(2) The curves shown in figure 3-4 are used
sampling is complete. However, if the PCI range of the
to select the minimum number of sample units that must
samples taken was found to be 40, it would be
be inspected. This will provide a reasonable estimate of
necessary to go back to figure 34. Start at 20 on the N
the true mean PCI of the section. The estimate is within
scale again, proceed vertically to the curve PCI
plus or minus 5 points of the true mean PCI about 95
range=40, and read 13 on the n scale. In this unusual
percent of the time.
When performing the initial
case it would be necessary to survey the additional 4
inspection, the PCI range for a pavement section (i.e.,
samples (9+4 = 13).
lowest sample unit PCI subtracted from the highest
sample unit PCI) is assumed to be 25 for asphalt
(b) Given: Portland cement concrete
concrete (AC) surfaced pavements and 35 for Portland
pavement section with N=30. Find: n.
cement concrete (PCC) surfaced pavements.
Answer: Start at 30 on the N scale, proceed vertical to
subsequent inspections, the actual PCI range
appropriate curve (PCI range=35) and read 15 on the n
(determined from the previous inspection) is used to
determine the minimum number of sample units to be
surveyed. As illustrated in figure 3-4, when the total
(c) Given: An AC or PCC pavement
number of samples within the section is less than five,
section with N<5. Find: n.
every sample unit should be surveyed. If N is greater
Answer: Survey all sample units.
than five, at least five sample units should be surveyed.
(3) Examples of first assumption for number
c. Selection of samples.
of sample units to be surveyed n follow:
sample units to inspect is as important as determining
(a) Given: Asphalt concrete pavement
the minimum number of samples (n) to be surveyed.
section with total number of sample units, N=20.
The recommended method for selecting the samples is
to choose samples that are equally spaced; however, the
first sample should be selected at random.
Answer: Start at 20 on the N scale (fig 3-4), proceed
technique, known as systematic sampling, is illustrated in
vertically to the appropriate curve (PCI range= 25) and
figure 3-5 and is briefly described below.
read 9 on the n scale. Nine sample units should be
surveyed. If the PCI range is found to be within 25 the
Figure 3-4. Determination of minimum number of sample units to be surveyed.