The use of blind or double-blind investigators in a study is done to avoid:

sampling error.
sample bias.
uncontrollable variables.
researcher bias.
random error.

2 answers

Ste/ty/andrew -- please use the same name for all of your posts.

What is YOUR answer?
The double blind method is an important part of the scientific method, used to prevent research outcomes from being 'influenced' by the placebo effect or observer bias. Blinded research is an important tool in many fields of research, from medicine, to psychology and the social sciences, to forensics.