# Contrast Types

**Deviation.** Compares the
mean of each level (except a reference category) to the mean of all
of the levels (grand mean). The levels of the factor can be in any
order.

**Simple.** Compares the mean of each level to the mean of a specified level.
This type of contrast is useful when there is a control group. You
can choose the first or last category as the reference.

**Difference.** Compares the mean of each level (except the first) to the mean of
previous levels. (Sometimes called reverse Helmert contrasts.)

**Helmert.** Compares the mean of each level of the factor (except the last)
to the mean of subsequent levels.

**Repeated.** Compares the mean of each level (except the last) to the mean of
the subsequent level.

**Polynomial.** Compares the linear effect, quadratic effect, cubic effect, and
so on. The first degree of freedom contains the linear effect across
all categories; the second degree of freedom, the quadratic effect;
and so on. These contrasts are often used to estimate polynomial trends.