Results 1 to 5 of 5

Math Help - Distribution help please. =[

  1. #1
    Newbie
    Joined
    Sep 2007
    Posts
    22

    Distribution help please. =[

    I can't figure this problem out. I left my maths book at school so I no longer have the formulas. :[

    Q6. The mass of baby guinea pigs four days after birth is a normally distributed variable with a standard deviation of 2 grams.

    a. What percentage of the guinea pigs are less than 4 grams below the mean weight?

    b. What percentage of the guinea pigs are more than 6 grams above the mean weight?

    I have my own graphics calculator too. I can get to a screen where you can enter these values...

    Lower:
    Upper:
    Standard Deviation:
    Mean:

    I defiantly know we had to use that part of the calculator. If you guys can tell me how to work it out, that would be great!

    Thanks in advance!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by Hikaru View Post
    I can't figure this problem out. I left my maths book at school so I no longer have the formulas. :[

    Q6. The mass of baby guinea pigs four days after birth is a normally distributed variable with a standard deviation of 2 grams.

    a. What percentage of the guinea pigs are less than 4 grams below the mean weight?

    b. What percentage of the guinea pigs are more than 6 grams above the mean weight?

    I have my own graphics calculator too. I can get to a screen where you can enter these values...

    Lower:
    Upper:
    Standard Deviation:
    Mean:

    I defiantly know we had to use that part of the calculator. If you guys can tell me how to work it out, that would be great!

    Thanks in advance!
    a. You're two standard deviations below the mean. So find Pr(Z < -2) and multiply the result by 100 to get percentage.

    b. You're three standard deviations above the mean. So find Pr(Z > 3) and multiply the result by 100 to get percentage.

    In both cases Z is a standard normal variable, mean = 0 and sd = 1.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Sep 2007
    Posts
    22
    Quote Originally Posted by mr fantastic View Post
    a. You're two standard deviations below the mean. So find Pr(Z < -2) and multiply the result by 100 to get percentage.

    b. You're three standard deviations above the mean. So find Pr(Z > 3) and multiply the result by 100 to get percentage.

    In both cases Z is a standard normal variable, mean = 0 and sd = 1.
    Ohhh that makes sense! =] I drew out a small graph/curvey thing and I get what you mean for the top half!

    However I'm curious; Why exactly is the mean 0? And why is the Standard Deviation 1? In the question it says the Standard Deviation is 2...
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by Hikaru View Post
    Ohhh that makes sense! =] I drew out a small graph/curvey thing and I get what you mean for the top half!

    However I'm curious; Why exactly is the mean 0? And why is the Standard Deviation 1? In the question it says the Standard Deviation is 2...
    You should have learned about the standard normal distribution.

    And you should have learned that Z = \frac{X - \mu}{\sigma} where Z is standard normal random variable and X is a normal random variable.

    Read this: Standard Normal Distribution (1 of 2)
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Senior Member
    Joined
    Mar 2008
    From
    Pennsylvania, USA
    Posts
    269
    Thanks
    37
    Hi,

    Heard of the 67-95-99.8 rule?

    Given a normal distribution, 67 percent of the guinea pigs will be within 1 standard deviation of the mean. 95 percent will be within two standard deviations, and 99.8 are within three SD's. It is just a shortcut that is helpful to memorize, unless you like reading through those Z tables with tiny font.

    -Andy
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 1
    Last Post: December 27th 2011, 02:08 PM
  2. Replies: 0
    Last Post: May 12th 2011, 04:43 PM
  3. normal distribution prior and posterior distribution proof
    Posted in the Advanced Statistics Forum
    Replies: 0
    Last Post: March 9th 2011, 07:12 PM
  4. Replies: 1
    Last Post: October 18th 2010, 02:41 PM
  5. Replies: 2
    Last Post: March 29th 2010, 03:05 PM

Search Tags


/mathhelpforum @mathhelpforum