The World of Computer Software/World_Of_Computer_Software-02-385-Vol-1of3.iso/e/estat21.zip/EASIDOC.ZIP/ESCHAP11.DOC

Text File | 1992-06-30 | 17KB | 397 lines

     Chapter 11: USING EASIGRAF WITH EASISTAT

     The easiest way to use EASIGRAF, and the purpose for
     which it is specifically designed, is to provide a
     graphical representation of some of EASISTAT's commands.
     The syntax to produce a graph is the same for each
     command and is very simple. In the description that
     follows we will assume that you are familiar with the
     EASISTAT documentation.

     A graphing option is available from six of the EASISTAT
     commands: CHISQ, ANOVA, BASICS, REGRESS, KOLMOGOROV and
     COMPONENTS. Each produces a different type of graph. To
     get one of these commands to produce a graph simply enter
     G as an option after specifying the command name. (The G
     is shown on the help screen by the relevant commands.)
     When this option is specified then EASISTAT will write a
     specially formatted graph file to disk and then will run
     EASIGRAF to display the graph by using EASISTAT's SYSTEM
     command. So all you need to do to get a frequency
     distribution graph of column 15 of EASISTAT's data is to
     type:

     BASICS G C15

     and EASISTAT will do the rest.

     Here are other examples:

     REGRESS G C15 C16
     -> scatter plot of C16 against C15

     ANOVA G C15
     -> vertical scatter plots of C15 in groups

     CHISQ G
     -> bar charts of cells in contingency table

     KOLMOGOROV G C15
     -> cumulative distributions of C15 in two groups

     COMPONENT G
     -> general option for graphing multiple columns of data

     Unless otherwise instructed EASISTAT will write a graph
     file called GRAPH.ME and will ask EASIGRAF to display it.
     If the G is followed immediately (no space) with a
     filename then EASISTAT will use this instead of GRAPH.ME:

     B GFRED.GRP C15
     -> creates graph file called FRED.GRP

     CHISQ GRAPH.ME
     -> creates graph file called RAPH.ME


     11:1. Problems running
     EASIGRAF from EASISTAT

     Although running EASIGRAF from EASISTAT is usually
     straightforward, there are a number of problems which may
     occur under some circumstances.

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     11:1.1 Bad command or file name

     If you get this error message then it simply means that
     EASISTAT cannot find EASIGRAF.EXE. The file called
     EASIGRAF.EXE must either be in the current working
     directory or somewhere on the DOS search path. Note that
     the current working directory is most likely to be the
     one containing your data files and such like, and not
     necessarily the one containing EASIGRAF.EXE (at least
     this is what we suggested in the installation
     instructions). Either you must copy EASIGRAF.EXE into the
     same directory as the one you wish to work in (using the
     COPY command from DOS), or you must change to the correct
     directory before using EASIGRAF (using the CHDIR
     command). If you have a hard disk then the best solution
     by far is to put EASIGRAF.EXE into a subdirectory which
     is somewhere on the search path - this is usually set up
     with a PATH command in the AUTOEXEC.BAT file, see the
     installation instructions for details.

     If you get this error message then EASISTAT will still
     have written the correct graph file to disk and there is
     nothing to stop you running EASIGRAF using EASISTAT's
     SYSTEM command and providing the full pathname for
     EASIGRAF. Here's what we mean:

     Suppose EASIGRAF.EXE and EASISTAT.EXE are on the hard
     disk C: in subdirectory ESTAT, but that the data being
     studied is on a disk in drive A:. The best way to start
     EASISTAT would be as follows:

     A:
     C:\ESTAT\EASISTAT

     If while using EASISTAT you want to draw a graph and type

     B GBCOL5.GRP C5

     then EASISTAT will write a graph file called BCOL5.GRP to
     the disk in drive A:. It will then try to run EASIGRAF
     but since EASIGRAF isn't on drive A: the following error
     message will appear:

     Bad command or file name

     and one returns to EASISTAT without seeing a graph. But
     the graph file is still there on disk so now one can
     type:

     SYSTEM C:\ESTAT\EASIGRAF BCOL5.GRP

     and the graph will appear.

     11:1.2 Unrecognised display type

     This situation is dealt with in the general section about
     problems with EASIGRAF.

     11:1.3 Out of memory

     If you get some kind of "Out of memory" error message
     then it means that there is not room in your computer's

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     memory for EASISTAT, EASISTAT's data, EASIGRAF,
     EASIGRAF's data and the EASIGRAF's menus all at once.
     However the graph file will have still been safely saved
     to disk and can be displayed later by running EASIGRAF on
     its own after you have quit EASISTAT.

