Metanalysis is a statistical technique where all data from all available studies of a topic are combined, regardless of data quality. The technique is used by researchers to get a maximum of statistical information without worrying about distortion in the results.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.