SET
statements:
set z1 = (x1 - mean(x1))/stddev(x1) set z2 = (x2 - mean(x2))/stddev(x2)
but this requires a lot of unnecessary typing. Instead, you could
DEFINE a function stand to perform this operation:
define stand(x) = (x - mean(x))/stddev(x)
The argument x to stand is a parameter that could be replaced
with any variable name. The SET statements above can now be
simplified to:
set z1 = stand(x1) set z2 = stand(x2)
SST functions can have many arguments. Each argument to a function should be separated by a comma. For example:
define f(x,y,z) = (x - y)/z
To use the above function in a SET statement, you might try:
set delprice = f(price2,price1,0.5)
The above example shows that constants (like 0.5) can be used as arguments to functions.
SST allows you to nest functions. That is, a function can be inserted in place of an argument to another function. For example:
define yhat(b1,b2,b3) = x1*b1 + x2*b2 + x3*b3 define error(y1,y2) = y1-y2 set e = error(depvar,yhat(1.0,0.0,-1.0))
Thus, it is possible to build up rather complicated expressions. Any SST
predefined function (e.g., cumnorm or mean) can also be used
as an argument in a user-defined function.
To obtain a listing of functions defined in an SST sesssion, use the
LIST command:
list func
and SST will display the names and definitions of all functions that
have been defined during the session. To inspect the definition of
a one or more functions, add these as arguments to the FUNC
subop:
list func[stand]
and SST reproduces the definition for stand.
pop15,
pop75, and dpi repeatedly in some subops. Define a variable
list vlist using the MACRO command:
macro vlist { pop15 pop75 dpi 2
Whenever you want to use this list of variables in a command, you just ask SST to expand the macro. For example:
reg dep[sr] ind[one @vlist]
is equivalent to:
reg dep[sr] ind[one pop15 pop75 dpi]
The @ symbol tells SST to expand the macro, i.e. to substitute the text of the macro in the command. Whenever you ask SST to expand a macro in a command, SST will display the expanded version of the command on the screen to remind you what the macro does.
More complicated macros may involve parameter lists and multiple
commands. For example, it is possible to define a macro to perform
two stage least squares. Suppose, for example, that we are interested
in estimating an equations with one endogenous and two exogenous
variables among the independent variables and that we have two
additional exogenous variables to be used as instruments. In the
first stage of two stage least squares, we regress the included
endogenous variable on all the exogenous variables and save the
predicted values. In the second stage we regress the dependent variable
on the predicted values from the first stage regression and the
included exogenous variables. To define a macro 2sls which
performs this operation in one shot, type:
macro 2sls(y1,y2,x1,x2,z1,z2) {
reg dep[y2] ind[x1 x2 z1 z2] pred[y2hat]
reg dep[y1] ind[y2hat x1 x2] coef[b2sls]
set e2sls = y1 - b2sls(1)*y2 - b2sls(2)*x1 - b2sls(3)*x2
calc stdev(e2sls)
}
The last two commands are necessary to calculate the standard error of the
regression for two stage least squares (the estimated standard errors from
the second stage regression need to be multiplied by the ratio of the last
value calculated and the reported standard error of the regression from the
second stage printout). To use the 2sls macro when price and
quantity are the endogenous variables, one and weather
are the included exogenous variables, and populat and dpi are
the excluded exogenous variables:
@2sls(quantity,price,one,weather,populat,dpi)
and the entire sequence of commands will be executed.
IF and WHILE commands that allow you
to write simple programs within SST.
The IF command is straightforward. You type `if' followed by a
logical expression (enclosed in parentheses) followed by a set of SST
commands enclosed in braces. For example, you might want to rerun a
regression with a variable deleted if its coefficient in the initial
regression was less than some predetermined value:
reg dep[y] ind[x1 x2 x3] coef[b1]
if(abs(b1(3) < 1.0) { reg dep[y] ind[x1 x2] 2
The second REG command (with x1 and x2 as independent
variables) will only be executed if the coefficient of x3 in the
first regression (b1[3]) is less than one in absolute value.
The use of WHILE loops is slightly more complicated. You type
`while' followed by a logical expression (again, enclosed in
parentheses) followed by a set of SST commands enclosed in braces. SST
evaluates the logical expression: if it is true, it performs specified set
of commands; otherwise it proceeds to subsequent commands. After the first
pass through the set of commands, it reevaluates the logical expression.
Again, if it is true it performs the commands. If the expression is now
false, it breaks. The major pitfall to avoid is creating an endless loop
that SST performs until you shut your computer off. The natural way to
control the loop is by resetting a variable that appears in the logical
condition within the WHILE loop.
An example may clarify the use of the WHILE loop. Suppose
you would like to estimate a regression ten times, each time adding
an additional observation to the sample used to compute the regression.
This can be done with a WHILE loop:
set counter=1; obs[1]
while(counter(1) <= 10) {
reg dep[y] ind[x1 x2] if[obsno < 20+counter(1)]
set counter=counter+1
}
The observation range for the regression is determined by the IF
subop. Initially, observations whose obsno index is less than 21 will
be used. Then the variable counter is incremented to two. The next
regression will use observations for which obsno is less than 22, and
so on. Eventually, the variable counter will exceed ten, and SST
will break out of the WHILE loop.
SCAT.
Currently, the supported procedure consists of timeplots, two-way
scatter, and matrix scatterplots (for three variables).
If you give the command:
scat var[y]
SST will plot the variable y (on the vertical axis) with its
observation number on the horizontal axis.
To obtain a scatterplot of a variable y against another variable
x, type:
scat var[y x]
The variable x will appear on the horizontal axis and the
variable y on the vertical axis.
To analyze interactions between three variables, use the SCAT
command to produce a matrix of two-way scatterplots:
scat var[x y z]
This will produce three scatterplots: y against x, z
against x, and y against z. To fit three scatterplots
onto the screen, it is necessary to reduce the size of each scatterplot,
but the simultaneous examination of the scatterplots may aid in the
detection of interesting interactions.
SST uses the variable names to label the axes and automatically
scales the axes to enhance readability. If you would like to add
a title to your scatterplot, use the LAB subop. For example:
scat var[gnp year] lab[U.S. Gross National Product, 1961-1984] \\ if[year >= 1960]
As usual, the IF and OBS subops may be used to restrict
the data range use for the SCAT command, as illustrated above.
We do not provide any special software for the printing of scatterplots. If you have the IBM monochrome card, depressing <Shift> and <PtrSc> simultaneously will dump the contents of the screen to your printer. Most graphics cards, such as the IBM Color Graphics Adapter or the Hercules Graphics Adapter, are equipped with screen dump programs. Consult the documentation for your graphics adapter to determine how to obtain hard copy of your graphics output.
