docs

[pspp] / doc / data-selection.texi

diff --git a/doc/data-selection.texi b/doc/data-selection.texi
index a8cb12d95364d95c0525ec06974cac9c7981fdfc..c2f3ab31a4ddf0fdd33797a83981acd820e35590 100644 (file)
--- a/doc/data-selection.texi
+++ b/doc/data-selection.texi
@@ -1,3 +1,12 @@
+@c PSPP - a program for statistical analysis.
+@c Copyright (C) 2017, 2020 Free Software Foundation, Inc.
+@c Permission is granted to copy, distribute and/or modify this document
+@c under the terms of the GNU Free Documentation License, Version 1.3
+@c or any later version published by the Free Software Foundation;
+@c with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
+@c A copy of the license is included in the section entitled "GNU
+@c Free Documentation License".
+@c

 @node Data Selection
 @chapter Selecting data for analysis
 
 @node Data Selection
 @chapter Selecting data for analysis
 


@@ -67,7 +76,7 @@ When @cmd{N OF CASES} is used along with @cmd{SAMPLE} or @cmd{SELECT
 IF}, the case limit is applied to the cases obtained after sampling or
 case selection, regardless of how @cmd{N OF CASES} is placed relative
 to @cmd{SAMPLE} or @cmd{SELECT IF} in the command file.  Thus, the
 IF}, the case limit is applied to the cases obtained after sampling or
 case selection, regardless of how @cmd{N OF CASES} is placed relative
 to @cmd{SAMPLE} or @cmd{SELECT IF} in the command file.  Thus, the

-commands @code{N OF CASES 100} and @code{SAMPLE .5} will both randomly
+commands @code{N OF CASES 100} and @code{SAMPLE .5} both randomly

 sample approximately half of the active dataset's cases, then select the
 first 100 of those sampled, regardless of their order in the command
 file.
 sample approximately half of the active dataset's cases, then select the
 first 100 of those sampled, regardless of their order in the command
 file.


@@ -92,7 +101,7 @@ transformation, permanently removing cases from the active dataset.
 The proportion to sample can be expressed as a single number between 0
 and 1.  If @var{k} is the number specified, and @var{N} is the number
 of currently-selected cases in the active dataset, then after
 The proportion to sample can be expressed as a single number between 0
 and 1.  If @var{k} is the number specified, and @var{N} is the number
 of currently-selected cases in the active dataset, then after

-@subcmd{SAMPLE @var{k}.}, approximately @var{k}*@var{N} cases will be
+@subcmd{SAMPLE @var{k}.}, approximately @var{k}*@var{N} cases are

 selected.
 
 The proportion to sample can also be specified in the style @subcmd{SAMPLE
 selected.
 
 The proportion to sample can also be specified in the style @subcmd{SAMPLE


@@ -101,15 +110,15 @@ The proportion to sample can also be specified in the style @subcmd{SAMPLE
 @enumerate
 @item
 If @var{N} is equal to the number of currently-selected cases in the
 @enumerate
 @item
 If @var{N} is equal to the number of currently-selected cases in the

-active dataset, exactly @var{m} cases will be selected.
+active dataset, exactly @var{m} cases are selected.

 
 @item
 If @var{N} is greater than the number of currently-selected cases in the
 
 @item
 If @var{N} is greater than the number of currently-selected cases in the

-active dataset, an equivalent proportion of cases will be selected.
+active dataset, an equivalent proportion of cases are selected.

 
 @item
 If @var{N} is less than the number of currently-selected cases in the
 
 @item
 If @var{N} is less than the number of currently-selected cases in the

-active, exactly @var{m} cases will be selected @emph{from the first
+active, exactly @var{m} cases are selected @emph{from the first

 @var{N} cases in the active dataset.}
 @end enumerate
 
 @var{N} cases in the active dataset.}
 @end enumerate
 


@@ -140,18 +149,45 @@ from the active dataset, unless @cmd{TEMPORARY} is in effect
 (@pxref{TEMPORARY}).
 
 Specify a boolean expression (@pxref{Expressions}).  If the value of the
 (@pxref{TEMPORARY}).
 
 Specify a boolean expression (@pxref{Expressions}).  If the value of the

-expression is true for a particular case, the case will be analyzed.  If
-the expression has a false or missing value, then the case will be
+expression is true for a particular case, the case is analyzed.  If
+the expression has a false or missing value, then the case is

 deleted from the data stream.
 
 Place @cmd{SELECT IF} as early in the command file as
 possible.  Cases that are deleted early can be processed more
 efficiently in time and space.
 deleted from the data stream.
 
 Place @cmd{SELECT IF} as early in the command file as
 possible.  Cases that are deleted early can be processed more
 efficiently in time and space.

+Once cases have been deleted from the active dataset using @cmd{SELECT IF} they
+cannot be re-instated.
+If you want to be able to re-instate cases, then use @cmd{FILTER} (@pxref{FILTER})
+instead.

