What are ceiling effects?
From PsychWiki - A Collaborative Psychology Wiki
Ceiling effects is when measurements of the dependent variable result in many very high or highest scores on the dependent variable, thus masking a potential effect of the independent variable. Ceiling effects may occur when giving examinations to students, when the exam is so easy that all the students correctly answered every test item.
Example / Application
Example: Ceiling effect on a bar graph
Application: This image shows signs of a ceiling effect, in that all the principles in this study rated above 4.0 on a 1-6 scale, creating high scores across the bar graph.
Lammers, W. J., and Badia, P. (2005). Fundamental of Behavioral Research. California: Thomson and Wadsworth.