Item numbers and Item Number Parsing for DMDX output files
Rationale for Meaningful Item
Numbers
The
primary reason for coding your within subject information into your item
numbers in your DMDX script is that it is much easier to do this work on the
front end (once in the item file) rather than having to reconstruct this
information by hand or other method for every subject at the end. In particular, this will be critical if you
use scrambling, but in all instances, it will save you time
Instructions for Creating Meaningful
Items Numbers in DMDX Script
Your
coding scheme should be clear and systematic so that you can easily identify
each of your trials and sort them for analysis.
Here are
some steps and rules for designing a code for your variables:
1. You
can use as many digits as you need to include information for all of your
variables. If you are running a psychophysioloigcal
experiment, you might want your item numbers match you event markers (the codes
that are sent to the amplifier to mark trial onset and identity). However, if
you are using Neuroscan or EGI amplifiers (and
probably others), your codes are limited to 3-digits, with a maximum value of
255 (i.e., 8-bit). Therefore, in some
instances you may code more information into your DMDX item numbers for
subsequent behavioral analysis than you can code into your event markers.
2. Each
place (10s,100s,1000s place, etc) should designate a
separate within subjects variable.
3. Each
TYPE of item should have the same item code, so that you can sort and average
all of the items that fall into a particular category of interest.
4.
Whatever coding scheme you develop, be sure to keep a reference handy! A good
idea would be to include it in comments at the top of your DMDX script as well.
An
example of a DMDX script with meaningful item
numbers is provided. If you care to run
the script you will need the happy, fearful and neutral
face bmps. The
example is for a 1-back working memory task involving emotional faces. Four factors about each trial are coded in
the four digit item numbers (item format ABCD)
A= trial
type (1=match, 2=non-match)
B =
emotion (1 = neutral, 2= happy, 3= fear)
C =
condition (1 = face, 2 = word)
D =
experiment block (1-4)
Instructions for Parsing Items in
DMDX Output
Once you
have designed your coding scheme, you can then use simple mathematical
operations (mod and trunc) in Excel or SPSS to parse
an item number in the output file into separate within subject variables.
Trunc returns the integer portion of a real
number. For example: Trunc (1.2) = 1; Trunc
(2.7) = 2
We can
also put expressions inside the ( ). For
example: Trunc(112 / 10) = 11; Trunc(191 / 100)
= 1
Mod
returns the integer remainder of a division operation: For example:
Mod(5,2) = 1;
Mod (11,4) = 3
The
basic strategy to extract a target position in a numeric string (e.g., 23543)
involves 2 steps:
1.
Isolate your target position by moving it to the ones column. Divide your
string of numbers by a factor of 10 (10, 100, 1000, etc), chosen so that the
"1" in the factor of 10 is in the same position as the position of
your target number. Take the "TRUNC" of this number.
For example,
if you have the number 23543, and you want to isolate the "4," you
would divide by 10, because the 4 is in the 10's place.
E.g., Trunc (23543/10) = 2354
2. Once
your target number is in the ones place, you just need to divide your number by
10, and take the "MOD" (or remainder) of this division. This will isolate the number "4."
E.g.,
Mod (2354/10) = 4
If,
however, you wish to isolate a 2-digit code (say 54, instead of 4), then you
would divide your number by 100 instead of 10.
E.g.,
Mod (2354/100) = 54
So the
rule is that if you want a 1-digit number, divide by 10; if you want a 2-digit
number, divide by 100; if you want a 3-digit number, divide by 1000, etc.
You can
also combine these two steps into one formula
Mod(Trunc(23543/10),10) = 4
However,
if you want to extract the first number in your item code (or the first two
numbers), you only need to use the "TRUNC" function:
E.g. Trunc (23543/1000) = 23, or Trunc
(23543/10000) = 2
A Quiz (or more examples)
What is
the formula to extract the 7 from 2455471
Mod(trunc(2455471 / 10),10)
What
about the 2?
Mod(trunc(2455471 / 1000000),10)
What
about the 1?
Mod(trunc(2455471 / 1),10) or
Mod(2455471,10)
What
about the 55?
Mod(trunc(2455471 / 1000),100)
Templates
An example
of a DMDX AZK output file from one
participant run through the above emotional face 1-back task is available. In addition, we provide an example of an excel spreadsheet with these data and columns
containing formulas to extract each within subject variable.
Finally,
here are the same examples if you process your data in SPSS (Original Data in SPSS; SPSS syntax file for parsing items)
Created
by Jessica Shackman
Last
updated