 
 When @cmd{SELECT IF} is specified following @cmd{TEMPORARY}
 (@pxref{TEMPORARY}), the @cmd{LAG} function may not be used
 (@pxref{LAG}).
 
 
 When @cmd{SELECT IF} is specified following @cmd{TEMPORARY}
 (@pxref{TEMPORARY}), the @cmd{LAG} function may not be used
 (@pxref{LAG}).
 

+@subsection Example Select-If
+
+A shop steward is interested in the salaries of younger personnel in a firm.
+The file @file{personnel.sav} provides the salaries of all the workers and their
+dates of birth.  The syntax in @ref{select-if:ex} shows how @cmd{SELECT IF} can
+be used to limit analysis only to those persons born after December 31, 1999.
+
+@float Example, select-if:ex
+@psppsyntax {select-if.sps}
+@caption {Using @cmd{SELECT IF} to select persons born on or after a certain date.}
+@end float
+
+From @ref{select-if:res} one can see that there are 56 persons listed in the dataset,
+and 17 of them were born after December 31, 1999.
+
+@float Result, select-if:res
+@psppoutput {select-if}
+@caption {Salary descriptives before and after the @cmd{SELECT IF} transformation.}
+@end float
+
+Note that the @file{personnel.sav} file from which the data were read is unaffected.
+The transformation affects only the active file.
+

 @node SPLIT FILE
 @section SPLIT FILE
 @vindex SPLIT FILE
 @node SPLIT FILE
 @section SPLIT FILE
 @vindex SPLIT FILE


@@ -175,7 +211,7 @@ When a list of variable names is specified, one of the keywords
 @subcmd{LAYERED} or @subcmd{SEPARATE} may also be specified.  If provided, either
 keyword are ignored.
 
 @subcmd{LAYERED} or @subcmd{SEPARATE} may also be specified.  If provided, either
 keyword are ignored.
 

-Groups are formed only by @emph{adjacent} cases.  To create a split 
+Groups are formed only by @emph{adjacent} cases.  To create a split

 using a variable where like values are not adjacent in the working file,
 you should first sort the data by that variable (@pxref{SORT CASES}).
 
 using a variable where like values are not adjacent in the working file,
 you should first sort the data by that variable (@pxref{SORT CASES}).
 


@@ -185,6 +221,51 @@ entire active dataset as a single group of data.
 When @cmd{SPLIT FILE} is specified after @cmd{TEMPORARY}, it affects only
 the next procedure (@pxref{TEMPORARY}).
 
 When @cmd{SPLIT FILE} is specified after @cmd{TEMPORARY}, it affects only
 the next procedure (@pxref{TEMPORARY}).
 

+@subsection Example Split
+
+The file @file{horticulture.sav} contains data describing the @exvar{yield}
+of a number of horticultural specimens which have been subjected to
+various @exvar{treatment}s.   If we wanted to investigate linear statistics
+of the @exvar{yeild}, one way to do this is using the @cmd{DESCRIPTIVES} (@pxref{DESCRIPTIVES}).
+However, it is reasonable to expect the mean to be different depending
+on the @exvar{treatment}.   So we might want to perform three separate
+procedures --- one for each treatment.
+@footnote{There are other, possibly better, ways to achieve a similar result
+using the @cmd{MEANS} or @cmd{EXAMINE} commands.}
+@ref{split:ex} shows how this can be done automatically using
+the @cmd{SPLIT FILE} command.
+
+@float Example, split:ex
+@psppsyntax {split.sps}
+@caption {Running @cmd{DESCRIPTIVES} on each value of @exvar{treatment}}
+@end float
+
+In @ref{split:res} you can see that the table of descriptive statistics
+appears 3 times --- once for each value of @exvar{treatment}.
+In this example @samp{N}, the number of observations are identical in
+all splits.  This is because that experiment was deliberately designed
+that way.  However in general one can expect a different @samp{N} for each
+split.
+
+@float Example, split:res
+@psppoutput {split}
+@caption {The results of running @cmd{DESCRIPTIVES} with an active split}
+@end float
+
+Unless @cmd{TEMPORARY} was used, after a split has been defined for
+a dataset it remains active until explicitly disabled.
+In the graphical user interface, the active split variable (if any) is
+displayed in the status bar (@pxref{split-status-bar:scr}.
+If a dataset is saved to a system file (@pxref{SAVE}) whilst a split
+is active, the split stastus is stored in the file and will be
+automatically loaded when that file is loaded.
+
+@float Screenshot, split-status-bar:scr
+@psppimage {split-status-bar}
+@caption {The status bar indicating that the data set is split using the @exvar{treatment} variable}
+@end float
+
+

 @node TEMPORARY
 @section TEMPORARY
 @vindex TEMPORARY
 @node TEMPORARY
 @section TEMPORARY
 @vindex TEMPORARY


@@ -194,9 +275,9 @@ TEMPORARY.
 @end display
 
 @cmd{TEMPORARY} is used to make the effects of transformations
 @end display
 
 @cmd{TEMPORARY} is used to make the effects of transformations

-following its execution temporary.  These transformations will
+following its execution temporary.  These transformations

 affect only the execution of the next procedure or procedure-like
 affect only the execution of the next procedure or procedure-like

-command.  Their effects will not be saved to the active dataset.
+command.  Their effects are not be saved to the active dataset.

