Jacobson et al.: Measurement errors in body size of Placopecten magellanicus 
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Simulated shell heights Simulated shell heights 
with and without errors with out errors and residuals 
Bias only 
Figure 1 
Rootograms (Tukey, 1977) showing hypothetical distributions of Atlantic sea scallop ( Placopecten 
magellanicus) shell-height (SH) measurements with and without simulated measurement errors. The 
black line in each panel shows the distribution of measurements with no errors (5-mm size bins). 
In the left column, bars show distributions of shell heights with measurement errors. In the right 
column, bars show residuals (measurement with no errors minus measurements with errors). For 
the “bias only” scenario (A and B), precise measurement errors were assumed with a bias of -4.1 
mm. For the “imprecision only” scenario (C and D) unbiased measurement errors were assumed 
with a standard deviation of 6.1 mm. For the “imprecision and bias” scenario (E and F), measure- 
ment errors were assumed with a bias of -4.1 mm and standard deviation of 6.1 mm. 
Materials and methods 
The SMAST sea scallop survey is conducted with video 
cameras mounted on a steel pyramid frame to provide 
a 3.24-m 2 view of the sea floor and associated macro- 
benthos (Stokesbury, 2002; Stokesbury et al., 2004). 
Video images are recorded at sea on high-resolution 
S-VHS videotape and then replayed in the laboratory 
where digitized images are created. All sea scallops 
are counted, and all clearly visible sea scallops (with 
the hinge and opposite edge visible) within the digi- 
tized images are measured to the nearest mm by using 
Image Pro Plus® software (Media Cybernetics, Inc., 
Bethesda, MD). 
