqqGGaaicqqGdbWqcqqGjbqscqqGGaaicqaIXaqmcqGGUaGlcqaI3aWncqGHsislcqaI1aqncqGGUaGlcqaI0aancqGGPaqkaaa@51FC@
    Salpingitis
    Yes No Total Proportion with salpingintis
Women with 1 sexual partner (n = 1,200) Use of IUD Yes 3 297 300 1·0%
   No 9 891 900 1·0%
   RR = 1.0 % 1.0 % = 1.0 MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacqqGsbGucqqGsbGucqGH9aqpdaWcaaqaaiabigdaXiabc6caUiabicdaWiabcwcaLaqaaiabigdaXiabc6caUiabicdaWiabcwcaLaaacqGH9aqpcqaIXaqmcqGGUaGlcqaIWaamaaa@3B08@ (95% CI 0.27-3.67)
    Salpingitis
    Yes No Total Proportion with salpingintis
Women with >1 sexual partner (n = 800) Use of IUD Yes 42 658 700 6·0 %
   No 6 94 100 6·0 %
   RR = 6.0 % 6.0 % = 1.0 MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacqqGsbGucqqGsbGucqGH9aqpdaWcaaqaaiabiAda2iabc6caUiabicdaWiabcwcaLaqaaiabiAda2iabc6caUiabicdaWiabcwcaLaaacqGH9aqpcqaIXaqmcqGGUaGlcqaIWaamaaa@3B1C@ (95% CI 0.43-2.29)