NAG CL Interface
g13dmc (multi_​corrmat_​cross)

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1 Purpose

g13dmc calculates the sample cross-correlation (or cross-covariance) matrices of a multivariate time series.

2 Specification

#include <nag.h>
void  g13dmc (Nag_CovOrCorr matrix, Integer k, Integer n, Integer m, const double w[], double wmean[], double r0[], double r[], NagError *fail)
The function may be called by the names: g13dmc, nag_tsa_multi_corrmat_cross or nag_tsa_multi_cross_corr.

3 Description

Let Wt = (w1t,w2t,,wkt) T , for t=1,2,,n, denote n observations of a vector of k time series. The sample cross-covariance matrix at lag l is defined to be the k×k matrix C^(l), whose (i,j)th element is given by
C^ij(l)=1nt=l+1n(wi(t-l)-w¯i)(wjt-w¯j),  l=0,1,2,,m, ​i=1,2,,k​ and ​j=1,2,,k,  
where w¯i and w¯j denote the sample means for the ith and jth series respectively. The sample cross-correlation matrix at lag l is defined to be the k×k matrix R^(l), whose (i,j)th element is given by
R^ ij (l) = C^ ij (l) C^ ii (0) C^ jj (0) ,   l=0,1,2,,m , ​ i=1,2,,k ​ and ​ j=1,2,,k .  
The number of lags, m, is usually taken to be at most n/4.
If Wt follows a vector moving average model of order q, then it can be shown that the theoretical cross-correlation matrices (R(l)) are zero beyond lag q. In order to help spot a possible cut-off point, the elements of R^(l) are usually compared to their approximate standard error of 1/n. For further details see, for example, Wei (1990).
The function uses a single pass through the data to compute the means and the cross-covariance matrix at lag zero. The cross-covariance matrices at further lags are then computed on a second pass through the data.

4 References

Wei W W S (1990) Time Series Analysis: Univariate and Multivariate Methods Addison–Wesley
West D H D (1979) Updating mean and variance estimates: An improved method Comm. ACM 22 532–555

5 Arguments

1: matrix Nag_CovOrCorr Input
On entry: indicates whether the cross-covariance or cross-correlation matrices are to be computed.
The cross-covariance matrices are computed.
The cross-correlation matrices are computed.
Constraint: matrix=Nag_AutoCov or Nag_AutoCorr.
2: k Integer Input
On entry: k, the dimension of the multivariate time series.
Constraint: k1.
3: n Integer Input
On entry: n, the number of observations in the series.
Constraint: n2.
4: m Integer Input
On entry: m, the number of cross-correlation (or cross-covariance) matrices to be computed. If in doubt set m=10. However it should be noted that m is usually taken to be at most n/4.
Constraint: 1m<n.
5: w[k×n] const double Input
On entry: w[(t-1)k+i-1] must contain the value for series i at time t, for i=1,2,,k and t=1,2,,n.
6: wmean[k] double Output
On exit: the means, w¯i, for i=1,2,,k.
7: r0[k×k] double Output
On exit: if matrix=Nag_AutoCov, r0[(j-1)k+i-1] contains the (i,j)th element of the sample cross-covariance matrix.
If matrix=Nag_AutoCorr, r0[(j-1)k+i-1], ij contains the (i,j)th element of the sample cross-correlation matrix and r0[(i-1)k+i-1] contains the standard deviation of the ith series.
8: r[k×k×m] double Output
On exit: if matrix=Nag_AutoCov, r[(l-1)k2+(j-1)k+i-1] contains the (i,j)th element of the sample cross-covariance matrix at lag l.
If matrix=Nag_AutoCorr, then it contains the (i,j)th element of the sample cross-correlation matrix lag l, for l=1,2,,m, i=1,2,,k and j=1,2,,k.
9: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument value had an illegal value.
On entry, k=value.
Constraint: k1.
On entry, n=value.
Constraint: n2.
On entry, m=value and n=value.
Constraint: m1 and m<n.
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
On entry, at least one of the series is such that all its elements are practically identical giving zero (or near zero) variance. In this case if matrix=Nag_AutoCorr all the correlations in r0 and r involving this variance are set to zero.

7 Accuracy

For a discussion of the accuracy of the one-pass algorithm used to compute the sample cross-covariances at lag zero see West (1979). For the other lags a two-pass algorithm is used to compute the cross-covariances; the accuracy of this algorithm is also discussed in West (1979). The accuracy of the cross-correlations will depend on the accuracy of the computed cross-covariances.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
g13dmc makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The time taken is roughly proportional to mnk2.

10 Example

This program computes the sample cross-correlation matrices of two time series of length 48, up to lag 10. It also prints the cross-correlation matrices together with plots of symbols indicating which elements of the correlation matrices are significant. Three * represent significance at the 0.5% level, two * represent significance at the 1% level and a single * represents significance at the 5% level. The * are plotted above or below the line depending on whether the elements are significant in the positive or negative direction.

10.1 Program Text

Program Text (g13dmce.c)

10.2 Program Data

Program Data (g13dmce.d)

10.3 Program Results

Program Results (g13dmce.r)