forecast using a four parameter logistic equation
 

I need to find x-values according to a four parameter logistic equation ( y =
(a-d)/[1+(x/c)^b]+d

How can I do this? I am not very familiar with solver. I have used XLSTAT
to find my four parameter curve and my a, b, c, and d values.

To be a little more descriptive (as I am really hoping to get some help here):

I am working with ELISA data. I have a calibration curve, therefore I have
concentrations(x, actually in LOG) and their response (y). I will generally
get a response (y) from my assay and want to find my estimated concentration
according to the calibration curve. (Like using the forecast function, but
it is not linear).

Can anyone help me with this?

Thank you,
Laura

Answer: forecast using a four parameter logistic equation
 

Hi Laura,

I can definitely help you with this! The four parameter logistic equation is a common method for curve fitting in ELISA assays. It allows you to estimate the concentration of an unknown sample based on the response of a calibration curve.

Here are the steps to find x-values according to a four parameter logistic equation:

1. Enter your a, b, c, and d values into Excel. You can put them in separate cells or in one cell separated by commas.
2. Create a column for your x-values. These should be the concentrations of your calibration standards.
3. In the next column, enter the four parameter logistic equation using cell references for your a, b, c, d, and x values. For example, if your a, b, c, and d values are in cells A1, A2, A3, and A4, respectively, and your x values are in column B starting in cell B2, the equation would be:
   
   
   Formula:

   ---

   =(A1-A4)/(1+(B2/A3)^A2)+A4 


   ---

4. Copy the equation down the column for all of your x values.
5. Now, when you have a response value (y) from your assay, you can use the solver function in Excel to find the corresponding x-value (concentration). To do this, go to the Data tab and click on Solver. Set the objective to the cell containing your four parameter logistic equation, and set the value to your assay response (y). Set the variable cells to the column of x-values you just created. Set the constraints to make sure the x-values are positive and within the range of your calibration standards. Then click Solve.
6. The solver will find the x-value that corresponds to your assay response (y) based on the four parameter logistic equation.

forecast using a four parameter logistic equation
 

You will need to use Solver for this. Email me privately (remove TRUENORTH)
and I will send a sample file
best wishes
--
Bernard V Liengme
Microsoft Excel MVP
http://people.stfx.ca/bliengme
remove caps from email

"Laura" wrote in message
...
I need to find x-values according to a four parameter logistic equation ( y
=
(a-d)/[1+(x/c)^b]+d

How can I do this? I am not very familiar with solver. I have used
XLSTAT
to find my four parameter curve and my a, b, c, and d values.

To be a little more descriptive (as I am really hoping to get some help
here):

I am working with ELISA data. I have a calibration curve, therefore I
have
concentrations(x, actually in LOG) and their response (y). I will
generally
get a response (y) from my assay and want to find my estimated
concentration
according to the calibration curve. (Like using the forecast function,
but
it is not linear).

Can anyone help me with this?

Thank you,
Laura

I am an employee of the MiraiBio Group of Hitachi Software Engineering.

We developed MasterPlex ReaderFit which is a curve-fitting software specifically designed for ELISA or dose-response analysis. It includes the very popular 4PL nonlinear regression model as well as the 5PL (including weighting options).

I have recently written a blog post on the 4 Parameter Logistic model equation which goes into pretty good details (including lots of images) in the context of ELISA and dose-response analysis.

There is a free fully functional 14-day free trial of MasterPlex ReaderFit. The product page contains a 1:43 minute demo video.

Allen Liu
MiraiBio

interesting. You created great equation . Hopefully it will be useful to all.

I too am doing ELISA with 4PL, I'm doing it on a 96 well microplate and found this page did all the analysis for me!

http://www.myassays.com/four-parameter-fit.assay

Check it out!

Hi Laura,

I have also looked for a four parameter logistic program in excel. I am now using the excel Add-in IVDfit. You can add your own formula in this program, so the function you are using can be fitted too. It can do 4PL, but also 5PL fits as well as spline, point to point and polynomial regressions. Because it is in excel it may be just the program for you.
You can download a trial version at www.ivdtools.com and see for yourself