Final answer:
The planned contrast that compares each category except the first (or last) to the overall experimental effect is known as the Helmert contrast.
Step-by-step explanation:
The planned contrast being referred to is the Helmert contrast. This type of contrast is used in statistical analysis when you want to compare each level of a categorical variable with the mean of the subsequent levels. Specifically, in a Helmert contrast, each category, except for the first (or in some versions, the last), is compared against the mean of the subsequent categories, effectively measuring the effect of each category relative to the average of those that follow. This is used to determine if there are significant differences as you move through the levels of the categorical variable, which could be meaningful in experimental and observational studies where categorical factors have an ordered relationship.