{smcl}
{* 09 Feb 2009}{...}
{cmd:help skilmack}
{hline}
{title:Title}
{p2colset 5 17 19 2}{...}
{p2col :{hi:skilmack} {hline 2}}Skillings-Mack test{p_end}
{p2colreset}{...}
{title:Syntax}
{p 8 16 2}
{cmd:skilmack}
{it:varname}
{ifin}{cmd:,} {opt i:d(varname)} {opt repeated(varname)} [{it:options}]
{synoptset 30}{...}
{synopthdr}
{synoptline}
{synopt:{opt c:ovariance}}use estimated covariance matrix in place of no-ties covariance matrix{p_end}
{synopt:{cmdab:f:orcesims(on}|{cmd:off)}}if and only if there are ties will simulations be run, unless overridden by this option{p_end}
{synopt:{opt reps(#)}}number of simulations; default is {cmd:reps(1000)}{p_end}
{synopt:{opt s:eed(#)}}specify initial value of random-number seed{p_end}
{synopt:{cmdab:n:otable(noties}|{cmd:tiescov}|{cmd:both)}}suppress output
table produced in no-ties section or in case of ties when {cmd:covariance}
option is used (or both){p_end}
{synoptline}
{p2colreset}{...}
{title:Description}
{pstd} {cmd:skilmack} implements the Skillings-Mack (SM) test, which is a
generalization of the Friedman test. It is particularly useful for an unbalanced/incomplete block design or in the presence of
missing data. Missing data can be missing by design or missing completely at
random.
{pstd} N.B. The SM test is equivalent to the Friedman test
when there are no missing data and is useful when there are many ties or equal ranks.
{pstd} Unlike with the {helpb friedman} command, the data are required to be in the
more usual long format, i.e., one column for the outcome measure, one for the
block identifier or ID, and one for the treatment or
within-block repeated variable.
{title:Options}
{phang} {cmd:id(}{it:varname}{cmd:)} is required and specifies the factor variable containing
the block identifiers.
{phang} {cmd:repeated(}{it:varname}{cmd:)} is required and specifies the factor variable
containing the treatment identifiers.
{phang} {cmd:covariance} specifies that the estimated covariance
matrix is used in place of the no-ties covariance matrix. The estimated
covariance matrix is the sample covariance matrix of the weighted sum of
centered ranks from the simulations.
{phang} {cmd:forcesims(on}|{cmd:off)} forces whether simulations are used.
Simulations will be run if and only if there are ties, unless overridden by
this option.
{phang} {opt reps(#)} sets the number of simulations. The default is
{cmd:reps(1000)}.
{phang} {opt seed(#)} specifies the random-number seed; time is used as the
default seed. This option allows an exact replication of the Monte Carlo
simulations.
{phang} {cmd:notable(noties}|{cmd:tiescov}|{cmd:both)} suppresses the output
table produced in the no-ties section or in the case of ties when the
{cmd:covariance} option is used (or both).
