If properly conducted can show cause and effect relationship

Observation in which subjects are selected and then their previous conditions or behaviors are determined

A dummy treatment used to see if something works

a group of people who receive a placebo, or receives the baseline treatment level

When an observed outcome is too large to just occur by chance

Which is not a principles of experimental design?

A group of experimental units or subjects that are similar in some way and are expected to respond the same to a treatment

When levels of one factor are associated with levels of another factor so their effects cannot be separated