contestada

The defect length of a corrosion defect in a pressurized steel pipe is normally distributed with mean value 30 mm and standard deviation 7.8 mm [suggested in the article "Reliability Evaluation of Corroding Pipelines Considering Multiple Failure Modes and Time-Dependent Internal Pressure" (J. of Infrastructure Systems, 2011: 216–224)].


What values separate the largest 80% from the smallest 20% of the defect length distribution.

Respuesta :

Answer:

[tex]z=-0.842<\frac{a-30}{7.8}[/tex]

And if we solve for a we got

[tex]a=30 -0.842*7.8=23.432[/tex]

So the value of height that separates the bottom 20% of data from the top 80% is 23.432.  

Step-by-step explanation:

Let X the random variable that represent the heights of a population, and for this case we know the distribution for X is given by:

[tex]X \sim N(30,7.8)[/tex]  

Where [tex]\mu=30[/tex] and [tex]\sigma=7.8[/tex]

For this part we want to find a value a, such that we satisfy this condition:

[tex]P(X>a)=0.80[/tex]   (a)

[tex]P(X<a)=0.20[/tex]   (b)

As we can see on the figure attached the z value that satisfy the condition with 0.20 of the area on the left and 0.80 of the area on the right it's z=-0.842

If we use condition (b) from previous we have this:

[tex]P(X<a)=P(\frac{X-\mu}{\sigma}<\frac{a-\mu}{\sigma})=0.20[/tex]  

[tex]P(z<\frac{a-\mu}{\sigma})=0.20[/tex]

But we know which value of z satisfy the previous equation so then we can do this:

[tex]z=-0.842<\frac{a-30}{7.8}[/tex]

And if we solve for a we got

[tex]a=30 -0.842*7.8=23.432[/tex]

So the value of height that separates the bottom 20% of data from the top 80% is 23.432.  

Otras preguntas