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Thread Subject:
A basic question

Subject: A basic question

From: Jerry

Date: 16 Jan, 2013 16:00:09

Message: 1 of 3

Hello,

I used this model [y = D+(A-D)/(1+(x/C)^B)] and predicted all parameters using a vector of 'x' and 'y'. For a specific value of 'y', I would like to calculate a value for the 'x' using this model [x =((((D+(A-D))/y)-1)^1/B)*C] derived from the original model. But the calculated 'x' is not what it should be. I am assuming there is something wrong with this model [x =((((D+(A-D))/y)-1)^1/B)*C]. May someone give me a comment?

Thanks,
Jerry

Subject: A basic question

From: someone

Date: 16 Jan, 2013 16:32:08

Message: 2 of 3

"Jerry " <jerrycholo@gmail.com> wrote in message <kd6iq9$mmv$1@newscl01ah.mathworks.com>...
> Hello,
>
> I used this model [y = D+(A-D)/(1+(x/C)^B)] and predicted all parameters using a vector of 'x' and 'y'. For a specific value of 'y', I would like to calculate a value for the 'x' using this model [x =((((D+(A-D))/y)-1)^1/B)*C] derived from the original model. But the calculated 'x' is not what it should be. I am assuming there is something wrong with this model [x =((((D+(A-D))/y)-1)^1/B)*C]. May someone give me a comment?
>
> Thanks,
> Jerry

Based on what little information provided
(I assume A,B,C, & D are scalars), I suggest two things:

1. Put parentheses around 1/B, i.e.

x =((((D+(A-D))/y)-1)^(1/B))*C
 
2. Try using element-by-element operators:

x =((((D+(A-D))./y)-1).^(1/B))*C

See if those help.

Subject: A basic question

From: Roger Stafford

Date: 16 Jan, 2013 18:27:10

Message: 3 of 3

"Jerry " <jerrycholo@gmail.com> wrote in message <kd6iq9$mmv$1@newscl01ah.mathworks.com>...
> I used this model [y = D+(A-D)/(1+(x/C)^B)] and predicted all parameters using a vector of 'x' and 'y'. For a specific value of 'y', I would like to calculate a value for the 'x' using this model [x =((((D+(A-D))/y)-1)^1/B)*C] derived from the original model. ....
- - - - - - - - - -
  There is an algebra error in your inverse. It should be

 x = C*((A-y)/(y-D))^(1/B)

Your expression with just 'y' in the denominator cannot be correct.

  A note of warning. If 1/B is a fractional power, you may get values of x which are not equal to the original x used to generate the y-value. Fractional powers can have multiple results. For example (-1)^(1/4) has four different results in the complex plane: (1+i)/sqrt(2), (1-i)/sqrt(2), (-1+i)/sqrt(2), and (-1-i)/sqrt(2).

Roger Stafford

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