extractFOSC  R Documentation 
Generic reimplementation of the Framework for Optimal Selection of Clusters (FOSC; Campello et al, 2013) to extract clusterings from hierarchical clustering (i.e., hclust objects). Can be parameterized to perform unsupervised cluster extraction through a stabilitybased measure, or semisupervised cluster extraction through either a constraintbased extraction (with a stabilitybased tiebreaker) or a mixed (weighted) constraint and stabilitybased objective extraction.
extractFOSC(
x,
constraints,
alpha = 0,
minPts = 2L,
prune_unstable = FALSE,
validate_constraints = FALSE
)
x 
a valid hclust object created via 
constraints 
Either a list or matrix of pairwise constraints. If missing, an unsupervised measure of stability is used to make local cuts and extract the optimal clusters. See details. 
alpha 
numeric; weight between 
minPts 
numeric; Defaults to 2. Only needed if classless noise is a valid label in the model. 
prune_unstable 
logical; should significantly unstable subtrees be
pruned? The default is 
validate_constraints 
logical; should constraints be checked for validity? See details for what are considered valid constraints. 
Campello et al (2013) suggested a Framework for Optimal Selection of
Clusters (FOSC) as a framework to make local (nonhorizontal) cuts to any
cluster tree hierarchy. This function implements the original extraction
algorithms as described by the framework for hclust objects. Traditional
cluster extraction methods from hierarchical representations (such as
hclust objects) generally rely on global parameters or cutting values
which are used to partition a cluster hierarchy into a set of disjoint, flat
clusters. This is implemented in R in function cutree()
.
Although such methods are widespread, using global parameter
settings are inherently limited in that they cannot capture patterns within
the cluster hierarchy at varying local levels of granularity.
Rather than partitioning a hierarchy based on the number of the cluster one
expects to find (k
) or based on some linkage distance threshold
(H
), the FOSC proposes that the optimal clusters may exist at varying
distance thresholds in the hierarchy. To enable this idea, FOSC requires one
parameter (minPts) that represents the minimum number of points that
constitute a valid cluster. The first step of the FOSC algorithm is to
traverse the given cluster hierarchy divisively, recording new clusters at
each split if both branches represent more than or equal to minPts. Branches
that contain less than minPts points at one or both branches inherit the
parent clusters identity. Note that using FOSC, due to the constraint that
minPts must be greater than or equal to 2, it is possible that the optimal
cluster solution chosen makes local cuts that render parent branches of
sizes less than minPts as noise, which are denoted as 0 in the final
solution.
Traversing the original cluster tree using minPts creates a new, simplified
cluster tree that is then postprocessed recursively to extract clusters
that maximize for each cluster C_i
the cost function
\max_{\delta_2, \dots, \delta_k} J = \sum\limits_{i=2}^{k} \delta_i
S(C_i)
where
S(C_i)
is the stabilitybased measure as
S(C_i) =
\sum_{x_j \in C_i}(\frac{1}{h_{min} (x_j, C_i)}  \frac{1}{h_{max} (C_i)})
\delta_i
represents an indicator function, which constrains
the solution space such that clusters must be disjoint (cannot assign more
than 1 label to each cluster). The measure S(C_i)
used by FOSC
is an unsupervised validation measure based on the assumption that, if you
vary the linkage/distance threshold across all possible values, more
prominent clusters that survive over many threshold variations should be
considered as stronger candidates of the optimal solution. For this reason,
using this measure to detect clusters is referred to as an unsupervised,
stabilitybased extraction approach. In some cases it may be useful
to enact instancelevel constraints that ensure the solution space
conforms to linkage expectations known a priori. This general idea of
using preliminary expectations to augment the clustering solution will be
referred to as semisupervised clustering. If constraints are given in
the call to extractFOSC()
, the following alternative objective function
is maximized:
J = \frac{1}{2n_c}\sum\limits_{j=1}^n \gamma (x_j)
n_c
is the total number of constraints given and
\gamma(x_j)
represents the number of constraints involving
object x_j
that are satisfied. In the case of ties (such as
solutions where no constraints were given), the unsupervised solution is
used as a tiebreaker. See Campello et al (2013) for more details.
As a third option, if one wishes to prioritize the degree at which the
unsupervised and semisupervised solutions contribute to the overall optimal
solution, the parameter \alpha
can be set to enable the extraction of
clusters that maximize the mixed
objective function
J = \alpha S(C_i) + (1  \alpha) \gamma(C_i))
FOSC expects the pairwise constraints to be passed as either 1) an
n(n1)/2
vector of integers representing the constraints, where 1
represents shouldlink, 1 represents shouldnotlink, and 0 represents no
preference using the unsupervised solution (see below for examples).
Alternatively, if only a few constraints are needed, a named list
representing the (symmetric) adjacency list can be used, where the names
correspond to indices of the points in the original data, and the values
correspond to integer vectors of constraints (positive indices for
shouldlink, negative indices for shouldnotlink). Again, see the examples
section for a demonstration of this.
The parameters to the input function correspond to the concepts discussed
above. The minPts
parameter to represent the minimum cluster size to
extract. The optional constraints
parameter contains the pairwise,
instancelevel constraints of the data. The optional alpha
parameters
controls whether the mixed objective function is used (if alpha
is
greater than 0). If the validate_constraints
parameter is set to
true, the constraints are checked (and fixed) for symmetry (if point A has a
shouldlink constraint with point B, point B should also have the same
constraint). Asymmetric constraints are not supported.
Unstable branch pruning was not discussed by Campello et al (2013), however
in some data sets it may be the case that specific subbranches scores are
significantly greater than sibling and parent branches, and thus sibling
branches should be considered as noise if their scores are cumulatively
lower than the parents. This can happen in extremely nonhomogeneous data
sets, where there exists locally very stable branches surrounded by unstable
branches that contain more than minPts
points.
prune_unstable = TRUE
will remove the unstable branches.
A list with the elements:
cluster 
A integer vector with cluster assignments. Zero indicates noise points (if any). 
hc 
The original hclust object with additional list elements

Matt Piekenbrock
Campello, Ricardo JGB, Davoud Moulavi, Arthur Zimek, and Joerg Sander (2013). A framework for semisupervised and unsupervised optimal extraction of clusters from hierarchies. Data Mining and Knowledge Discovery 27(3): 344371. \Sexpr[results=rd]{tools:::Rd_expr_doi("10.1007/s1061801303114")}
hclust()
, hdbscan()
, stats::cutree()
Other clustering functions:
dbscan()
,
hdbscan()
,
jpclust()
,
optics()
,
sNNclust()
data("moons")
## Regular HDBSCAN using stabilitybased extraction (unsupervised)
cl < hdbscan(moons, minPts = 5)
cl$cluster
## Constraintbased extraction from the HDBSCAN hierarchy
## (w/ stabilitybased tiebreaker (semisupervised))
cl_con < extractFOSC(cl$hc, minPts = 5,
constraints = list("12" = c(49, 47)))
cl_con$cluster
## Alternative formulation: Constraintbased extraction from the HDBSCAN hierarchy
## (w/ stabilitybased tiebreaker (semisupervised)) using distance thresholds
dist_moons < dist(moons)
cl_con2 < extractFOSC(cl$hc, minPts = 5,
constraints = ifelse(dist_moons < 0.1, 1L,
ifelse(dist_moons > 1, 1L, 0L)))
cl_con2$cluster # same as the second example
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