If you are trying to determine whether a sample is above background, the more appropriate term would be the decision level where : (Embedded image moved to file: pic18515.pcx) DL in cps(or cpm) 1.645 value corresponds to a 5% false alarm rate. Other Equations are available in NUREG-1400 Jack Priest Health Physicist Davis Besse NPS jmpriest@firstenergycorp.com 419-321-7809