# Some methods for performing nondestructive testing of bridges use

The following 2x2 factorial data from problem 2 in section 4.3 show times for 2 fluids to drain from a container
using tubes of 2 diameters.
4.50
4.00
Ln(Time)
3.50
3.00
Water
2.50
Glycol
2.00
1.50
0.15
0.2
0.25
Diameter
0.3
0.35
(a) Does there appear to be an interaction between fluids and diameters in the Ln scale? Why?
The responses are not reasonably parallel indicating an important interaction. The p-value of 0.249 for the
interaction effect in the ANOVA table below confirms this interaction. The fluid differences in the Ln scale are
different for the smaller and larger diameter tubes with more of a fluid difference for the smaller diameter tube.
(b) What does this interaction or lack of interaction tell us about consistency of span effect for the 2 materials
in the original time scale?
In the original scale the ratios of times for glycol:water would be larger for the smaller diameter tube.
(c) Why would it not be useful to compare the main effect mean for 0.188” diameter values averaged over both
liquids to the mean for 0.314” diameter values averaged over both liquids?
The diameter effect is different for the two liquids. A combined effect wouldn’t match the effect in either
liquid. We need to summarize diameter effects separately for water and glycol.
Below is some info from Excel. There were 3 measurements in each treatment group for 18 total
measurements.
ANOVA
Source of Variation
Fluids
Sample
Diameter Columns
Interaction
Within
Total
SS
0.901
6.975
0.223
0.00819
df
1
1
1
8
8.107
11
MS
0.901
6.975
0.223
0.00102
(c) What is the value of the pooled standard variance S p2 ?
The pooled standard variance is 0.00102.
F
880.1
6815.9
217.9
P-value
1.81E-09
5.17E-13
4.36E-07
The means in the Ln scale are as shown below.
Fluid
Water
Water
Water
Water
Water
Water
Glycol
Glycol
Glycol
Glycol
Glycol
Glycol
Diam
0.188
0.188
0.188
0.314
0.314
0.314
0.188
0.188
0.188
0.314
0.314
0.314
Time
21.12
21.11
20.80
6.60
6.04
5.92
51.25
46.03
46.09
7.85
7.91
7.97
Mean
St Dev
CV
SD/Mean
21.01
0.1819
0.0087
6.19
0.3630
0.0587
47.79
2.9966
0.0627
7.91
0.0600
0.0076
ln(Time)
3.050
3.050
3.035
1.887
1.798
1.778
3.937
3.829
3.831
2.061
2.068
2.076
Mean
Similar
to CV
St Dev
3.045
0.0087
1.821
0.0579
3.866
0.0616
2.068
0.0076
(d) For water the difference between 0.188&quot; and 0.314&quot; means in Ln scale = 3.045 – 1.821 = 1.224.
What is the corresponding ratio of times? 3.40
(e) Find a 2-sided 95% confidence interval for the difference in the Ln scale.
Ln Scale
Lower Limit
1.16
Ln Scale
Upper Limit
1.28
(f) Find a 2-sided 95% confidence interval for the ratio of times to drain the container.
Ratio
Lower Limit
3.20
Ratio
Upper Limit
3.61
According to problem 2 in section 4.3, a particular fluid model says that the time to drain should be inversely
proportions to the 4th power of the diameters. What ratio of drain times would be predicted by this model? Is
the confidence interval consistent with this ratio?
Model  ratio = 7.78. This ratio is clearly not in the confidence interval. We have very good evidence for
rejecting the model.