The eigenvalues for successive factors can be displayed in a simple line plot. Cattell (1966) proposed that this scree plot can be used to graphically determine the optimal number of factors to retain.

The scree test involves finding the place where the smooth decrease of eigenvalues appears to level off to the right of the plot. To the right of this point, presumably, you find only "factorial scree" - "scree" is the geological term referring to the debris that collects on the lower part of a rocky slope. Thus, no more than the number of factors to the left of this point should be retained.

For more information on procedures for determining the optimal number of factors to retain, see Reviewing the Results of a Principal Components Analysis in the Overviews for the Factor Analysis module and How Many Dimensions to Specify in the Multidimensional Scaling module.