gate_tail.Rd
These methods aim to set a onedimensional gate (cutpoint) near the edge of a peak in
the density specified by a channel of a flowFrame
to isolate the tail population.
They allow two approaches to do this, both beginning by obtaining a
smoothened kernel density estimate (KDE) of the original density and then utilizing either
its first or second derivative.
gate_tail(fr, channel, filterId = "", num_peaks = 1, ref_peak = 1, strict = TRUE, tol = 0.01, side = "right", min = NULL, max = NULL, bias = 0, ...)
fr  a 

channel  the channel from which the cytokine gate is constructed 
filterId  the name of the filter 
num_peaks  the number of peaks expected to see. This effectively removes any peaks that are artifacts of smoothing 
ref_peak  After 
strict 

tol  the tolerance value used to construct the cytokine gate from the derivative of the kernel density estimate. See details. 
side  On which side of the density do we want to gate the tail, the

min  a numeric value that sets the lower boundary for data filtering 
max  a numeric value that sets the upper boundary for data filtering 
bias  a numeric value that adds a constant to the calculated cutpoint(threshold). Default is 0. 
...  additional arguments used in calculating derivative. See details. 
method  the method used to select the cutpoint. Either "first_deriv" or "second_deriv". See details. 
auto_tol  when TRUE, it tries to set the tolerance automatically. See details. 
adjust  the scaling adjustment applied to the bandwidth used in the first derivative of the kernel density estimate 
a filterList
containing the gates (cutpoints) for each sample with
the corresponding rectangleGate
objects defining the tail as the positive population.
The default behavior of the first approach, specified by method = "first_deriv"
,
finds valleys in the first derivative of the KDE and uses the lowest such valley
to place the cutpoint on the steep right shoulder of the largest peak in the original density.
The default behavior of the second approach, specified by method = "second_deriv"
,
is to find peaks in the second derivative of the KDE and use the largest such peak
to place the cutpoint at the point on the right shoulder of the largest
peak in the original density where it is most rapidly flattening (the first derivative is rapidly
growing less negative).
Both approach can be significantly modified from defaults with a number of optional
arguments. The num_peaks
argument specifes how many peaks should be found
in the smoothened KDE and ref_peak
specifies around which peak the gate's
cutpoint should be placed (starting from the leftmost peak). Setting the side
argument to "left" modifies the procedure to put the cutpoint on the left side of the
reference peak to isolate a left tail. The max
and min
arguments allow for
prefiltering extreme values in the channel of interest (keeping only observations with
channel values less than max and/or more than min). The bandwidth used for kernel density
estimation can be proportionally scaled using adjust
(e.g. adjust = 0.5
will
use a bandwidth that is half of the default). This allows for tuning the level of
smoothing applied in the estimation.
Lastly, the tol
, auto_tol
, and bias
arguments allow for adjustments
to be made to the cutpoint that would otherwise be returned. tol
provides a tolerance value
that the absolute value of the KDE derivative at the cutpoint must be under. If the derivative
at the original cutpoint is greater than tol
in magnitude, the returned cutpoint will be the first point
to the right of the original cutpoint (or to the left in the case of side = "left"
) with corresponding
derivative within tol
. Thus in practice, a smaller value for tol
effectively pushes the cutpoint
further down the shoulder of the peak towards the flat tail. tol
is set to 0.01 by default
but setting auto_tol = TRUE
will set the tolerance to a reasonable estimate of
1% of the maximum absolute value of the first derivative of the KDE. tol
and
auto_tol
are only used for method = "first_deriv"
. Additionally, the bias
argument allows for directly shifting the returned cutpoint left or right.
It is also possible to pass additional arguments to control the calculation
of the derivative, which will have some effect on the resulting cutpoint determination,
but this should usually not be needed. By default the number of grid points for the derivative
calculation will be 10,000, but this can be changed with num_points
. The default
bandwidth can also be directly adjusted with bandwidth
, where the final value used
will be given by adjust*bandwidth
if (FALSE) { gate < gate_tail(fr, Channel = "APCA") # fr is a flowFrame }