 
 The only specification on @cmd{TEMPORARY} is the command name.
 
 
 The only specification on @cmd{TEMPORARY} is the command name.
 


@@ -206,31 +287,30 @@ procedure-like commands.
 
 Scratch variables cannot be used following @cmd{TEMPORARY}.
 
 
 Scratch variables cannot be used following @cmd{TEMPORARY}.
 

-An example may help to clarify:
+@subsection Example Temporary

 
 

-@example
-DATA LIST /X 1-2.
-BEGIN DATA.
- 2
- 4
-10
-15
-20
-24
-END DATA.
+In @ref{temporary:ex} there are two @cmd{COMPUTE} transformation.  One
+of them immediatly follows a @cmd{TEMPORARY} command, and therefore has
+effect only for the next procedure, which in this case is the first
+@cmd{DESCRIPTIVES} command.

 
 

-COMPUTE X=X/2.
+@float Example, temporary:ex
+@psppsyntax {temporary.sps}
+@caption {Running a @cmd{COMPUTE} transformation after @cmd{TEMPORARY}}
+@end float

 
 

-TEMPORARY.
-COMPUTE X=X+3.
+The data read by the first @cmd{DESCRIPTIVES} procedure are 4, 5, 8,
+10.5, 13, 15.  The data read by the second @cmd{DESCRIPTIVES} procedure are 1, 2,
+5, 7.5, 10, 12.   This is because the second @cmd{COMPUTE} transformation
+has no effect on the second @cmd{DESCRIPTIVES} procedure.   You can check these
+figures in @ref{temporary:res}.

 
 

-DESCRIPTIVES X.
-DESCRIPTIVES X.
-@end example
+@float Result, temporary:res
+@psppoutput {temporary}
+@caption {The results of running two consecutive @cmd{DESCRIPTIVES} commands after
+         a temporary transformation}
+@end float

 
 

-The data read by the first @cmd{DESCRIPTIVES} are 4, 5, 8,
-10.5, 13, 15.  The data read by the first @cmd{DESCRIPTIVES} are 1, 2,
-5, 7.5, 10, 12.

 
 @node WEIGHT
 @section WEIGHT
 
 @node WEIGHT
 @section WEIGHT


@@ -251,10 +331,10 @@ procedures.  Use of keyword @subcmd{BY} is optional but recommended.  Weighting
 variables must be numeric.  Scratch variables may not be used for
 weighting (@pxref{Scratch Variables}).
 
 variables must be numeric.  Scratch variables may not be used for
 weighting (@pxref{Scratch Variables}).
 

-When @subcmd{OFF} is specified, subsequent statistical procedures will weight all
+When @subcmd{OFF} is specified, subsequent statistical procedures weight all

 cases equally.
 
 cases equally.
 

-A positive integer weighting factor @var{w} on a case will yield the
+A positive integer weighting factor @var{w} on a case yields the

 same statistical output as would replicating the case @var{w} times.
 A weighting factor of 0 is treated for statistical purposes as if the
 case did not exist in the input.  Weighting values need not be
 same statistical output as would replicating the case @var{w} times.
 A weighting factor of 0 is treated for statistical purposes as if the
 case did not exist in the input.  Weighting values need not be


@@ -267,3 +347,27 @@ the next procedure (@pxref{TEMPORARY}).
 
 @cmd{WEIGHT} does not cause cases in the active dataset to be
 replicated in memory.
 
 @cmd{WEIGHT} does not cause cases in the active dataset to be
 replicated in memory.

+
+
+@subsection Example Weights
+
+One could define a  dataset containing an inventory of stock items.
+It would be reasonable to use a string variable for a description of the
+item, and a numeric variable for the number in stock, like in @ref{weight:ex}.
+
+@float Example, weight:ex
+@psppsyntax {weight.sps}
+@caption {Setting the weight on the variable @exvar{quantity}}
+@end float
+
+One analysis which most surely would be of interest is
+the relative amounts or each item in stock.
+However without setting a weight variable, @cmd{FREQUENCIES}
+(@pxref{FREQUENCIES}) does not tell us what we want to know, since
+there is only one case for each stock item. @ref{weight:res} shows the
+difference between the weighted and unweighted frequency tables.
+
+@float Example, weight:res
+@psppoutput {weight}
+@caption {Weighted and unweighted frequency tables of @exvar{items}}
+@end float