one group pre post test design maturation history regression attrition testing instrumentation observer bias demand characteristic double blind study placebo effects null effect ceiling and floor effects a study in which a researcher recruits one group of participants measured them on a pretest exposes them to treatment intervention or change and the measures them on a post test a threat to internal validity that occurs when an observed change in an experimental group could have emerges more or less spontaneously over time a threat to internal validity that occurs when it is unclear whether a change in the treatment group is caused by the treatment of by a historical event that affects everyone or almost everyone in the group a threat to internal validity related to regression toward the mean by which any extreme finding is likely to be closer to its own typical or mean level the next time it is measured with or without the experimental treatment or intervention in a repeated measures experiment or quasi experiment a threat to internal validity that occurs when a systematic type of participant drops out of a study before it ends more than once in a repeated measures experiment or quasi experiment a kind of order effect in which scores change over time just because participants have take the test a threat t internal validity that occurs when a measuring instrument changes over time from having been used before a bias that occurs when observers expectations influence that interpretation of the subjects behavior or the outcome of the study cues that lead participants to guess a study s hypothesis or goals a study in which neither the participants nor the researchers who evaluate them known who is in the treatment group and who is in the comparisons a control group that is exposed to an inert treatment a finding that an independent variable did not make a difference in the dependent variable that there is no significant covariance between the two an experimental design problem in which independent variable groups score almost the same on a dependent variable such that all scores fall at the high end of their possible distribution an experimental design problem in which independent variable groups score almost the same on a dependent variable such that all scores fall at the low end of their possible distribution