{smcl}
{* 16August2003}{...}
{hline}
help for {hi:corrtab}
{hline}
{title:Correlation analysis}
{p 8 16}{cmd:corrtab}
[{it:varlist}]
[{cmd:weight} {it:fweight aweight}]
[{cmd:if} {it:exp}]
[{cmd:in} {it:range}]
[ {cmd:,}
{cmdab:o:bs}
{cmdab:sig}
{cmdab:b:onferroni}
{cmdab:sid:ak}
{cmdab:cwd:eletion}
{cmdab:vs:ort(}{it:varname numeric}{cmd:)}
{cmdab:v:ars(}{it:#}{cmd:)}
{cmdab:a:bove(}{it:#}{cmd:)}
{cmdab:p:rint(}{it:#}{cmd:)}
{cmdab:sor:t}
{cmdab:sp:earman}
{cmdab:t:label}
{cmdab:c:label}
{cmdab:all:label}
{cmdab:format}
]
{title:Description}
{p}{cmd:corrtab} displays Pearson or Spearman rank correlations
for {it:varlist}. The default calculation of individual correlation coefficients
is made independently and thus the display contains the pairwise coefficients.
Optionally, casewise deletion can be requested. The sample {it:n} and a test of
independence are also reported optionally. String variables are automatically
omitted from the analytic processing. Multiple, duplicative {it:varlist} designations
can be selected to enable full capture of intended variables; duplicate
variables specified in {it:varlist} are removed before processing.
{title:Remarks}
{p}{cmdab:corrtab} provides a rapid display of correlations formatted for
easy reading and for copying to reports and manuscripts. {cmd:corrtab} is
meant for use when the number of column variables is 8 or fewer, although it
could display many more column variables depending on font and linesize.
The user should experiment. The number of column variables that will be
displayed properly depends also on the length of the labels in column 1.
{p}{cmdab:corrtab} optionally makes use of advanced labeling systems to provide
clear and useful display suitable for the screen and for word-processors
(see below).
{title:Options}
{p}{cmd:spearman} specifies Spearman correlations. The default is to calculate
Pearson correlations.
{p}{cmd:obs} adds a line to each row of the display reporting the number of
observations used in calculating the correlation coefficient.
{p}{cmd:sig} adds a line to each row of the display reporting the significance
level of each correlation coefficient.
{p}{cmd:print(}{it:#}{cmd:)} specifies the significance level for printing of correlation
coefficients. Coefficients with significance levels larger than {it:#} are
left blank. {cmd:print(10)} or {cmd:print(.1)} would list only coefficients
significant at the 10% level or better.
{p}{cmd:bonferroni} makes the Bonferroni adjustment to calculated significance
levels. This affects printed significance levels and the {cmd:print()} option.
{cmd:corrtab, print(.05) bonferroni} prints coefficients with
Bonferroni-adjusted significance levels of .05 or less.
{p}{cmd:sidak} makes the Sidak adjustment to calculated significance levels.
This affects printed significance levels and the {cmd:print()} option.
{cmd:corrtab, print(.05) sidak} prints coefficients with Sidak-adjusted
significance levels of .05 or less.
{p}{cmd:vars(}{it:#}{cmd:)} specifies that the first {it:#} variables on the {it:varlist} are
to be correlated with all of the variables on the {it:varlist}. This produces
{it:#} columns of correlations. There is no limit
to the number of variables specified, but a difficult to read display occurs
when the number of variables exceeds the width of the screen. Not specifying {cmd:vars()}
results in all variables being displayed.
{p}{cmd:sort} requests the {it:varlist} be reported in sorted order. If {cmd:vars()}
is specified the first {it:#} variables will not be sorted.
{p}{cmd:above(}{it:#}{cmd:)} specifies the minimum absolute level of correlation coefficients
to be printed. Coefficients with smaller coefficients are left blank.
{cmd:above(.5)} would list only coefficients of 0.5 or greater
or -0.5 or less.
{p}{cmd:cwdeletion} removes observations with missing values in the {it:varlist}
from the calculations.
{p}{cmd:vsort()} sorts the correlation coefficient in descending order according
to a selected variable in the column list. This option works only when {cmd:obs}
and/or {cmd:sig} are not used. That is, it works for the simple display of
coefficients only.
{p}{cmd:tlabel} makes use of the {cmd:tlabel} system (if used) to provide detailed
labels for column 1 (see below).
{p}{cmd:clabel} places labels in the column names using {cmd:char} {it:varname}{cmd:[varname]},
according to {cmd:list}'s {cmd:subvarname} option (see below).
