Time Course Study of Blood Pressure in Children
Over a Three-Year Period
Bogalusa Heart Study
A N T O N I E W. V O O R S , M.D.,
D R . P.H.,
LARRY S. WEBBER, P H . D . ,
AND G E R A L D S. BERENSON,
M.D.
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SUMMARY Indirect blood pressure (BP) was measured by mercury sphygmomanometer on 3524 children,
ages 5-14 years, representative of a defined geographic population. Year 1 to Year 4 correlations ranged from
0.52 to 0.63 systolic blood pressure (SBP) and from 0.23 to 0.45 diastolic 4th phase blood pressure (DBP4).
Some 55% of those originally in the upper decile remained in the uppermost two deciles 3 years later. All
children ages 5, 8, 11, and 14 years were reexamined annually, and the mean year-to-year standard deflation
(SD) within the child was computed to be 5.5 mm Hg SBP and 5.9 mm Hg DBP. After 3 years, for all available
children (n = 2601) the age-specific cross-sectional SD was found to be 8.7-9.7 mm Hg SBP and 7.6-8.0 mm
Hg DBP. These data enabled us to quantify toe regression to the mean due to within-child variation for the uppermost and the lowest decile BP, using the Gardner-Heady model. These children had an average BP at examination that differed from the expected level (adjusting for regression to the mean due to withu-child variation) by 1 mm Hg SBP and DBP. Quantification of the degree to which children's BP "tracks" is important
for early diagnosis and intervention in high BP. (Hypertension 2 (suppl I): I-102-M08, 1980)
KEYWORDS
•
children
• blood pressure tracking
V
ARIATIONS in blood pressure (BP) levels of
children over time need to be elucidated;
their relationship to the development of
adult essential hypertension is poorly understood at
the present time. Although essential hypertension is
stated to be rare in children,1-' there is evolving
evidence'-4 that essential hypertension begins in
childhood. It may be assumed that prevention of
hypertension would be more successful if the disease
process were prevented early. Hence, for early
diagnosis and prevention we need to know the time
course of BP levels and whether "tracking" occurs
(i.e., to what extent a child's BP level will persist in
rank with respect to peers and whether a child's
relatively high BP level will also persist). In this regard
we do know that BP levels during adolescence and
young adulthood have been found to be predictive of
later hypertension.*"10
From the Departments of Preventive Medicine, Biometry, and
Medicine, Louisiana State University Medical Center, New
Orleans, Louisiana.
Supported by funds from the National Heart, Lung, and Blood
Institute of the USPHS, Specialized Center of Research —
Arteriosclerosis (SCOR-A), HL151O3.
Address for reprints: Gerald S. Berenson, M.D., Louisiana State
Medical Center, 1542 Tulane Avenue, New Orleans, Louisiana
70112.
•
hypertension
To help clarify an understanding of the early onset
of essential hypertension, data are needed from an entire geographic population of free-living children
rather than from hospital-based patients and nonrepresentative selected individuals. Standardized
methodology for data collection is also required. Pertinent information is now available from the
community-wide examination of children as performed in the Bogalusa Heart Study on a biracial population.
For practical purposes an important question in a
longitudinal population study is: What happens over
time to the children at the extremes of the BP distribution? At least three considerations are important
in answering this question:
1. What were the circumstances under which the
BP was taken? Was it taken under basal-like
conditions? How often were BP measurements repeated during the visit? How large
was the observer error? How were the BP
measurements conducted from other technical viewpoints? What were the measurements of other determinants of BP levels?
2. What was the regression toward the mean occurring in children of the upper decile? After
adjusting for regression to the mean, will the
subsequent BP still be high?
1-102
BLOOD PRESSURE TIME COURSE IN CHILDREN/ Voors et al.
3. What is the significance of an exaggerated
change in rank, especially toward higher
levels? Will some children suddenly change
their rankings and track at a different level?
Materials and Methods
Population
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The eligible population consisted of all children,
aged 5 through 14 years, residing in Ward 4 of
Washington Parish (County), Louisiana, including the
town of Bogalusa. Of this eligible population, 93%
participated in the study during the first year, totaling
3524 children, 63% white and 37% black.
Bogalusa is a semi-rural, one-industry (paper and
chemical) community typical of many small towns in
the southern United States. Based on the 1970 U. S.
