Jacobson et al.: Measurement errors in body size of Placopecten mage/lanicus 
239 
shell height was 201 mm for video, 193 mm for measur- 
ing boards, and 192 mm for calipers. 
Bland-Altman plots for experiment 2 show that mea- 
suring board shell heights were more accurate than 
video measurements, and that bias in video and mea- 
suring board data was relatively constant across the 
range of shell heights in experiment 2 (Fig. 4). However, 
relatively large outliers sometimes occurred in video 
measurements at 80-130 mm SH (Fig. 4). 
Video and measuring-board shell-height compositions 
in experiment 2 were similar in terms of skewness with 
gj=-0.41 for video measurements and -0.47 for measur- 
ing boards compared to -0.46 for calipers (Table 2). The 
video shell-height distribution was more peaked with 
g 2 ~- 0.65 compared to g 2 =- 0.85 for measuring boards, 
and g 2 =-0.84 for calipers (Table 2). Video measurement 
errors were skewed to the left (g 1 =-0.60) compared to 
measuring-board errors which were nearly symmetrical 
(^ 1 =-0.05). The distribution of errors for measuring 
boards was flatter (g 2 ~- 0.85) and video measurement 
errors were more peaked (g 2 - 1-84) than would be ex- 
pected for normal distribution. The error distribution 
for measuring boards had a nearly flat mode about 
5-mm wide because shell heights are automatically 
truncated by measuring boards to the next lowest 5-mm 
shell-height bin. 
On a proportional basis, meat weights calculated from 
shell heights in experiment 2 were much less accu- 
rate than the original shell-height measurements. In 
Table 1 
Summary of size-composition data and measurement 
errors for 182 tile measurements (height and width from 
91 tiles, each 48.5x48.5 mm) by video equipment in exper- 
iment 1. 
Statistic 
Video 
Measurements and measurement errors 
Bias 
-2.2 
Standard deviation 
2.7 
Square root of the mean squared error 
3.5 
Skewness (gq) 
-0.28 
Kurtosis (g 2 ) 
-0.53 
Measurements 
Minimum 
38.3 
5% quantile 
41.2 
95% quantile 
50.1 
Maximum 
50.1 
Average 
46.3 
Percent bias 
-5% 
Coefficient of variation 
6% 
Percent square root of the mean squared 
error 7% 
particular, %RMSE values for meat weights were 71% 
and 74% for video and measuring boards, respectively 
(Table 3), compared to 30% and 31% for the original 
Table 2 
Summary statistics for shell-height composition data and measurement errors (in mm) from 172 uniquely identified Atlantic sea 
scallop ( Placopecten magellanicus) shell valves in experiment 2. “NA” means that a statistic is not applicable. 
Statistic 
True shell height (calipers) 
Video 
Measuring boards 
Shell heights and measurement errors 
n measurements used 
172 
670 
344 
n omitted 
0 
18 
0 
Bias 
NA 
-4.5 
-0.6 
Shell heights 
Minimum 
38.5 
34.3 
37.5 
5% quantile 
54.8 
48.8 
52.5 
95% quantile 
149.6 
147.3 
147.5 
Maximum 
192.0 
200.6 
192.5 
Average 
109.9 
106.5 
109.3 
Percent bias 
NA 
-4% 
-1% 
Standard deviation 
33.5 
33.1 
33.6 
Coefficient of variation 
30% 
31% 
31% 
Square root of the mean squared error 
NA 
33.4 
33.6 
Percent square root of the mean squared error 
NA 
30% 
31% 
Skewness (g x ) 
-0.46 
-0.41 
-0.47 
Kurtosis (g 2 ) 
-0.84 
-0.65 
-0.85 
Measurement errors 
Standard deviation 
NA 
6.1 
1.7 
Square root of the mean squared error 
NA 
7.6 
1.8 
Skewness (gj) 
NA 
-0.60 
-0.044 
Kurtosis (g 2 ) 
NA 
1.84 
-0.85 
