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A brief summary of frequency distribution shape is as follows:
Positively skewed: Most of the scores will be to the left with few high value scores forming a positive "tail" on the right hand side of the distribution. The mean is greater than the median because the high value scores pull the mean to the right while not affecting the median all that much.
Negatively skewed: Most of the scores will be to the right with few low value scores forming a negative "tail" on the left hand side of the distribution. The mean is less than the median because the low value scores pull the mean to the left while not affecting the median all that much.
Symmetrical: Scores are distributed such that the left hand side and the right hand side of the distribution look similar, for example a "bell" curve. The mean and the median are approximately equal.
Bimodal: A distribution with two distinct "peaks", generally caused by two different groups of data mixed together in the one frequency distribution. Outlier(s): A score or a very small set of scores that "lie out" a significant distance from the rest of the scores. Outliers are extreme values that need to be investigated further as to why they occurred.
Clustering: May be related to bimodal or multimodal distributions. Scores that form subgroups (called "clusters") separated by small gaps. Clusters are different to outliers in that the clusters can be close to one another but at the same time clearly separated from each other.