Census, the per capita income of Bogalusa blacks was
55% less than that for whites ($1199 versus $2673
respectively). Furthermore, almost one-half (45.3%)
of all black families earned less than the U.S. Census
poverty level as compared to 15.5% of all white
families."
Children 5, 8, 11, and 14 years of age at the first examination were reexamined annually. A total of 868
children (65% of the 1326 eligible) yielded complete
data during all four annual examinations. In each 1
year of the study, absenteeism was low and not limited
to any particular age, race, or sex group.
During the fourth year (Year 4), all age groups were
reexamined (2601 children, 74% of those originally examined; fig. 1).
1-103
TABLE 1. Measured Variables Entered into the Multiple
Regression Equation, Bogalusa Heart Study, 1973-1977
Description
Dependent
Mean of
mercury
Mean of
mercury
variables
6 SBP by
sphygmomanometer
6 DBP by
sphygmomanometer
Independent variables
Blood pressure
Mean of 6 SBP by mercury
sphygmomanometer
Mean of 6 DBP by
mercury
sphygmomanometer
Demographic
Age (exact calendar age)
Anthropometric
Body height
Ponderosity index* =
weight/height'
Triceps skinfold
thickness*
Laboratory
Fasting plasma glucose
Blood hemoglobin
Unit of
Year of
measurement measurement
mm Hg
4
mm Hg
4
mm Hg
1
mm Hg
1
years
4
cm
4
kg/cm*
4
mm
4
mg/dl
g/dl
4
4
'Logarithmic transformation.
General Examination Procedure
Because the objective of the examination procedure
was to assess risk factor variables for coronary artery
disease and hypertension, various characteristics were
measured in both Year 1 and Year 4, some of which
are listed in table 1. All measurements followed
methods outlined in detailed protocols. 1 2 "
Blood Pressure Measurements
The methods used in the BP measurements were
according to a rigid, randomizing protocol and have
been described previously.14 Briefly, we measured
relaxed, sitting right-arm BP by mercury sphygmo-
YEAR
OF
STUDY
1
2
3
4
AGE YEARS
manometer six times per visit on each child (three
measurements were made by each observer; there was
a waiting time between observers of 15 minutes). In
addition, but not analyzed here, three BP measurements were obtained on each child with the Physiometric recorders (Sphygmetrics, Inc., Woodland
Hills, California, Model USM-105). Much attention
was given to observer training, adherence to the
protocol, and measurement in a relaxed state. Cuff
bladders were as large as possible to fit the individual
upper arm length and circumference.
One of the eight BP observers in the study produced
deviant diastolic blood pressure (DBP) readings on the
mercury sphygmomanometer during Year 4, and
therefore all DBP observations made by this observer
during Year 4 were removed from the study data. This
has resulted in using only three instead of six BP
measurements in obtaining an individual average on
some children.
© 6 7 © 9 10@12 13(
©
© © ©
© ® @ @
5 6 7 © 9 ]Q@\2 13(M)15 1 6 (
FIGURE 1. Design of Bogalusa Heart Study for schoolaged children, 1973-1977.
Birariate Comparisons for Annual Reexaminees
Pearson product moment coefficients of correlation
of BP between successive years are explored in various
configurations, and annual measurements of systolic
blood pressure (SBP) of children originally in the top
deciles are related to the rankings of those levels from
the other children at consecutive years.
1-104
HIGH BLOOD PRESSURE IN THE YOUNG
Correlates of Blood Pressure Levels
To assess BP correlates, individual values were
entered as the dependent variable into a multiple
regression analysis, and other measured characteristics were entered as independent variables. A ponderosity index (weight/height8) was chosen as a measure
of obesity for theoretical and morphological reasons.18 This ponderosity index was used instead of the
body weight as an independent variable to avoid apparent artifacts resulting from the highly positive correlation between height and weight. Logarithmic
transformation was applied to this index and to triceps
skinfold thickness to approach Gaussian distributions
(table 1).
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Regression of the Initially High Blood Pressure
toward the Mean
To compute the amount of regression toward the
population's mean BP as experienced by a child above
the 90th percentile, we used the statistical model of
Gardner and Heady.18 The model requires knowledge
of the interchild and intrachild variability. The interchild variability was obtained by computing the crosssectional standard deviation for each 2-year age group
in Year 4. The intrachild variability was computed by
pooling the intrachild variances for the 868 children
examined annually for each period.
