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Jul
10

Reverse engineering contingency (2×2) table from Odds Ratio (OR)

Given the odds ratio (OR), we will calculate the individual cells in the contingency table (a,b,c,d).

In yellow, I’ve highlighted what is known.
a,b,c, and d are unknown and what we want to calculate.

Odds Ratio = (a/c) / (b/d)

Cases Controls Total
Exposed a b total_exposed
(a+b)
Unexposed c d total_unexposed
(c+d)
Total total_cases
(a+c)
total_controls
(b+d)
total_participants

If you’re getting the OR from a paper, the paper usually has total_exposed, total_unexposed, total_cases,and total_participants.

In that case, you can derive a, b, c, and d.

Solving for a:

Cases Controls Total
Exposed a total_exposed – a total_exposed (a+b)
Unexposed total_cases – a total_unexposed – total_cases + a total_unexposed (c+d)
Total total_cases (a+c) total_controls (b+d) total_participants

So now, the equation for OR can be written in terms of a and the known numbers :

OR = (a * d) / (b * c)
OR = (a * (total_unexposed – total_cases + a)) / ((total_exposed – a) * (total_cases – a))

If you have the values for OR, total_exposed, total_unexposed, total_cases, and total_controls, you can solve for a</i> using the quadratic formula.

Once you solve for a, solving for b, c, and d is trivial.

Try it out!

 Deriving cells of 2×2 Contingency Table from Odds Ratio:

Enter values in yellow cells

Condition

Odds
Ratio
Absent Present Totals
  Group 1  
  Group 2  
Totals

   

I came across this problem when reading an Alzheimer’s paper.

Looking at ApoE ε4 carriers (n=452), smokers have an OR of 1.97 for dementia compared to non-smokers.

Because this was a population study, I wanted to know how many smokers got dementia, and how many non-smokers got dementia. If I got the individual cells, I could calculate this.

Out of the 452 ApoE ε4 carriers, 207 were smokers (45.8% of 452) and 31 had dementia (6.9% of 452).

From this,

  • OR = 1.97
  • total_exposed = 207
  • total_unexposed = 245
  • total cases (those with dementia) = 31
  • total controls (without dementia) = 421

I plugged in the above calculator to get:

Dementia Non-Dementia Total
Smoking 19 188 207
Non-smoking 12 233 245
Total 31 421 452

In this population-based study, 9% (19/207) of the smokers had dementia while 5% (12/245) of the nonsmokers had dementia.

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