Such observer bias effects are near-universal in human data interpretation under expectation and in the presence of imperfect cultural and methodological norms that promote or enforce objectivity.
[3] The classic example of experimenter bias is that of "Clever Hans", an Orlov Trotter horse claimed by his owner von Osten to be able to do arithmetic and other tasks.
As a result of the large public interest in Clever Hans, philosopher and psychologist Carl Stumpf, along with his assistant Oskar Pfungst, investigated these claims.
As an example, researchers compared performance of two groups given the same task (rating portrait pictures and estimating how successful each individual was on a scale of −10 to 10), but with different experimenter expectations.
It might be thought that, due to the central limit theorem of statistics, collecting more independent measurements will improve the precision of estimates, thus decreasing bias.