{p}{cmd:alllabel} places labels in columns and rows using
{cmd:char} {it:varname}{cmd:[varname]}, according to {cmd:list}'s {cmd:subvarname} option (see below).
{p}{cmd:format} Default is %9.3f. Increase both f and d (%.f.df) to handle
large number of observations and/or increased decimal format.
{title:User-defined labels}
{p}By default, {cmd:corrtab} uses variable names for column and row labels.
However, variable names are not always appropriate or appropriately formatted.
Specific labels for correlation display create several problems. The primary
problem is that column labels must be short enough that they don't waste
display space. Row labels can be longer and provide more information.
User-defined labels provide the opportunity to make word-processor-ready
tables as well as correlation tables that are easy to read and work with.
{p} There are two systems available. The first ({cmd:tlabel}) was first used
in the program {cmd:fsum} (see {help fsum} if installed). {cmd:tlabel} user-defined labels
are actually variable characteristics in the form of
{cmd:char }{it:varname}{cmd:[tlabel]} {it:description}.
See help for {help char}. Characteristics (labels) are saved with the data set.
They can be entered from the keyboard with the {cmd:char} command. Since such
labels will probably be used repeatedly, they can be entered in a do file or
program and called when needed. An example of do file commands is shown directly
below:
{p 4 8}{inp:. char haq_disa[tlabel] "HAQ (0-3)"} {p_end}
{p 4 8}{inp:. char sex[tlabel] "Sex (% male)"} {p_end}
{p 4 8}{inp:. char age[tlabel] "Age (years)"} {p_end}
{p 4 8}{inp:. char ethorig[tlabel] "Ethnic origin (code)"} {p_end}
{p}As an aid, the programs {cmd:tlabel} and {cmd:tlablist} are provided.
{p}The second system uses {cmd:clabel}. In Stata 8, an option was provided
to the {cmd:list} command {help list} to make use of {cmd:char {it:varname}[varname]}
to label columns. {cmd:corrtab} makes use of this option, as well. Examples of
labels altered for the shorter {cmd:clabel} system are:
{p 4 8}{inp:. char haq_disa[varname] HAQ} {p_end}
{p 4 8}{inp:. char sex[varname] Sex} {p_end}
{p 4 8}{inp:. char age[varname] Age } {p_end}
{p 4 8}{inp:. char ethorig[varname] Ethnicity} {p_end}
{p}The dual labeling system is optional. Its main value is in the circumstance
where the same variables and labels are used repeatedly. In this instance it
saves time and improves screen and word-processor formatting and readability.
{title:Examples}
{p 4 8}{inp:. corrtab} {p_end}
{p 4 8}{inp:. corrtab price weight mpg displ} {p_end}
{p 4 8}{inp:. corrtab price weight mpg displ, sig var(2) sort} {p_end}
{p 4 8}{inp:. corrtab price weight mpg displ, sig obs var(2) above(0.5) sp cwd sort} {p_end}
{p 4 8}{inp:. corrtab price weight mpg displ, sig obs vsort(price)} {p_end}
{p 4 8}{inp:. corrtab mpg re* p* *igh*,sig bon tlabel clabel} {p_end}
{p 4 8}{inp:. corrtab price weight mpg displ,all} {p_end}
{p 4 8}{inp:. corrtab price weight mpg displ, t c} {p_end}
{p 4 8}{inp:. corrtab haq pain glb fatigue age totin,v(3) t c vsort(haq)} {p_end}
Pearson correlations
+---------------------------------------------------------+
| Variable HAQ Pain Global |
|---------------------------------------------------------|
| HAQ (0-3) 1.000 0.598 0.588 |
| Pain (0-10) 0.598 1.000 0.665 |
| Global severity (0-10) 0.588 0.665 1.000 |
| Fatigue (0-10) 0.527 0.608 0.604 |
| Total Income (US dollars) -0.337 -0.223 -0.249 |
| Age (years) 0.131 -0.036 0.024 |
+---------------------------------------------------------+
{title:Acknowledgements}
{p}{cmd:corrtab} is a Stata 8 program that is an upgrade from the Stata 5
version of {cmd:pwcorrs}.
{p}Nick Cox made very helpful suggestions.
{title:Author}
Fred Wolfe, National Data Bank for Rheumatic Diseases, Wichita, KS
fwolfe@arthritis-research.org
{title:Also see}
{p 0 19}On-line: help for {help pwcorr}, {help corr}, {help pwcorrs}
if installed, {help fsum} if installed. {help matrixof} if installed.
{p_end}