document.write( "Question 730580: I am trying to calculate the 95% confidence interval for a given odds ratio (OR) using the formulas \n" );
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document.write( "a = exposure and outcome present \n" );
document.write( "b = exposure and outcome absent \n" );
document.write( "c = no exposure and outcome present \n" );
document.write( "d = no exposure and outcome absent \n" );
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document.write( "OR = (a/c)/(b/d) which can be rewritten as OR = (a)x(d)/(b)x(c)\r \n" );
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document.write( " 95% CI for the OR = [1n (OR) +/- 1.96√1/a + 1/b + 1/c + 1/d)]\r \n" );
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document.write( "OR = (285)x(111)/(98)x(268) = 1.21\r \n" );
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document.write( "So I used a natural logarithm table to find the ln for the OR I calculated. ln for 1.21 = 0.1906\r \n" );
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document.write( "95% CI = [1n (OR) +/- 1.96√(1/285 + 1/98 + 1/268 + 1/111)] = 0.1906 +/- 0.1567 \n" );
document.write( " = 0.0339 to 0.3473 = exp(0.0330) to exp(0.3473)\r \n" );
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document.write( "So, \n" );
document.write( "OR (1.21) [95% CI = 1.03 to 1.42]\r \n" );
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document.write( "I think that is correct. My problem is, how do I know what the natural log is for an OR that is less than 1.0? I have an OR = 0.95 and I don't know how to calculate the 95%CI for that OR. The natural logarithm tables are for numbers 1.0 and higher.\r \n" );
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document.write( "Thanks you! \n" );
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