detection limits, censored observations, and holding time violations for chemistry analytes - USEPA/SuRGE GitHub Wiki

detection limit flags

Measured concentrations that are below the method detection limit (MDL) will be flagged as "ND" and reported as the value of the MDL. Values that exceed the method detection limit (MDL) but are less than the reporting limit (RL) will be flagged as "L". Procedures vary across labs for defining a RL. We assume equivalency between the terms "quantification limit" and "reporting limit". If neither value is reported by the lab, we assume the RL is equal to the lowest non-zero standard in the standard curve.

flags and duplicates

Each chemistry analyte will have a xx_flag field (i.e., no2_flag). In cases where one duplicate was was flagged for a detection limit issue, but the other was not, the aggregated value will not be flagged (see issue #12). In cases where duplicates have different flags, the aggregated value will have the flag associated with the higher concentration sample. For example, one duplicate is flagged ND and the other L, the aggregated value is flagged L (see issue #12).

summary

Condition	Value Reported	Flag
< MDL	MDL	ND
>MDL and < RL	Measured concentration	L
>RL	Measured concentration
Holding time violation	Measured concentration	H
Shipping issue	Measured concentration	S