I describe my use of the Frequency Domain System Identification toolbox (FDIDENT) to fit this transfer function in CSWG elog #11205. FDIDENT is a third party Matlab toolbox which provides tools for identifying linear dynamic single-input/single-output (SISO) systems from time response or frequency response measurements. The toolbox is free for non-profit use.

https://www.mathworks.com/products/connections/product_detail/product_35570.html

http://home.mit.bme.hu/~kollar/fdident/

A stable, but non-minimum phase, model without delay – compatible with a Linear Time Invariant (LTI) representation -- results in a best fit for a 22 order numerator and 28 order denominator model, m2228. The model is compared to the measurement data in the attached bode plot.

I attach several new parts of this high frequency characterization in order to facilitate incorporating the uncertainty in any future transfer function fitting. 

I attach three new text files:
    "..._tf.txt" -- a copy of the originally attached text file, columns are 
        [freq re(A) im(A)]
    "..._coh.txt" -- an export of the (prefiltered) coherence, columns are 
        [freq iEXCCoh PCALCoh]
    "..._relunc.txt" -- an export of the combined relative uncertainty on the transfer function, columns are 
        [freq sigma_A]

Computing the uncertainty on this actuation strength was a bit of a challenge. 
Remember, the above measure of the actuation strength of the UIM stage, A, is a combination of two transfer functions, as described in P1500248, Section V. In this aLOG they're referred to as "PCAL2DARM" where we use the photon calibrator as a reference actuator, and "iEXC2DARM" where the suspension stage under test is used as the actuator. Typically, the iEXC2DARM transfer function has the lowest coherence. 
Even worse, I've combined many data sets of both transfer functions covering different frequency regions each with a different number of averages.
Thus form the uncertainty, I've taken each frequency region's data set, and 
    - Filtered both iEXC and PCAL transfer functions for data points in which the iEXC TF has coherence greater than 0.95,
    - Created a relative uncertainty vector for each iEXC and PCAL transfer functions using the standard B&P equation,
        sigma_TF(f) / TF = sqrt( (1-C(f)) / (2 N C(f)) )
    where C(f) is the coherence, and N is the number of averages (N was 10 for swept sine TFs, 25 for broad band TFs)
    - Concatenated the data sets to form the overall transfer function, A,
    - Combined the two uncertainty vectors in the standard way,
        sigma_A / A = sqrt((sigma_iEXC / iEXC)^2 + (sigma_PCAL / PCAL)^2)
    - Sorted the collection of 
        [frequency complextf iexccoh pcalcoh sigma_A]
    by frequency.
    - Exported the uncertainty.

Note that one only needs one column of uncertainty, for the absolute uncertainty in magnitude is just
    |sigma_A| = abs(A) * (sigma_A / A)
and the absolute uncertainty in phase is
    /_ sigma  = 180/pi * (sigma_A / A)

I attach a plot of the magnitude and its uncertainty for demonstrative purposes, so that when the files are used, you can compare your plots of this against mine to be sure you're using the data right. Note that I've multiplied the uncertainty by a factor of 10 for plotting only so that it's visible.

I've updated and committed the function that's used to process this data, and it can be found here:
    /ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/ER10/H1/Scripts/FullIFOActuatorTFs/
    process_H1SUSETMY_L1_HFDynamicsTest_20161117.m