Topical use is the main advantage for dermatological therapy due to the outer-layer features of skin. Consequently numerous preparations are available for the dermatosis. However, the efficacy evaluation of different preparations accurately is always very difficult. The traditional and accepted method until now it is also popular in practice) to observe treatment effect of cream is to apply one kind of drug to a group of patients with a specific disease who received different systemic approaches that might influence the effect of topical appliance. Here we show 3 very severely affected psoriatic patients, with several years of history, whose lumber regions were marked into six different areas being applied with six different kinds of cream, respectively in the following seven days. To our great surprise, most of the creams have similar effects, even the vehicle cream without any medicine component. I coined the SPSS rule which stands for the acronym of same patient, same site or symmetrical site for different creams. This method avoids the influences of different systemic medicine, patients’ personal response, anatomical sites and disease duration on the efficacy evaluation of topical preparations. I propose it be implemented in the future evaluation of topical preparations.