Interpolation

[Strugln_Student]

I have a problem involving interpolation. I have two columns of data that vary depending on input parameters. I would like to find an iterpolated value within the second column based on data from the first column. I want to keep from adding a trend line, and would rather have the computer take an interpolation value I select, search the 50+ data points in the first column, find values that are directly above and below, interpolate, and return the interpolated value from the second column. It is not a problem to have the fist column in ascending order, but I don't want to interpolate by trend line, rather linearly between the closest greater than and less than points.

I have search the internet and read something about an Interpol() function, but can not find it in excel as a function or an add-in. I could only find one page about it and could not get access.

Any assistance you can give me will be greatly appreciated. Thanks.

[Strugln_Student]

I have this same problem on another set of data, but I am able to fit a trend line to it and use the equation to solve for points, but the other set of data does not have a trend line anywhere close to matching the data set.

[Strugln_Student]

If you don't understand my problem please tell me so I can try to clarify. If you understand, but can't think of anything that would work please tell me, or if I am being a complete newb please tell me.

[BenjieLop]

Maybe, you should go back to the basics of interpolation. For example, if your first column has values in the following cells:

A1
A2
A3

and your second column has the following entries

B1
B2 --- (this is the value that you want interpolated)
B3

Without going through the algebraic process, your formula to determine B2 is

=B1-(B1-B3)*(A1-A2)/(A1-A3)

Hope this helps.

[Strugln_Student]

Because of the nature of the spread sheet the data changes, and therefore the location of the data points directly above and directly below the value I want to interpolate moves about the range of data. I was hoping there was a function or an add on that would do it, but after much searching I have found nothing. I have got it to work by using the vlookup function, but that only returns a value that is closest to the interpolated value, so I had to determine if the value vlookup returned was greater that or less than the look up value and then find the value closest on the other side of the data point by more equations. It turns out to be several equations to make it work, but it does. Seems like it would be a pretty easy and useful function, but excel does not have it. Thanks for your help.