If del C-13 has not been measured for a given sample, it can be estimated on the basis of thousands of such measurements that have already been reported. However, the estimate contributes an additional degree of uncertainty that is reflected by an error term in the correction formulae. Corrections for isotopic fractionation in commonly dated materials are summarized below:

It is important to note that the formulae for bison collagen and human collagen yield only minimum corrections. In the case of bison, one cannot know, unless del C-13 has been measured, the proportion of C4 plants that comprised the animal’s diet. The estimated value, -20 parts per mil, yields an adequate correction only if the animal never consumed C4 plants. Likewise, the estimated value for human collagen, -19 parts per mil, yields an adequate correction for humans that consumed no marine resources, no C4 plant-eating bison, and no corn. Increases in any of these dietary resources would enrich the C-13 ratio above -19 and render the age correction too small by 16 years for every part per mil change in the ratio.

Further reading:

Stuiver, M. and Polach, H.A.
1977 Reporting of 14C data. Radiocarbon 19(3): 355-363.