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Mean and median in a plot

 
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bigger



Joined: 06 Jan 2007
Posts: 7

PostPosted: October 3, 2007 4:29 AM    Post subject: Mean and median in a plot Reply with quote

Doing econometrics.

I can go to Y= and write the following formula:
(15/64)*(2x^(3/2)-x^2)

Plotted with x=0 and x=4

Adjust the plot window to show these values as x min and x max.

I get a nice plot and can make the TI89 calculate the max (lower bound = 0, upper bound = 4) as 2.25 ; 0.395.

Any suggestions on how to figure a) Mean and b) Median?

I can use this trick to do mean: In the plot window I select f5:Math -> ff(x)dx and entering the max as the shading value. I get a number above 0.5 (this number is the proportion of the area shaded) therefore I know 2.25 is too high so I enter a lower value. Using hit-and-run I deduce that in order to get 0.5 I have to enter the value of aprox 2.14.
However – it is a lot of guesswork. I would prefere calculating the result rather than guessing it.

BTW – I also tried this on a Casio CFX-9950GB. While my trusty TI89 did the shading and calculations in a few seconds the Casio took about 40 seconds for shading and calculating the shaded area. Considering this method might require a minumum of 3 tries then that‘s a lot of time wasted!
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mellamokb



Joined: 10 May 2007
Posts: 124

PostPosted: October 22, 2007 9:27 PM    Post subject: Reply with quote

Hi bigger,

I'm assuming your expression is in y1(x), if not, adjust my code accordingly.

For the minimum and maximum values, you can have the calculator determine the exact values using the following functions:

Note: OUT denotes the calculators output, and <= is the less than/equal to key (single character).

Code:

fMin(y1(x),x)|0<=x and x<=4
OUT: x=0 or x=4
fMax(y1(x),x)|0<=x and x<=4
OUT: x=9/4



For the mean, I believe it is just the area divided by the width of the interval, which can be done as follows:

Note: _/ is the integral [F3]+[2] or [2ND]+[7].

Code:

_/(y1(x),x,0,4)/(4-0)
OUT: 1/4



The median involves finding the point at which half of the area is to the left and half of the area is to the right. This is equivalent to solving two equal integrals, the first having bound [0, b] and the second having bound [b, 4]:

Code:

solve(_/(y1(x),x,0,b)=_/(y1(x),x,b,4),b)|0<=b and b<=4
OUT: b=2.1641353083



However, since we know the area to be _/(y1(x),x,0,4)=1, then it is equivalent to finding the point b where the area is 1/2, because that would be half the area:

Code:

solve(_/(y1(x),x,0,b)=1/2,b)|0<=b and b<=4
OUT: b=2.1641353083



Hope this helps!

~ mellamokb
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bigger



Joined: 06 Jan 2007
Posts: 7

PostPosted: November 12, 2007 12:42 PM    Post subject: Thanks Reply with quote

Thanks mellamokb!

This is exactly what I was looking for.

Bigger
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