     11:1.4 Syntax errors

     It is possible that you might get an error message from
     EASIGRAF complaining about the syntax of its data,
     although we hope that this should not happen. EASISTAT
     should provide files in the correct syntax. Check you
     have not made an error using EASISTAT, then try examining
     the graph file with a word processor or text editor like
     EDIT to see if you can see what the problem is.


     11:2. The BASICS command

     When the G option is selected from the BASICS command a
     graph file is written to disk containing three columns of
     figures. In the first column is a set of values for the
     variable studied. The second column contains the number
     of items having that value and the third column is a
     cumulative total of the number of items having the stated
     value or below.

     The graph when first displayed plots the first column
     along the X axis and the second on the Y axis, producing
     a frequency distribution for the variable. If instead the
     third column is plotted against the first then a
     cumulative distibution is produced. To do this go to the
     "Data" menu (type "D"), then select the XY columns option
     (type "5") and select columns 1 and 3 (type "1,3" then
     keep pressing ENTER till you arrive back at the main
     menu). The cumulative frequency distribution will then be
     displayed. (To see the original graph again select
     columns 1 and 2 to be plotted against each other.)

     No key is needed for this data. Either graph may be
     displayed as a line graph instead of a bar chart by
     typing "L" when the main menu is showing. However you
     should note that the line will be drawn only between
     values which are actually present in the data table, so
     any values with zero frequency will not result in the
     line going down to zero. This will be clear by comparing
     the line and bar graphs on the screen.

     There are a other few points that are worth noting. This
     kind of frequency diagram can work very well for measures
     for which only a small number of values are possible, for
     example for social class (which only takes 5 values).
     However for continuous variables or ones which can take
     many values, the situation will be that each row of the
     data set will have a different value, and all the points
     on the frequency diagram will have a frequency of 1,
     since only one row has that value. If you try to produce
     a frequency distribution with this sort of data you will
     produce a fairly meaningless graph, and you may even
     think that the programs are not working correctly. The
     solution would be to break the continous data down into
     ranges and then produce a histogram. This can be done

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     either by producing another column using the DERIVE
     command, or by using the CHISQ command to produce a
     contingency table one column wide with each row
     representing an appropriate range of values.

     Another thing that can be a problem with producing a
     histogram is that the bars of the graph have a default
     width of 1. This will be inappropriate if the variable
     graphed takes large or small values. To change this bar
     width you must enter a constant value for the third
     "dimension" of the graph. This is is done by entering a
     negative number for the Z axis. For example to produce a
     histogram with bar width of 0.5 one would type "D5", then
     "1,2,-0.5" and then press ENTER twice. To get a bar width
     of 100 type "D5", then "1,2,-100" then ENTER twice.

     Sometimes the autoscaling will mean that half of the
     first or last column is not displayed. If this occurs and
     is a problem then you should rescale the graph to
     slightly change the minimum and maximum values for the X
     axis. To do this first type "A7" and you will see the old
     values for the X axis. Then you might want to backspace
     over this line and enter for example "-1,60,1" instead of
     "0,60,1". This reduces the mininum X value to negative 1
     and prevents you losing the left half of the column whose
     centre is at zero. Press ENTER to leave the Y axis
     unchanged and then again to return to the  main menu.


     11:3. The CHISQ command

     The rows and columns of the contingency table are output
     as the rows and columns of a graph file, which then
     produces a histogram of the values of the rows and
     columns in the contingency table. This is accomplished by
     EASIGRAF using the "Group by rows and columns" option to
     organise the data. The bars of the histogram are grouped
     according to the row of the table which they represent,
     and are shaded according to the column of the table.
     Appropriate titles and a key are provided. Each group of
     bars is titled according to the condition used to specify
     the row, while the key relates the shading to the
     conditions specifying the columns. Selecting the
     "Exchange" option (by typing "X") swaps round whether the
     values are grouped into rows or columns of the table.

     It is possible to create a table with just one column and
     in this way an ordinary histogram of one variable can be
     produced. This is especially useful when the variable is
     continuous and takes many different values, since in
     these circumstances the BASICS command does not produce a
     very helpful graph. Instead, the CHISQ command can be
     used to gather the values into different ranges and a
     histogram with one bar for each range can be drawn. An
     example application of this to the GHQ variable was
     provided in the section on the CHISQ command in the
     EASISTAT documentation.


