Two essential features of all statistically designed experiments are (a) compare several treatments; use the double-blind method. (b) compare several treatments; use chance to assign subjects to...

Two essential features of all statistically designed experiments are

(a) compare several treatments; use the double-blind method.

(b) compare several treatments; use chance to assign subjects to treatments.

(c) always have a placebo group; use the double-blind method.

(d) use a block design; use chance to assign subjects to treatments.

(e) use enough subjects; always have a control group.