A number representing the typical score attained by a group of subjects.
Measures of central tendency: Indices representing the average or typical score attained by a group of subjects; the most commonly used in educational research are the mean and the median.
Researcher bias - halo (all will be good) or pitchfork - all will be bad
Observation bias -
goal is to slect participants who are REPRESENTATIVE of the entire population
groups are identified, not indiv.: 5th grader classes in APS, then randomly selct a certain # from each class (cluster)