While not an expected source of error in this particular design, nearby material influence is a very important error factor to consider. In fact this possible error influenced the preliminary design approach. As previously mentioned, metal springs were intentionally avoided in the mechanical design aspect. The reason stems from the fact that many metal springs are composed of highly-permeable materials with low reluctance. Magnetic fields travel the path of least reluctance from the north to south poles of the magnet. This can either concentrate or divert your field in such a way that helps or hurts your design. When considering material influence, magnetic simulation and bench test are necessary.
To illustrate the concept of how material influences the field, Figure 2-34 shows a SOT-23 with a magnet suspended above. In the left example is a mu metal cylinder wrapping around the device. Observe that the B field concentrates within this cylinder and the aerial view of the device below illustrates that the field observed by the Hall element is basically 0 mT. By comparing to the unshielded example on the right, notice that at least 30 mT were diverted away from being measured.