Project Management in the Oil and Gas Industry

 
C O V
S
X
. .
 
(2.11)
As another example, assume there is a third concrete mix at another site 
to provide concrete strength after 28 days of 500 kg/cm
2
. When you take 
three samples, it gives the results of strength after 28 days as 510 kg/cm
2
, 
500 kg/cm
2
, and 490 kg/cm
2
. When calculating the arithmetic mean and 
standard deviation, we find the following results: 
• Arithmetic mean = 500 kg/cm
2
• Standard deviation = 8.16 kg/cm
2
So, when comparing between concrete from one site with a mean con-
crete strength of 300 kg/cm
2
and concrete from a second site with a mean 
concrete strength of 500 kg/cm
2
and the two sites have the same standard 
deviation as the above example, the coefficients of variations are as follows:
• Coefficient of variation of the first site = 0.03 
• Coefficient of variation of the second site = 0.02 
We note that the second site has a coefficient of variation less than the 
first location. That is, the standard deviation to the arithmetic average is 
less at the second site than at the first site. This means that the second site 
mix concrete has a higher quality. Therefore, the coefficient of variation is 
the standard quality control of concrete and the closer to zero, the better 
the quality control.
To define a practical method of probability distribution, assume we have 
test results for 46 cube crushing strengths, as shown in Table 2.7, and we 
need to define the statistical parameters for these numbers.
The raw data is collected in groups and the number of samples with a 
value between the range for every group is called frequency. The frequency 
table from the raw data in Table 2.8 is tabulated in Table 2.9.
The data from Table 2.9 is presented graphically in Figure 2.5. To analyze 
the data, make a cumulative descending table, as shown in Table 2.8. This 
table is presented graphically in Figure 2.6. From Table 2.10, one can find 
that for this set of data, sample results of concrete strength can obtain the 
cumulative descending data. To understand this table, the probability of 
having a concrete strength that is less than 300kg/cm
2
is nine percent.

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Project Economic Analysis 53