48,452 research outputs found
Minimizing the effect of sinusoidal trends in detrended fluctuation analysis
The detrended fluctuation analysis (DFA) [Peng et al., 1994] and its
extensions (MF-DFA) [Kantelhardt et al., 2002] have been used extensively to
determine possible long-range correlations in self-affine signals. While the
DFA has been claimed to be a superior technique, recent reports have indicated
its susceptibility to trends in the data. In this report, a smoothing filter is
proposed to minimize the effect of sinusoidal trends and distortion in the
log-log plots obtained by DFA and MF-DFA techniques
Sequential drain amylase to guide drain removal following pancreatectomy
BACKGROUND:
Although used as criterion for early drain removal, postoperative day (POD) 1 drain fluid amylase (DFA) ≤ 5000 U/L has low negative predictive value for clinically relevant postoperative pancreatic fistula (CR-POPF). It was hypothesized that POD3 DFA ≤ 350 could provide further information to guide early drain removal.
METHODS:
Data from a pancreas surgery consortium database for pancreatoduodenectomy and distal pancreatectomy patients were analyzed retrospectively. Those patients without drains or POD 1 and 3 DFA data were excluded. Patients with POD1 DFA ≤ 5000 were divided into groups based on POD3 DFA: Group A (≤350) and Group B (>350). Operative characteristics and 60-day outcomes were compared using chi-square test.
RESULTS:
Among 687 patients in the database, all data were available for 380. Fifty-five (14.5%) had a POD1 DFA > 5000. Among 325 with POD1 DFA ≤ 5000, 254 (78.2%) were in Group A and 71 (21.8%) in Group B. Complications (35 (49.3%) vs 87 (34.4%); p = 0.021) and CR-POPF (13 (18.3%) vs 10 (3.9%); p < 0.001) were more frequent in Group B.
CONCLUSIONS:
In patients with POD1 DFA ≤ 5000, POD3 DFA ≤ 350 may be a practical test to guide safe early drain removal. Further prospective testing may be useful
A maximum likelihood based technique for validating detrended fluctuation analysis (ML-DFA)
Detrended Fluctuation Analysis (DFA) is widely used to assess the presence of
long-range temporal correlations in time series. Signals with long-range
temporal correlations are typically defined as having a power law decay in
their autocorrelation function. The output of DFA is an exponent, which is the
slope obtained by linear regression of a log-log fluctuation plot against
window size. However, if this fluctuation plot is not linear, then the
underlying signal is not self-similar, and the exponent has no meaning. There
is currently no method for assessing the linearity of a DFA fluctuation plot.
Here we present such a technique, called ML-DFA. We scale the DFA fluctuation
plot to construct a likelihood function for a set of alternative models
including polynomial, root, exponential, logarithmic and spline functions. We
use this likelihood function to determine the maximum likelihood and thus to
calculate values of the Akaike and Bayesian information criteria, which
identify the best fit model when the number of parameters involved is taken
into account and over-fitting is penalised. This ensures that, of the models
that fit well, the least complicated is selected as the best fit. We apply
ML-DFA to synthetic data from FARIMA processes and sine curves with DFA
fluctuation plots whose form has been analytically determined, and to
experimentally collected neurophysiological data. ML-DFA assesses whether the
hypothesis of a linear fluctuation plot should be rejected, and thus whether
the exponent can be considered meaningful. We argue that ML-DFA is essential to
obtaining trustworthy results from DFA.Comment: 22 pages, 7 figure
Minimal Synthesis of String To String Functions From Examples
We study the problem of synthesizing string to string transformations from a
set of input/output examples. The transformations we consider are expressed
using deterministic finite automata (DFA) that read pairs of letters, one
letter from the input and one from the output. The DFA corresponding to these
transformations have additional constraints, ensuring that each input string is
mapped to exactly one output string.
We suggest that, given a set of input/output examples, the smallest DFA
consistent with the examples is a good candidate for the transformation the
user was expecting. We therefore study the problem of, given a set of examples,
finding a minimal DFA consistent with the examples and satisfying the
functionality and totality constraints mentioned above.
We prove that, in general, this problem (the corresponding decision problem)
is NP-complete. This is unlike the standard DFA minimization problem which can
be solved in polynomial time. We provide several NP-hardness proofs that show
the hardness of multiple (independent) variants of the problem.
Finally, we propose an algorithm for finding the minimal DFA consistent with
input/output examples, that uses a reduction to SMT solvers. We implemented the
algorithm, and used it to evaluate the likelihood that the minimal DFA indeed
corresponds to the DFA expected by the user.Comment: SYNT 201
Detrended fluctuation analysis for fractals and multifractals in higher dimensions
One-dimensional detrended fluctuation analysis (1D DFA) and multifractal
detrended fluctuation analysis (1D MF-DFA) are widely used in the scaling
analysis of fractal and multifractal time series because of being accurate and
easy to implement. In this paper we generalize the one-dimensional DFA and
MF-DFA to higher-dimensional versions. The generalization works well when
tested with synthetic surfaces including fractional Brownian surfaces and
multifractal surfaces. The two-dimensional MF-DFA is also adopted to analyze
two images from nature and experiment and nice scaling laws are unraveled.Comment: 7 Revtex pages inluding 11 eps figure
- …