Controls are used in analytical testing in order to provide a reference against which experimental results can be compared.  In order to act as a referent, a control material is subjected to the same or similar conditions as a sample material to be tested.  The ability to compare experimental results with the results of measuring a control material enables the assessment of the significance of the experimental results. 
The precise amount of an analyte detected in a control material can change depending on the particular analytical instrument used to assay the control material, as well as on the particular chemical reaction to which the control material is subjected in order to measure the analyte.  To determine the amount of an analyte which a user can expect to detect in a control material (the recovery value) with a particular analytical instrument using a particular chemical method, a control substance is typically tested independently with that instrument and method by a number of different laboratories (see, e.g., Brion, E., et al., “ Evaluation of Commutability of Control Materials,” Clin Chem Lab Med, 40:625-630 (2002)).  The target recovery value for the control is then assigned based on an average of the values determined by such laboratories.