The following will describe concepts that affects the quality of primary data studies particularly their ability to yield unbiased and precise estimate of treatment effects and other findings

1 Prospective, rather than retrospective design 2 • Experimental rather than observational 3 Controlled, rather than uncontrolled 4 Contemporaneous control groups 5 Internal control groups rather than external ones 6 Allocation concealment 7 Randomized assignment of patients 8 Blinding of patients 9 Large enough sample size to detect true treatment effects 10 Minimal patient drop-outs or loss