This is the 2007 version. Click here for the 2017 chapter 01 table of contents.

Measurement and Observer Effects

In observational research, results can be changed or biased by the act of measurement itself. This is called a measurement effect .

What are measurement effects? What is an observer effect? How can it distort observational data?

One type of measurement effect is called an observer effect. It occurs when subjects alter their behavior because an observer is present. For example, a developmental psychology student might decide to study parent/child interactions by bringing video equipment into homes and recording the behavior of parents and children. The presence of an observer is likely to change the behavior of both parents and children. Similar problems affect every television documentary about particular societies or cultures: it is always possible that the presence of a camera changes behavior.

What are unobtrusive measures, and why are they used?

One way to avoid observer effects is to use unobtrusive measures (Webb, Campbell, Stanley & Sechrest, 1966) in which the subject is not aware of being observed or tested. An unobtrusive study of parent/child interactions in a department store might use video records from security cameras. Unobtrusive measures are used to prevent the observer effects that may occur when a person knows he or she is being observed.

Webb, Campbell, Stanley, and Sechrest told how a museum wanted to measure which of its exhibits was attracting the most interest from visitors, but they did not want to bother visitors with a questionnaire. They came up with a clever unobtrusive measure: they examined the amount of wear on the floor tiles by each exhibit. When the tiles in front of an exhibit showed more wear and tear, they could assume more people were stopping there to look at the exhibit.

What unobtrusive measure was used in a museum?

The German researcher Eibl-Eibesfeldt used a form of unobtrusive measure in his studies of native cultures. During some of his movie making, Eibl-Eibesfeldt put a mirror in front of the lens of his camera. In that case, the camera photographed everything off to the side rather than things in front of it. Eibl-Eibesfeldt would obtain permission from a family to film their everyday behaviors. Then he would point his camera away from them and start the camera running.

How did Eibl-Eibesfeldt minimize observer effects with a "somewhat sneaky" technique?

The people knew enough about cameras to assume he was not photographing them if the camera was pointed a different direction, so they "acted natural" and Eibl-Eibesfeldt was able to photograph them interacting in natural ways while they thought they were off-camera. This was somewhat sneaky but probably essential for getting an accurate record of un-self-conscious behavior and avoiding observer effects.

Prof. Eibl-Eibesfeldt put the film archive of his right-angle camera documentary work at this URL:


Write to Dr. Dewey at

Don't see what you need? Psych Web has over 1,000 pages, so it may be elsewhere on the site. Do a site-specific Google search using the box below.

Custom Search

Copyright © 2007-2011 Russ Dewey