(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.

For

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

scale.

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

than five, at least five sample units should be surveyed.

(3) Examples of first assumption for number

c. Selection of samples.

Determining specific

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.

This

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.

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