The invention relates to a reference plant which has been selected to: a) not express a medicinally active compound or group of compounds; yet b) express, at least substantially qualitatively, most other non medicinally active compounds present in a therapeutically active comparator plant. The reference plant can be used to generate a reference extract with a reference chemical profile which resembles that of the comparator plant less the active compound or group of compounds and may thus be used as a placebo or to otherwise test the hypothesis that the active compound or compounds are responsible for an extracts perceived medicinal activity.