The Counting Horse, sound file.
The case of Clever Hans was so influential that Pfungst’s finding came to be called the Clever Hans effect. Further research showed that nearly everyone will display some signals, even if they are actively trying not to do so.
The observer effect can change the outcome of some studies so badly that preventing problems led to the creation of of double-blind tests. In a double-blind test, neither the subjects or the people running the experiment know what idea they are researching. Only the researcher — who is not involved with the subjects — knows the real reason behind the experiment.
Some experiments go even further, and use a triple-blind design. The subjects don’t know what is being researched, the people running the experiment don’t know either, and the researcher interpreting the collected information also is not informed about the real object of the study. This is to prevent bias* at every step in collecting, reporting, and analyzing the evidence. It is especially useful for avoiding “cherry picking” or confirmation bias.†