Produces Jackknife estimates and standard errors (R.W. Payne).

PRINT = *string*

Controls printed output (estimates, vcovariance); default
esti

DATA = *variates*, *factors* or *texts*

Data vectors from which the statistics are to
be calculated

ANCILLARY = *any type*

Other relevant information needed to calculate the
statistics

VCOVARIANCE = *symmetric matrix*

Saves the variance-covariance matrix for
the statistics

LABEL = *texts*

Texts, each containing a single line, to label the statistics

ESTIMATE = *scalars*

Saves the Jackknife estimate for each statistic

SE = *scalars*

Saves Jackknife estimates of the standard errors

PSEUDOVALUES = *variates*

Saves the Jackknife pseudo-values

ACCELERATION = *scalars*

Saves the acceleration parameter for bias-corrected
and accelerated bootstrap confidence intervals

The Jackknife provides a way of decreasing bias and obtaining standard errors in
situations where the standard methods might be expected to be inappropriate. The basic
form of the Jackknife method works by calculating the statistic (or statistics) of interest
omitting each data value in turn. Thus, if there are *n* data values, *n* "partial estimates" *T*_{-1}
... *T*_{-n} are obtained (where *T*_{-j} is the estimate omitting value *j*). These are combined with
the estimate *T* obtained from all the data, to produce *n* pseudo-values:

*P _{j}* =

The Jackknife estimate of the statistic is given by the mean of the pseudo-values, and the standard error by the standard error of the mean of the pseudo-values.

The Jackknife can be shown to eliminate the term proportional to 1/*n* from a bias of the
form

*T* = *t* + *a*/*n* + O(1/*n*^{2})

where *t* is the true value of the estimate and O(1/*n*^{2}) is a term of order one divided by the
square of the number of observations (Quenouille 1956). However, it is not appropriate
in all situations. In particular the statistic needs to be "smooth" (small changes in the data
set should cause only small changes in the statistic); it will not work for example with
medians or order statistics. Further details and advice are given by Miller (1974), Bissell
& Ferguson (1975), Hinkley (1983) and Efron & Tibshirani (1993).

The data for JACKKNIFE are provided as a list of vectors (variates, factors or texts)
using the DATA option. From this, new vectors are formed omitting each unit of the original
vectors in turn, and a subsidiary procedure RESAMPLE is called to calculate the statistics.
Other relevant information can be provided for passing to RESAMPLE, in any type of data
structure, using the ANCILLARY option. To use JACKKNIFE, you need to provide a version
of RESAMPLE to calculate the particular statistics that you require. The default RESAMPLE
procedure, which accompanies JACKKNIFE in the library, merely prints details of the
syntax (also described in the *Methods* Section).

A label should be provided for each statistic, using the LABEL parameter; by default, there is assumed to be a single statistic labelled simply as Statistic. The estimates, their standard errors and variates of corresponding pseudo-values for each statistic can be saved by the ESTIMATE, SE and PSEUDOVALUES parameters, respectively. Also, if there is more than one statistic, a variance-covariance matrix can be saved for the estimates using the VCOVARIANCE option.

Printed output is controlled by the PRINT option, with settings estimates for the estimates and their standard errors, and vcovariance for the variance-covariance matrix; by default PRINT=estimates.

The jackknife is also required for the calculation of bias-corrected and accelerated confidence limits for bootstrap statistics (as given by the BOOTSTRAP procedure). The necessary acceleration quantities can be saved using the ACCELERATION parameter. For details see Efron & Tibshirani, 1993, Section 14.3.

Options: PRINT, DATA, ANCILLARY, VCOVARIANCE.

Parameters: LABEL, ESTIMATE, SE, PSEUDOVALUES, ACCELERATION.

The original papers describing the Jackknife technique are by Quenouille (1949, 1956) and by Tukey (1958). Good expository accounts are provided by Hinkley (1983) or Bissell & Ferguson (1975).

JACKKNIFE needs a subsidiary procedure RESAMPLE to calculate the statistics of
interest. RESAMPLE has an option, DATA, which is used to supply the data vectors (variates,
factors or texts) from which the statistics are to be calculated. (On the first occasion that
RESAMPLE is called, these will be the original vectors as supplied to JACKNIFE, in order to
calculate the estimate *T*; subsequently, they will be new vectors containing all except one
of the units.) Other relevant information can can be supplied through the ANCILLARY
option, which corresponds to the ANCILLARY option of JACKKNIFE itself. RESAMPLE can
be called by the BOOTSTRAP procedure, and it then also has an AUXILIARY
option, but this is not relevant to JACKKNIFE.

There are two parameters: STATISTICS supplies a list of scalars to store the estimates of each statistic, and EXIT a list of scalars which should be set to zero or one according to whether or not each statistic could be estimated successfully with the supplied data vectors. If the value of EXIT is not calculated in RESAMPLE, JACKKNIFE assumes that the calculations succeeded. This example shows a version of RESAMPLE which calculates the correlation between two variates.

PROCEDURE [PARAMETER=pointer] 'RESAMPLE'

OPTION 'DATA', " (I: variates, factors or texts) data

vectors from which to calculate the

statistics; no default"\

'ANCILLARY'; " (I: any type of structure) other

relevant information needed to

calculate the statistics "\

MODE=p; TYPE=!t(variate,factor,text),*;\

SET=yes,no; LIST=yes; DECLARED=yes; PRESENT=yes

PARAMETER 'STATISTIC', " (O: scalars) to save the calculated

statistics "\

'EXIT'; " (O: scalars) to save an exit code

to indicate failure (EXIT[i]=1) or

success (EXIT[i]=0) when calculating

each STATISTIC[i]"\

MODE=p; TYPE='scalar'; SET=yes

CALCULATE STATISTIC[1] = CORRELATION(DATA[1]; DATA[2])

& EXIT[1] = STATISTIC[1]==C('missing')

ENDPROCEDURE

If any of the data vectors is restricted, JACKKNIFE will use only the units that are not restricted for any of the vectors.

Bissell, A.F. & Ferguson, R.A. (1975). The jackknife - toy, tool or two-edged weapon. *The
Statistician*, 24, 79-100.

Efron, B. & Tibshirani, R.J. (1993). *An Introduction to the Bootstrap*. Chapman & Hall,
London.

Hinkley, D. (1983). Jackknife methods. In: *Encyclopedia of Statistics, Volume 4* (ed: S. Kotz,
N.L. Johnson & C.B. Read). Wiley, New York.

Miller, R.G. (1974). The jackknife - a review. *Biometrika*, 61, 1-15.

Quenouille, M.H. (1949). Approximate tests of correlation in time series. *Journal of the
Royal Statistical Society, Series B*, 11, 18-44.

Quenouille, M.H. (1956). Notes on bias in estimation. *Biometrika*, 61, 353-360.