4. The studies had to provide sufficient information to estimate the Odds ratio (OR) and 95 % confidence intervals (CI)
5. The studies were original studies of risk factors that had adequate control groups and reported the actual numbers of patients
6. The studies had to be independent in order to avoid giving double weight to some studies
1. The study does not focus on risk factors
2. Conference abstracts
3. Not an original study (e.g., editorial or literature review)
4. Studies in those without general population of infected and non-infected group