{title:Remarks}
{phang} The following data are taken from Brady (1969):
Dysfluencies under each condition
+-----------------+
| id R A N |
|-----------------|
| 1 3 5 15 |
| 2 1 3 18 |
| 3 5 4 21 |
| 4 2 . 6 |
| 5 0 2 17 |
| 6 0 2 10 |
| 7 0 3 8 |
| 8 0 2 13 |
+-----------------+
{pstd}Use {helpb reshape} to reshape the data into the long format; this is,
after prefixing each condition with "score" by using {helpb rename}, type
. {cmd:reshape long score, i(id) j(cond) string}
+--------------------+
| id cond score |
|--------------------|
| 1 A 5 |
| 1 N 15 |
| 1 R 3 |
|--------------------|
| 2 A 3 |
| 2 N 18 |
| 2 R 1 |
|--------------------|
| 3 A 4 |
| 3 N 21 |
| 3 R 5 |
|--------------------|
| 4 A . |
| 4 N 6 |
| 4 R 2 |
|--------------------|
| 5 A 2 |
| 5 N 17 |
| 5 R 0 |
|--------------------|
| 6 A 2 |
| 6 N 10 |
| 6 R 0 |
|--------------------|
| 7 A 3 |
| 7 N 8 |
| 7 R 0 |
|--------------------|
| 8 A 2 |
| 8 N 13 |
| 8 R 0 |
+--------------------+
The SM results from the above data are
. {cmd:skilmack score, id(id) repeated(cond)}
Weighted Sum of Centered Ranks
cond | N WSumCRank SE WSum/SE
-------+-------------------------------------
A | 7 -1.73 3.74 -0.46
N | 8 13.12 3.87 3.39
R | 8 -11.39 3.87 -2.94
---------------------------------------------
Total 0
Skillings Mack = 13.281
P-value (No ties) = 0.0013
N.B. As P-value <0.02, it is likely to be conservative (unless n large).
Consider obtaining a p-value from a simulated null distribution of SM -
see options.
{pstd} N.B. A large negative {cmd:WSumCRank} (or {cmd:WSum/SE}) means a low ranking
(e.g., 1) because of typically low scores. This was the case for condition
{cmd:R},
which had the fewest dysfluencies.
{bf: Simulations and ties}
{pstd} Simulations are preferable for obtaining more accurate small p-values
if the sample size is not large, because the p-value from the chi-squared
approximation is likely to be conservative here (Skillings and Mack
1981).
{pstd} If there are ties (equal ranks), average ranks are assigned, e.g., 1.5,
1.5, 3. Assigning average ranks is perhaps the most common way of dealing with
ties. However, one may prefer to force ranks to be randomly assigned when they
are tied. (This can effectively be done by adding a small random amount to
each score.)
{pstd} The SM statistic can be calculated when there are ties; however, the p-value calculated from the assumed chi-squared null distribution
becomes more and more conservative the more ties there are. To provide a
more accurate p-value, simulations are used to approximate the distribution
of SM values under the null hypothesis, and conditional on the particular
missing-data structure and tied rankings.
{pstd} A dataset is simulated by sorting on random numbers, for each
individual, to randomly shuffle which data point belongs to which repeat. The
sorting on random numbers is not applied where there are missing data to
preserve the missing-data structure.
{pstd} With the {cmd:covariance} option, the SM statistic can be redefined by
estimating the covariance matrix of the weighted sums of centered ranks and
using this in place of the covariance matrix (which is accurate when there are
no ties, but not when there are many ties). A new table is produced with
different standard errors, and a new SM statistic and p-value are calculated.
The tables can be suppressed by using the {cmd:notable} option.
{bf: Definition of SM statistic}
{pstd} SM = A' (sigma0^-1) A , where A is a column vector of all but one of
the weighted [by sqrt(12/ si+1), where si is the number of measures for person
i] sums of centered ranks, and (sigma0^-1) is any generalized inverse of the
covariance matrix.
{title:References}
{phang}
Brady, J. P. 1969. Studies on the metronome effect on stuttering. {it:Behaviour Research and Therapy} 7: 197-204.
{phang}
Skillings, J. H., and G. A. Mack. 1981. On the use of a Friedman-type
statistic in balanced and unbalanced block designs. {it:Technometrics} 23: 171-177.
{title:Authors}
{pstd}Mark Chatfield{p_end}
{pstd}Medical Research Council{p_end}
{pstd}Human Nutrition Research{p_end}
{pstd}Cambridge, UK{p_end}
{pstd}mdc_england@hotmail.com{p_end}
{pstd}Adrian Mander{p_end}
{pstd}Medical Research Council{p_end}
{pstd}Human Nutrition Research{p_end}
{pstd}Cambridge, UK{p_end}
{title:Also see}
{psee}
Online: {helpb friedman} (if installed) {p_end}