Fig. 1. Precision versus accuracy.
Sources of errors [Textbox 2.]
Recognized sources of error in morphometry include three broad classes of observational errors:
1. Random errors, which occur irregularly and hence are unpredictable. Such errors arise in three different ways: random oscillations of the apparatus, mechanical vibrations, and minor positional changes of the subject at every single measurement. This type of error results in dissimilar outcomes, which can be detected by replicated observations. Random error primarily affects precision.
2. Systematic errors, which can be subdivided into a) observational error, which arises from an individual’s bias, unclear description of measuring procedures, lack of proper setting of the equipment, or false data recording due to parallax errors (Seifert, 2002); b) instrumental error caused by factors such as imperfect calibration, etc., and c) environmental error that can be ascribed to the effects of the external conditions on the measurements, e.g., temperature, illumination, etc. Systematic errors primarily influence a measurement’s accuracy, but these sources are predictable.
3. Gross errors, arising from false readings, mistakes in recording data by an observer (e.g., reading or recording 88 instead of 38), or mistakenly set magnification. This type of error seriously affects both precision and accuracy. This source of error can be eliminated by careful reading or recording. This type of error can also be recognized post hoc via comparing the repeated measurements in a pairwise matrix scatterplot (Baur & Leuenberger, 2011).