SUPP
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PERCENTILE
95th
120
50th
110 -
100
5th
-
1973 - 1974, N-2594
90
1976 - 1977. N-2574
5
6
T
7
1
8
1
9
1
10
1 1 1
11 12 13
1 1
14 15
>
r
16 17
AGE, YEARS
Tracking of Blood Pressure in Children Originally in the
Extreme Deciles
FIGURE 2.
Comparison of percentiles of systolic blood
pressure taken by mercury sphygmomanometer, 3 years
apart in children aged 5-17 years.
In the study population, with two complete
examinations 3 years apart, children in the upper and
lower BP deciles (mean of six sphygmomanometer
measurements) were observed after 3 years. After
reducing the regression toward the mean (due to intrachild variability) to zero in a statistical adjustment, we
compared the average observed BP to that expected
from observations made at Year 1 after adjustments.
PERCENTILE
95th
80-
P~O
Results
Mean BP levels by age for Years 1 and 4, shown in
figures 2 and 3, are quite similar. Sex differences
became apparent after 13 years of age, but are not
consequential in the present computations.
Bivariate Comparisons for Annual Reexaminees
Correlation coefficients for readings from Year 1 to
Year 2 ranged from 0.62 to 0.73 SBP and from 0.42 to
0.55 DBP (table 2). Only a slight drop over the 3-year
period was noted in the correlation coefficients by
Year 4. In fact, the correlation coefficients of readings
3 years apart arc only slightly lower than those from
some of our other studies of readings 1 hour apart
(0.83 SBP and 0.78 DBP) or 1 month apart (0.81 SBP
and 0.65 DBP10).
A tendency is apparent for elevated readings in
children at one point in time to remain high. The
yearly SBP distributions for 191 children initially 5
years of age (fig. 4) show some regression to the mean,
50th
70-
60-
s
•
o
50
5
6
7
8
9
•1973 - 1974, N-2594
0 1 9 7 6 - 1977, N-2578
10 11 12 13 14 15 16 17
AGE, YEARS
FIGURE 3.
Comparison of percentiles of diastolic (4th
phase) blood pressure taken by mercury sphygmomanometer, 3 years apart in children aged 5-17 years.
BLOOD PRESSURE TIME COURSE IN CHILDREN/Voors et al.
TABLE 2. Product Moment Correlation Coefficients for Blood
Pressure Readings over S Successive Years, Bogalusa Heart
Study, 197S-1977
Age
(Yearl)
Year 2
Year 1 with
Year 3
Year 4
r
r
r
n
SBP
5
191
8
246
11
270
14
161
0.64
0.73
0.68
0.62
0.59
0.67
0.64
0.60
5
191
8
246
11
270
14
161
0.42
0.48
0.55
0.52
TABLE 4. Stepwise Multiple Regression of Blood Pressure
Reexamined After Three Years in Children, Ages 8-17 Years,
Bogalusa Heart Study, 1973-1977
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Source*
Regression
coefficient
Partial
correlation
coefficient
SBP
Intercept
Systolic, Year 1
Height, Year 4
Fasting glucose
W/H 1 , Year 4f
66.87
0.51*
0.25»
0.10*
12.48«
(R» = 0.54)J
0.52*
0.44*
0.12»
0.11'
DBP
Intercept
Height, Year 4
Diastolie, Year 1
Triceps skinfoldf
5.12
0.13»
0.37*
4.58*
0.47*
0.06b
0.31°
(R1 = 0.38)t
0.14*
0.36*
0.12*
0.09*
0.07b
0.05°
BP
0.58
0.62
0.63
0.52
DBP
0.35
0.44
0.43
0.40
0.45
0.41
0.44
0.23*
Age
Fasting glucose
Blood hemoglobin
•p < 0.05; all other correlation coefficients, p < 0.0001.
but most of the levels tend to remain high in
succeeding years. A slight improvement is seen if we
use the average of 2 or 3 years to establish a baseline
distribution.
