Difference

Difference or reverse Helmert contrasts. Compares categories of an independent variable with the mean of the previous categories of the variable. The general matrix form is

   mean (     1/k       1/k       1/k  ...  1/k)
  df(1) (      -1         1         0  ...    0)
  df(2) (    -1/2      -1/2         1  ...    0)
      .                   .
      .                   .
df(k-1) (-1/(k-1)  -1/(k-1)  -1/(k-1)  ...    1)

where k is the number of categories for the independent variable. For example, the difference contrasts for an independent variable with four categories are as follows:

( 1/4   1/4   1/4   1/4)
(  -1     1     0     0)
(-1/2  -1/2     1     0)
(-1/3  -1/3  -1/3     1)