 This can occur, for example, when the lake already contains a small number of labeled fish from the previous field experiments . In such situation, the following expression can be employed: where  RA is the ratio of the native-to-labeled fish in the lake , RB is the ratio of the native-to-labeled fish in the lot of nB marked fish which are added to the pond, and RAB is the ratio of the native-to-labeled fish captured during the second visit.
<meaning-changed>  This can occur, for example, when the lake already contains a small number of labeled fish from the previous field experiments . In such situation, the following expression can be employed: where  RA is the ratio of the native-to-labeled fish in the lake , RB is the ratio of the native-to-labeled fish in the lot of nB marked fish which are added to the pond, and RAB is the ratio of the native-to-labeled fish captured during the second visit.
 This can occur, for example, when the lake already contains a small number of labeled fish from the previous field experiments ; and vice versa, where the amount of marked fish added contains a small number of unmarked fish. In a laboratory setting, an unknown (the "lake") may contain a quantity of a compound that is naturally present in dominant ("blue") and lesser ("yellow") isotopic forms. A standard that is enriched with the lesser isotopic form may then be added to the unknown, which can be subsequently analyzed. Keeping to the fish analogy, the following expression can be employed: where RA is the ratio of the native-to-labeled fish in the lake , RB is the ratio of the native-to-labeled fish in the lot of nB marked fish which are added to the pond, and RAB is the ratio of the native-to-labeled fish captured during the second visit.