A researcher is studying the effect of three different row spacing on the yields of four different varieties of cotton. He randomly assigns each row spacing-variety combination to six 50' x 50' plots. Within each plot he randomly selects 12 plants and weighs the cotton yield of the individual plants
a) Is this an observational study or an experimental study
b) what type of experimental design is this
c) what is or are the treatment factor or factors
d) what are the experimental units
e) what are the measurement units