Simplicity of conducting the study is to as ability to test large numbers of participants is to

To complete the statement above:

Simplicity of conducting the study is to archival research; as ability to test large numbers of participants is to survey. 

Archival research will inquire about including essential sources held in files, a Special Collections library, or another storehouse. 
 Archival research is a technique for gathering information from sources that as of now exist. Normal cases of documented research sources are evaluation records or overview information that was gathered previously.