Observations for all four age groups (table 3) show
that, of the 85 children who were in the highest decile
in Year 1,41 (48%) remained there in Year 2, while 57
(67%) were in the highest two deciles. By Year 4, 28
(33%) were still in the top decile. If BP readings for 3
years are averaged, of the 87 who were in the top
decile, 38 (44%) remained in the top decile in Year 4.
We note a better persistence of ranking, as expected, if
we restrict the selection to those children who were in
the highest decile in Year 1 and Year 2 and Year 3.
Some 16 of 27 (59%) remain in the highest decile in
Year 4, and 21 (78%) were in the highest two deciles.
Correlates of Blood Pressure Levels
The strongest correlate of BP levels among those
listed in table 1 was the pressure observed 3 years
1-105
*For variables entered into the equation, see table 1.
These variables are listed in order of acceptance by the
model.
tLogio.
^Multiple correlation coefficient squared; n = 2518 (SBP)
and n = 2522 (DBP); the remaining of the 2601 children
had missing observations.
•p < 0.0001.
bp < 0.001.
°p < 0.01.
previously as analyzed in this data set (table 4). For
SBP, this variable alone accounted for 42% of the
pressure variability at Year 4. In total, all entered
variables could account for 54% of the SBP and 38%
of the DBP mercury sphygmomanometric BP variation. A measure of tracking is the coefficient of partial
correlation of BP in Year 1 and Year 4, controlling for
all other significant independent variables. This
coefficient was 0.52 for SBP and 0.36 for DBP. In the
regression analysis, at Year 4, the height, some
obesity index, and the fasting plasma glucose showed
additional independent correlation with BP.
TABLE 3. Number of Children in Highest Deciles for Systolic Blood Pressure, Bogalusa Heart Study, 197S-1977
Year 2
Highest decile
n
Year 1 only
85
Average of 2 yrs
86
Average of 3 yrs
87
Each of 2 yrs
41
Each of 3 yrs
27
9+10
n (%)
57
(67)
10
n (%)
41 (48)
Decile
Year 3
9 + 10
10
n (%)
n (%)
Year 4
10
9 + 10
n (%)
in
(%)
58
(68)
40 (47)
47
(55)
28
(33)
68
(79)
43 (•50)
56
(65)
35
(41)
61
(70)
38
(44)
30
(73)
20
(49)
21
(78)
16
(59)
38
(93)
27 (66)
1-106
HIGH BLOOD PRESSURE IN THE YOUNG
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JULY-AUGUST,
1980
YEARl(Ag«3yt<B»,N-l91)
I TW OECILE, YUR I
19On
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TO
80
90
100
IK)
120
130
70 80 90 100 110 120 130 70
SYSTOUC BLOOO PRESSURE (mm Hg)
80
90
K»
110
120 130
4. Distribution of systolic blood pressure for children aged 5 years who were in the top decile at
Year I, as followed at Years 2-4 and compared to the distribution for the other children.
FIGURE
Regression of Initially High Blood Pressure toward the Mean
The pooled intrachild standard deviation as
obtained from the annual reexaminations was 5.5 mm
Hg SBP and 5.9 mm Hg DBP. If these results are
applied to the cross-sectional standard deviation for
the various age groups of observation (for example,
for ages 8-9 years at Year 4 it was 9.1 mm Hg SBP
and 8.0 mm Hg DBP), the intcrchild standard deviation can be computed by taking the square root of the
difference in variance. The resulting interchild standard deviations are, for ages 8-9 years, 7.2 mm Hg
SBP and 5.4 mm Hg DBP.
Assuming that BP values have Gaussian distributions, we can use the model of Gardner and
Heady18 to obtain the average regression in pressure
upon reexamination as experienced by the children
ranking above the 90th percentile during the first examination. This regression for ages 8-9 years is 5.9
mm Hg SBP and 7.7 mm Hg DBP (see Appendix).
These results are typically based on means of six
sphygmomanometer measurements per examination
on a child. Using common estimates of BP variation
for children of various BP levels is justified because we
found no relationship between level and variability.10
Tracking of Blood Pressure for Children Originally in the
Extreme Deciles
In the study population with two complete
examinations 3 years apart, children in the upper and
lower BP deciles measured by mercury sphygmoma-
nometer at the first examination were identified, and
their BP similarly observed during the examination at
Year 4. Also, adjusted levels from the upper and lower
deciles of the original examination were computed
after taking into account regression toward the mean
due to intrachild variability. The latter are values
statistically adjusted by reducing this regression
toward the mean to zero (table 5).
