Both physical and neurological attributes of dogs contribute to their ability to detect extremely small concentrations of specific scents and their ability to differentiate between odours. The accuracy of these detections is determined by the concept of scent groups. A scent group refers to a specific mixture of scents that comprise the overall odour of an individual. A dogs acuity to a scent group is so refined they are able to discriminate humans by odour and can even match certain scents to specific body parts of an individual. Scent discrimination is most proficient while a human odour is fresh and becomes more difficult once an odour starts to fade.