There are many types of evidence,
but not all are equally reliable.

Systematic reviews and meta-analyses*
are considered to be the least biased forms of evidence synthesis.

This is true, if they follow a systematic review process –
procedures designed to reduce risk of bias*
and increase transparency*.

To reduce risk of bias in our database
we followed the systematic review process.  
We are also transparent about what we did –
technical report on search and data extraction
 can be downloaded here.

*meta-analyses are systematic reviews that do some number-crunching using statistics
*risk of bias he risk of overestimating or underestimating the true effect
*transparency means being open and explicit about all details of your work