Calculating P(Y<$\sqrt{X}$) for Joint PDF of X and Y

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In summary: Also, you need to split the region where x > y into two parts for the two different upper limits on y. In summary, the problem involves finding the probability that Y is less than the square root of X, given a joint pdf for X and Y. The correct integration region is a trapezoidal shape with vertices at (0,0), (1,0), (1,1), and (2,2). After setting up the integral for this region and solving, the correct answer is 1/3. However, one should be careful and double check the integration region before solving the problem.
  • #1
mynameisfunk
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Homework Statement


Suppose that the joint pdf of [itex] X [/itex] and [itex]Y[/itex] is
[tex] f(x,y)= (8/3)xy , 0<x<1,0<y<2, x<y<2x [/tex]
Compute [tex] P(Y<\sqrt{X}) [/tex]

Homework Equations

The Attempt at a Solution


[tex] \int_0^1 \int_x^{\sqrt{x}} (8/3)xy dy dx = (4/3) \int_0^1 x - x^2 dx = 2/9[/tex]
 
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  • #2
mynameisfunk said:

Homework Statement


Suppose that the joint pdf of [itex] X [/itex] and [itex]Y[/itex] is
[tex] f(x,y)= (8/3)xy , 0<x<1,0<y<2, x<y<2x [/tex]
Compute [tex] P(Y<\sqrt{X}) [/tex]

Homework Equations




The Attempt at a Solution


[tex] \int_0^1 \int_x^{\sqrt{x}} (8/3)xy dy dx = (4/3) \int_0^1 x - x^2 dx = 2/9[/tex]

Wrong answer: start over. Be very careful about the integration region or regions---always draw a picture first, before writing down your integrals.
 
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  • #3
I want to add to this post. Sorry for the double posting. I know this solution I posted above can't be right. I tried switching the order of integration from dydx to dxdy and I get 1/3. One of my classmates suggested doing a bivariate transformation, which I haven't tried but I am a little confused as to why I wouldn't be able to just go ahead and compute this directly.
 
  • #4
Here is the picture I drew. Am I not drawing this right?
 

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  • #5
mynameisfunk said:
Here is the picture I drew. Am I not drawing this right?

Not quite right: for small x > 0 you have the wrong upper limit on y (but it is OK for larger x).
 
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Related to Calculating P(Y<$\sqrt{X}$) for Joint PDF of X and Y

1. How do you calculate P(Y<$\sqrt{X}$)?

To calculate P(Y<$\sqrt{X}$), you will need to know the probability density function (PDF) of both Y and X. The formula for calculating this probability is P(Y<$\sqrt{X}$) = ∫ f(y)∫ f(x) dx dy, where f(y) and f(x) are the PDFs of Y and X, respectively. This integral can be solved using mathematical software or by hand if the PDFs are known.

2. What is the significance of P(Y<$\sqrt{X}$) in scientific research?

P(Y<$\sqrt{X}$) is often used in research to determine the likelihood of a particular event happening. It can be used to calculate the probability of a certain outcome given a set of data or to compare the probability of different outcomes. In scientific research, this calculation can help researchers make informed decisions and draw conclusions based on statistical significance.

3. Can P(Y<$\sqrt{X}$) be negative?

No, P(Y<$\sqrt{X}$) cannot be negative. Probability values must be between 0 and 1, with 0 indicating impossibility and 1 indicating certainty. If a calculated probability is negative, it is most likely due to a mistake in the calculation or an incorrect assumption in the data.

4. How does the relationship between Y and X affect P(Y<$\sqrt{X}$)?

The relationship between Y and X can greatly impact the value of P(Y<$\sqrt{X}$). If Y and X are highly correlated, then P(Y<$\sqrt{X}$) will be closer to 1, meaning there is a higher likelihood of Y being less than the square root of X. If Y and X are not correlated, the value of P(Y<$\sqrt{X}$) will be closer to 0, indicating a lower likelihood of this event occurring.

5. What other factors should be considered when calculating P(Y<$\sqrt{X}$)?

Aside from the relationship between Y and X, other factors that should be considered when calculating P(Y<$\sqrt{X}$) include the sample size, the distribution of the data, and any assumptions made in the calculation. These factors can greatly influence the accuracy and reliability of the calculated probability and should be carefully evaluated in any scientific analysis.

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