Accuracy versus precision

Soil carbon should be measured as accurate and precise as possible. The concept of accuracy and precision is illustrated in Figure 1,2 by a simple example.

Figure 1. Accuracy is the proximity of measurement results to the true value; precision, the repeatability, or reproducibility of the measurement (http://en.wikipedia.org/wiki/Accuracy_and_precision).

Figure 2. Soil carbon measurement should target high precision and accurate measurements (Case 1). Case 1: all measurements are close to the true value  (accurate) and all measurements are close to each other or less variation among measurements  (precise); Case 2: the measurement is precise but all measurements are far from the actual value (center) which makes it biased and inaccurate; Case 3: most measurements are reassembly close to the true value (accurate) but not precise; Case 4: all measurements far from the true value (less accurate) and there is high variation between measurements (less precise).

See table 1 for major sources of uncertainties in SOC measurement.
Table 1. Source of uncertainty in SOC measurement