An experimental design is of resolution 
if no 
factor effect is aliased with another
effect containing less than 
factors. When the fractional factorial
design is developed, the defining relation
is composed of roman letters.
As an example, 
design
is constructed with the defining relation 
or 
. In this case, the minimum number of
letters in the defining relations represents the resolution of design. Hence,
design is called a
resolution III.
1. Resolution III designs. These
are designs in which no main effects are aliased with any other main effect, but
main effects aliased with two-factor interactions and two-factor interactions
may be aliased with each other. A design with is of resolution III 
. These are main effects only
designs.
2. Resolution IV designs. These
are designs in which no main effect is aliased with any other main effect or
with any two-factor interaction, but two-factor interactions are aliased with
other. A 
design with
is of resolution IV
. These are main effects
clear designs.
3. Resolution V designs. These
are designs in which no main effect or two-factor interaction is aliased with
any other main effect or two-factor interaction, but two-factor interactions are
aliased with three-factor interactions. A 
design with 
is of resolution V 
. These are two-factor interactions clear
designs.