After adjusting the 3 years' difference in mean SBP
and DBP (mean for all children of that age group), we
found that those children initially in the highest decile
had average observed examination pressures differing
from the expected by 1 mm Hg SBP and DBP, as did
those in the lowest decile (fig. 5).
Discussion
Measurement of valid and replicable indirect BP in
children is fraught with pitfalls, but reasonable
measurements are obtainable when attention is paid to
methodology, even in an office setting (unpublished
observations). In our study, we attempted to arrange
nearly optimal conditions for measuring children's
BP. We obtained readings that approached known
resting levels for children in this age group;14 the
observed levels were comparable to the basal pressures
observed by Shock." Such basal-like levels have increased predictive power of later hypertension according to Smirk" and Harlan et al.* The measurement
replicability was increased by measuring the BP three
times each at three observer stations, two of which
BLOOD PRESSURE TIME COURSE IN CHILDREN/Voors et al.
1-107
TABLE 5. Blood Pressures Observed in Year 4 for Children from Extreme Deciles in Year 1 (Expected Values
are Adjusted for Year and for Regression Toward the Mean), Bogalusa Heart Study, 1973-1977
BP
SBP
DBP
Year 4
Age (yrs)
n
8-9
44
10-11
12-13
14-15
16-17
50
56
64
44
8-9
43
10-11
12-13
14-15
16-17
55
56
62
45
Observed
Year 1
Upper Decile
Expected*
Year 4
Observedf
Year 4
112.4
112.0
114.8
119.5
123.8
106.5
110.0
116.6
119.6
122.1
106.4
109.6
117.1
118.7
118.8
72.4
69.6
74.2
77.9
80.0
65.0
65.0
72.9
73.3
76.6
65.0
66.8
74.0
74.7
73.4
± 3.0
=•= 2.5
± 2.3
* 3.1
± 2.5
±
±
±
±
±
2.7
1.8
1.8
1.5
2.5
n
Observed
Year 1
Lower Decile
Expected*
Year 4
82.8
82.2
84.0
87.0
91.0
88.8
92.3
96.9
98.5
101.4
46.5
46.0
48.7
51.4
53.8
53.5
56.6
61.9
63.3
65.2
43
52
58
63
44
45
48
57
63
44
Observedf
Year 4
88.9 =•= 1.9
92.3 ± 2.4
97.6 * 1.9
100.4 ± 1.8
101.5 ± 2.2
52.8
55.1
62.1
63.1
66.4
=fa 2.2
=*• 2.2
rfa 1.9
± 1.7
± 2.2
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•Measurement of Year 1, adjusted for year and for regression toward the mean. Cross-sectional standard
deviations at Year 4 were as follows for systolic/diastolic pressures: ages 8-9: 9.07/8.01; ages 10-11: 8.75/
7.57; ages 12-13: 9.69/7.91; ages 14-15: 9.39/7.90; ages 16-17: 8.87/7.78 mm Hg.
fMean =*= 2 SE.
stations arc included in these analyses (the data showing this increase in replicability are not presented
here), and by using the 4th rather than the 5th
Korotkoff phase for diastolic assessment." Careful
selection, training, testing, and retesting of the observers was conducted according to a protocol especially designed to obtain valid measurements.4
Possible observer bias was further minimized by daily
randomization of observers and of children in the examination sequence, and by a supervisory system of
blind reexamination by the same observer of children
at the end of each examination flow.14 Even so, bivariate comparisons for annual reexaminations of the upper decile showed a considerable dispersion. We infer
that this was due to limiting the examinations on
which assignment to the upper decile was based to 1
examination day only. Hence, there is an obvious
need to reduce intrachild variation by multiple measurements.
The observation of an independent relationship
between BP level and intrachild BP variability, and
the absence of any strong correlation of this variability
with other measured characteristics,10 enabled us to
use a subsample of children for measuring the intrachild variation during annual reexaminations.
