---| 9 |-----------------------------------------

20    -----------------------|
      |                      |
      |                      |
      |                      |
      |                      |
      |                      |
      |               +      |
      |               +      |
10    -               +      |
      |               +      |
      |               +      |
      |      *        +      |
      |      *        +      |
      |      *        +      |
      |      *        +      |
      |      *        +      |
      |      *        +      |
0     -------|--------|------|
             boys     girls   
              mean IQ