Much of my time has
been spent on providing just-in-time help of stat methods, and
SPSS.
I developed a lot of my soapboxes by seeing how
people were shooting themselves in the foot.
My use of SYSMIS is this.
There is a value for a variable such as the system was unable to
follow my instructions.
I use the occurrence of a SYSMIS value as a clue in going back to
find out what was wrong with my instructions (syntax).
(Over the years I have come to believe that it is almost always
the instructions that are to blame.).
I then go back and change the instruction so that they provide for
the special cases that caused the original SYSMIS.
Hence, in this instance
value labels varlist -999 'missing to
avoid dividing by zero' .
in public policy issues and program evaluation it is very common
to do what in auditing is called "referencing" . This is a
process where another person goes over the whole data gathering,
transformation, and analysis, to check that the statements made
are consistent with what was done. I would not want legislation
or a court case to rely on my statements unless I exercised due
diligence.
In addition to redrafting the syntax to provide for the special
cases, I also redraft syntax to include comments that explain why
there are warnings.
These go along with my soapbox about readability. Except for my
first few years of using computers, by 1974 or so, I came to value
human factors criteria, such as readability of syntax and
listings, over machine efficiency. YMMV but I find it much more
saving of my time in the long run to use syntax that explains what
is going on.
BTW even when I am the only one who is going to see the syntax,
emphasizing readability also helps me when I am inevitably
interrupted during an analysis. Even a short effort such as
replying to a post on this list may be subject to phone calls,
calls of nature, meals, etc. Foe example, I had two phone calls
from my physicians while responding to this post.
I also have a soapbox about about carefully differentiating the
different kinds of user missing values including things like
'missing to avoid dividing by zero' . I find this very useful
in the reasoning that the statistics are in support of. But that
is another story.
Art Kendall
Social Research Consultants
On 4/26/2013 10:03 AM, David Marso [via SPSSX Discussion] wrote:
Art,
What is your concept of SYSMIS and do you think it should ever
remain in a data file?
Isn't the very essence of system missing an entity that cannot be
assigned a value by the 'system'?
I would consider division by zero as a candidate for treatment as
SYSTEM MISSING rather than USER MISSING.
Interesting, you have been using SPSS 11 years longer than I have
(1983).
However, I did spend 11 years working at SPSS in the trenches (6
doing teksport,5 as a Full time Consultant)- Teksport was
existence in the belly of the beast-answering all sorts of 'exotic
questions' (Macro, INPUT PROGRAM, MATRIX etc -yeah, all the
weirdness that nobody else wanted to touch with a 10 foot pole-
).
In the early days of my taking on the 'exotic' I probably learned
more from the users calling in than they learned from me. My very
first exposure to macro was someone trying to do:
!LET !arg2=!arg1 + 1.
Well, that had me baffled. Well it is beyond obvious now, but
believe it or not, once upon a time I was a rank newbie too.
I didn't pop out of my mother with a full blown SPSS manual in my
brain ;-)
There was even a special folder in the support database called
"Marso Madness". I wanted to call it "Doing Unnatural Acts with
SPSS" but that got vetoed ;-).
I LEARNED what I know today by being exposed to all the weird
things 'weird' people try to do with SPSS for 6 years, 8 hours a
day.
I seriously doubt that support will even consider answering those
sorts of questions any more. Maybe I'll call them for the heck of
it and screw with their minds with some easy but actual 'how do I
do this' question. and see how long it takes them to refer me to
the consulting department ;-)))
Art
Kendall wrote
I admit it is a
soapbox
topic.
You surely are not a beginner and despite having used
SPSS since
1972 I still learn from your
posts.
However, people who follow the
list or search the archives
are at least
to some degree learners.
To aid debugging and for
quality assurance review I try to have
people catch
all instances of sysmis, redraft the
syntax, and
provide for that condition in the
transformations.
Then assign a missing value that can
be labelled.
Redrafting would not be finished
until all sysmis
had been changed to to user
missing with
value labels
for example in this instance I would
put
value labels varlist -999
'missing to avoid
dividing by zero' .
Art Kendall
Social Research Consultants
On 4/25/2013 1:47 PM, David Marso [via SPSSX
Discussion] wrote:
In principle I agree with the sysmis on right side
for the most part.
However in this case it is a direct correlate to the
notion that
the SYSTEM could NOT carry out some operation namely
an attempted
division by 0 (which would indeed end up as SYSMIS in
normal
calculations after spewing a host of warning messages.
Protecting
the division with DO IF will render the same result.
So, my using
SYSMIS in this case is deliberate and significant in
reflecting
the inability of the system to caary out the
computation.
--
In MATRIX such an misadventure results in a fatal
untrappable
error and SPSS takes its ball and goes home (end of
game), leading
to unhappy emails and or phone calls.
My shameless hack attempts to capitalize on the
cranked up
energizer bunny properties of MATRIX vector efficiency
while
gracefully avoiding division by 0. It is crucial to
mop up
afterwards internally to avoid ludicrous results but
it is easier
than protecting EVERY DIVISION in MATRIX with the
following
awkward monstrosity.
DO IF (zero_check_matrix).
COMPUTE newarray = oldarray / value.
ELSE.
COMPUTE
newarray=MAKE(NROW(oldarray),NCOL(oldArray),0).
LOOP #=1 TO NROW(oldarray).
LOOP ##=1 TO NCOL(oldArray).
COMPUTE newarray(#,##)=oldarray(#,##)/value.
END LOOP.
END LOOP.
END IF.
This also ends up being a PITA because one must take
great care to
treat the resulting 0 appropriately in subsequent
calculations.
In the HACK mode one can test for HUGE values which
are
distinguishable from 0's which can legitimately arise
from 0
numerators .
Yeah, there are trade offs, and I am nowhere near
solving every
weird thing that can crop up in complex situations.
But, I do stand by my using SYSMIS for this situation
;-)
--
Art
Kendall wrote
OOPS! sysmis on
right side of
assignment operator.
RECODE ALL (LO THRU 1.0 = COPY)
(ELSE -999 = ).
missing values all (-999).
Art Kendall
Social Research Consultants
On 4/25/2013 12:15 PM, Richard Ristow [via
SPSSX
Discussion]
wrote:
At 09:43 AM 4/25/2013, David Marso wrote:
>If the numbers are all positive then
you can nuke
the very
small fudge factor
>and remove the recode.
>If you have negative values then you
will need to
modify the
recode (I'm
>sure you can sort that).
>RECODE ALL (1.000001 THRU HI=SYSMIS).
I think a cleaner way to do this RECODE,
sidestepping
having to
choose a fudge factor, is
RECODE ALL (LO THRU 1.0 = COPY)
(ELSE = SYSMIS).
But actually, I think the dividing-by-zero
problem is
a strong
reason
to do the computation in a transformation
program,
rather than
MATRIX.
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