Estimate of the latter variation allowed us to quantify
the resulting regression toward the mean, according to
the Gardner-Heady model. Previously we have tentatively applied this model to our population examined at Years 1 and 2.10
After this regression toward the mean was taken
into account, the tracking of resting BP in children
originally above the 90th percentile became quantifiable. We found that their average BP level dropped
only 1 mm Hg SBP and DBP during the reexamination after 3 years. The importance of BP levels obtained 3 years earlier in determining the present level
was also borne out by a stepwise multiple regression
analysis where 54% of the SBP variability was deter-
mined. From a practical standpoint, three or four
serial BP determinations (weekly or monthly) appear
to approach a reproducible level for a given child.
Reexamination of children with high BP levels, as
found in the study population, revealed only few
130
SYSTOLIC
120
nso
< too
1 90
tw
DIASKXIC ( 4 t h ph<u«)
70
60
• - • Eipected from Ytor I
after adjustments
50
o-o Observed at Year 4
8-9
10-11
12-13
14-15
16-17
Agt at Examination, Ymr 4
FIGURE 5. Blood pressures {mean ± 2 SEJ observed at
Year 4 for children from extreme deciles at Year 1; mean expected values are computed from observation at Year I, adjusted for year and regression toward the mean due to intrachild variation. (Variation of expected values has been
equated with Year 1 variation for orientation.)
1-108
HIGH BLOOD PRESSURE IN THE YOUNG
potentially secondary hypertensives. The consistency
of ranking in the highest BP decile for two examinations 3 years apart is high. This observation
contributes to the probability that primary hypertension begins early in life, and is detectable at that
time by careful BP measurement and evaluation.
Our study design is not addressed to the question of
whether any individual child shows an abrupt change
toward higher BP levels, which then becomes consistent. Hence, this remains likely in accelerated
hypertension, but in the present literature it does not
appear to be the general rule in the expression of essential hypertension.
References
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I,
HYPERTENSION, VOL
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JULY-AUGUST,
1980
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Korotkoff phases in mercury sphygmomanometry of children.
Prev Med 8: 492, 1979
Appendix
Regression to the Mean Blood Pressure
If we use Gardner and Heady's notation," assuming Gaussian
distribution of BP, we find L = the 90th percentile pressure, M " the
mean pressure, and ( = the cross-sectional standard deviation of
pressure. Therefore <t>\ (L - y.)/t\ = 0.1754 according to the frequency function of the normal distribution. Further, c = v V + &,
where a = the interchild standard deviation of pressure, and & => the
intrachild standard deviation of pressure.
A child found to be above the 90th percentile BP level during the
first examination has an expected BP level of X of
E(X | X > L) = n +
TV\ (L
-
n)/t\
where
FI(L - M)/<) = *I(L - f)/(\/0.\
- 0.1754/0.1 = 1.754
and the expected value of the BP level x is
E(x |X > L) =
M
+ (1-754) o*/i.
Systolic Blood Pressure
For the children ages 8-9 years in Year 4, we obtained the following estimates:
i = 9.098 mm Hg
«' = 82.7736 mm1 Hg
S = \/3O.516O = 5.5241 mm Hg
a = v/52.2576 = 7.2289 mm Hg
E(X |X > L) = ii + (1.754)t =
M
+ 15.9579 mm Hg
E<x |X > L) = n + (1.754) 52.2576/9.098 = n
+ (1.754) (5.7439) = M + 10.0747 mm Hg.
The difference between these two expectations represents the expected regression of SBP toward the mean and amounts to 5.8832
mm Hg.
Diastolic (4th Phase) Blood Pressure
For the same children, the estimates are as follows:
t = 8.045 mm Hg
t1 = 64.7220 mm1 Hg
6 = -s/35.2886 = 5.9404 mm Hg
a = V29.4334 = 5.4253 mm Hg
E(X I X>L) =• n + (1.754)t = n + 14.1109 mm Hg
E(x I X>L) »
M
+ (1.754)
=
M
+ (1.754) (3.6586)
= ^ + 6.4172 mm Hg.
The difference between these two expectations represents the expected regression of DBP toward the mean and it 7.6937 mm Hg.
Time course study of blood pressure in children over a three-year period. Bogalusa Heart
Study.
A W Voors, L S Webber and G S Berenson
Hypertension. 1980;2:I102-108
doi: 10.1161/01.HYP.2.4_Pt_2.I102
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