But then he imagines computing the correlation between mind and body from a sample of working actors. To get work, his actors have been selected by casting directors only if they have a high value for the sum of both mind and body. He has marked these working, observed actors with small triangles in the plot. The remaining aspiring non-working and unobserved actors are marked with small circles. The plotted pattern of triangles for the working, observed actors has a definite negative correlation suggesting, wrongly, that either the more able actors are not much to look at or the most attractive actors are dunces in their acting ability. Although this might fit some stereotypical caricatures, it is entirely due to the selective sampling based on the definition that produced our observed sample. He continues with an SAT example as well.
His scatterplot can be improved. Measurements with equal variability should be plotted in a square plot. The plotting characters for the unobserved and observed actors could be more pronounced. See an example below.