     11:4. The ANOVA command



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     The ANOVA command divides the values up into groups
     according to the conditions specified and values from
     each group go into different columns of the graph file.
     (Every value goes into a row of its own so there is only
     one value per row.)  EASIGRAF then groups the data in the
     graph file by columns and displays a vertical scatter
     chart of points in each group. The mean of each group can
     be displayed as a horizontal line by selecting the
     "Regression and mean" option from the main menu (by
     typing "R") and then selecting "Toggle mean" (type "3"
     and then press ENTER).

     Although no graphing option is supplied for the TTEST
     command of EASISTAT, if it is desired to represent an
     unpaired t test graphically then instead an ANOVA command
     can be performed with just two groups (this is equivalent
     to an unpaired t test).


     11:5. The REGRESS command

     The values of the first variable go into the first column
     of the graph file and those of the second into the
     second. The option to organise the data as XY columns is
     used by EASIGRAF to plot the second value against the
     first in a scatter chart. To see the regression line
     select the  "Regression and mean" option from the main
     menu (by typing "R") and then select "Toggle Y on X"
     (type "1" and then press ENTER). Selecting "Exchange" (by
     typing "X") will swap over the two variables.


     11:6. The KOLMOGOROV command

     This graph file is the most complicated. The first column
     contains the values under consideration. The second and
     sixth columns contain the number of items having that
     value in each of the two groups. The third and seventh
     columns contain the cumulative total of items having that
     value or less, and the fourth and eighth columns contain
     the percentages of each group having a given value and
     the fifth and ninth columns contain the cumulative
     percentages. The option to organise the data as XY
     columns is used by EASIGRAF to plot the fifth and ninth
     columns against the first to produce two overlaid plots
     of the cumulative percentage totals for each group,
     expressed as a fraction of the total number of items in
     each group. (The Kolmogorov-Smirnov statistic is the
     maximum vertical distance between these two lines.) Other
     interesting plots are of the two absolute distributions
     (type "D5", then "1,2" then ENTER and "1,6" then ENTER
     twice), of the two cumulative distributions (type "D5",
     then "1,3" then ENTER and "1,7" then ENTER twice) and of
     the two percentage distributions (type "D5", then "1,4"
     then ENTER and "1,8" then ENTER twice). Other
     combinations can also be plotted.


     11:7. The COMPONENTS command

     Unlike the other commands, the graph produced has nothing
     to do with the command itself and indeed if the "G"

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     option is selected with this command then the principal
     component analysis is not even performed. Instead, this
     syntax is used simply as a convenient way to specify a
     number of columns of data to be written to a graph file.
     Any number of columns can be selected and they are simply
     written into the graph file as they are. By default the
     first three columns are assigned to the X, Y and Z axes
     of the graph and the option to organise the data as XY
     columns is used to produce a scatter diagram but it is
     anticipated that the data could be used in many different
     ways, including three-dimensional plots and multiple X-Y
     plots. Some examples of the flexibility of this command
     can be seen when GDEMO is used to drive EASISTAT (by
     running EASISTAT and then entering INPUT GDEMO).

     Sometimes it is desired to plot all the values from one
     column in the EASISTAT data table in one vertical line
     and those from another column in another line. To do this
     use the COMPONENTS command to output the data from the
     required columns into a graph file, and then change the
     data organisation to be grouped by columns, by pressing
     "D4" then ENTER. This differs from the graph produced by
     the ANOVA command where although the values from
     different groups are plotted in vertical lines, the
     values actually all come from the same column of the data
     table. Instead, one might be thinking more about the
     relationship between two columns of values, perhaps in a
     way more similar to the paired t test (although the
     pairing itself cannot be displayed).


     11:8. General points

     EASIGRAF works well with EASISTAT. EASISTAT is used to
     produce the basic data to be graphed and the titles,
     graph type and scale can then be edited and adjusted by
     EASIGRAF to produce quite a helpful graph. We recommend
     that you give each graph file a specific name (rather
     than just leaving it to be called GRAPH.ME). Then you can
     produce a number of graph files during a session using
     EASISTAT and afterwards you can use EASIGRAF to "fine-
     tune" them.

     If you have a very large data set then there is unlikely
     to be room for EASIGRAF and EASISTAT to run at once and
     you will have to just use EASISTAT to write graph files
     to disk and then use EASIGRAF to view them later. If you
     are running EASISTAT in batch mode (from a command file)
     then when the "G" option is selected a graph file will be
     written to disk but EASIGRAF will not be run
     automatically. The reason for this is that EASIGRAF runs
     mainly in interactive mode (from the keyboard) and so it
     would not run unattended. If you wish to use an EASISTAT
     command file to display a graph then you can easily do so
     by having it run EASIGRAF explicitly using the SYSTEM
     command, e.g.:

     R GTIM C15 C16
     SYSTEM EASIGRAF TIM




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