Biomedical Engineering Reference
In-Depth Information
of the products. The intervals that represent the total variability were just under 4σ
units in width, σ representing the estimated standard deviation (SD) and μ
representing the estimated mean in
MMAD
for that product. This interval was spe-
cifically defined as
m
+
(. )[ (. )]
196
s
-
m
-
1 96
s
=
392
.
s
(8.3)
in which 1.96
σ
is the width of an interval that would contain, approximately, 95%
of the individual
MMAD
values. This figure of merit is a comparison of the vari-
ability associated with estimating the average
MMAD
to the total variability in the
individual
MMAD
measurements. In all cases, the variation in estimating the aver-
age
MMAD
through the
LPM
/
SPM
ratio was consistently severalfold smaller than
the variability expected from individual measurements, indicating the consistent
ability to adequately detect difference in
MMAD
within the normal observed vari-
ability of the product.
In contrast, the grouped stages consistently exhibited poorer performance.
Groups 1 and 3 were consistently unable to detect difference in
MMAD
within this
range, and groups 2 and 4 exhibited inconsistent performance with respect to being
able to detect differences. Even in those cases where one of these metrics might be
judged adequate to detect changes in
MMAD
, its performance was inferior to the
corresponding
LPM
/
SPM
metric.
The results in Table
8.5
and Fig.
8.12
confirm that for the eight products exam-
ined, the
LPM
/
SPM
ratio consistently out-performed all stage groupings with
respect to a measurement related to the central tendency of the APSD (as reflected
in the values of
MMAD
). Note that the CI stage
d
50
in this table refers to the cut-
point size of the stage separating
LPM
from
SPM
. The two MSA figures of merit
also confirmed the early observations concerning groups 1 and 3, i.e., that these
stage groupings contain virtually no information related to the central position of
the APSD.
The values obtained for the discrimination index indicate that the
LPM
/
SPM
ratio is consistently able to detect differences in
MMAD
of the order of tenths of
microns. While groups 2 and 4 exhibited some ability to detect changes in
MMAD
,
the performance was both inconsistent and always inferior to the
LPM
/
SPM
ratio
for the same product.
Another graphical approach to comparing performance is to plot
MMAD
pre-
dicted by the metric through the regression model versus the actual
MMAD
as deter-
mined through the full-resolution CI results. The comparison was limited to
comparing the ratio metric to only the group 2 and group 4 results since the other
two groupings consistently appear unable to offer any information about the
MMAD
.
The results of this analysis are depicted in Figs.
8.13
,
8.14
,
8.15
,
8.16
,
8.17
,
8.18
,
8.19
, and
8.20
and again clearly demonstrate the consistent superiority of the ratio
metric over stage groups with respect to measuring changes in the
MMAD
, i.e., any
shift in the average particle size of the emitted aerosol from any OIP.
Search WWH ::
Custom Search