10 October 2007

Replace No Response trials in DMDX: avoid missing data points

As experimenters, we often want the participant to respond to all trials. A non-response is a missing data point. Missing data points are not good. Therefore, non-responses are not good.

Here is a simple experiment in which participants have to respond to an item by pressing the shift keys:
<ep><fd 1><d 50><vm 800,600,600,8,60><dfs 36><dbc 210210210><cr><rcot><t 3000><id "keyboard"><s 4><g 2><cr><eop>
11111 "Beginning of the block" <ms% 1200>;
+1 "Item 1" <ms% 1200 > * ;
-2 "Item 2" <ms% 1200 > * ;
-3 "Item 3" <ms% 1200 > * ;
+4 "Item 4" <ms% 1200 > * ;
0 "THE END" l;

Long response window = no No Response trials = no missing data points
Lidija's solution to this problem was to give her participants a very long response window - for example, 30 seconds. This would be achieved by changing the value of <t N> (currently set for 3 seconds) in the header to 30000 (which is 30 seconds). This almost certainly eliminates the occurrence of No Response trials. However, how would you interpret a 10 second response? Did the participant remember what they heard/saw? Were they guessing? Was it a genuine response? It is very hard to say. You could hope for the best. But, in experiments, things usually go wrong rather than better than you had hoped for.

In order to avoid very long response times, it is often desirable to replace missing trials. This means that participants may only have a small response window, and that No Response trials are re-presented again and again until the participant makes a response.

The question then becomes: when will the No Response trial be re-presented?

Re-present a No Response trial immediately
In DMDX, it is very easy to re-present the last trial if a No Response was recorded. Simply add this code <binr N> to the end of an item before the semi-colon. Here is an example:
e.g. before +1"Item 1"<ms% 1200>*;
e.g. after +1"Item 1"<ms% 1200>*<binr 1>;

This will cause DMDX to loop back to item 1 over and over until the participant responds. You would add <binr 2> to the end of trial 2 and so forth. However, this technique has one major disadvantage: it will continuously present the No Response trial again and again in consecutive trials until the participant responds. The danger is that participants might figure out that they can see/hear the same trial again and again just by not responding. This is also not good.

Re-present a No Response trial at the end of a block of trials
A more useful option might be to re-present No Response trials at the end of a block of trials. Unfortuantely, re-presenting missed trials at the end of a block is very difficult in DMDX. However, I have figured out how to do it :) It is possible through the use of counters (a big thank you goes out to Arman for all of his advice and patience re: counters).

Think of it this way: In an experiment where participants respond by pressing either the left or right shift key, for every + trial, there are three possible outcomes:
left shift = Wrong
right shift = Correct
no response =  No Response

Note: for - trials, left shift would be Correct, right shift would be Wrong.

Create a counter for each response. This means you need (3 * no. of items per block) counters - in my case 3 * 4 = 12 counters. Counters are initialised using the <set cN> keyword. Now, we need keywords that will increment a counter depending on the participant's response.

<incic N> increments n when a Correct response is registered.
<incinr N> increments n when a No Response response is registered.
<inciw N> increments either when a Wrong response is registered, or when a No Response is registered. Note: this is very tricky. I only figured this out by looking in the DMDX help manual. Pay close attention to this sentence:

"In keeping with the precedent set by early branching code wrong responses include no responses."

So, No Response trials increment both <incinr n> and <inciw n> keywords. This is very important.

Lets run through the three possible outcomes and see how they affect the incrementing of the three counters for a trial. I have coloured the Correct counters blue, the Wrong counters red, and the No Response counters yellow.

So, if there's a Correct response in trial 1,
c11=1   c21=0   c31=0.
If the participant makes a Wrong response in trial 2,
c12=0   c22=1   c32=0.
If the participant makes a No Response in trial 3,
c13=0   c23=1   c33=1. This is because a No Response is counted by the <incinr n> and the <inciw n> keywords.

At the end of the block, we need to check if there were any No Response trials, using some fancy math:
999 <branchif 11111, (c11 + c12 + c13 + c14 + c21 + c22 + c23 + c24 - c31 - c32 - c33 - c34 .LT. 4)>;

The .LT. 4 means less than 4 - because I have 4 trials in this example, and want to check whether I have received 4 responses (either Correct or Wrong). If the result of the fancy math above is less than 4 (my number of trials), then it means that there was a No Response trial in this block, and the <branchif -11111 comes into effect, branching back to trial 11111 (at the start of the block).

Now that we have branched back to the beginning of the block, we need to add conditions to each trial to determine if it was a No Response trial on the previous attempt, in order to decide whether it should be re-presented. We insert this code before every trial:
999 <branchif 999, (c11 + c21 - c31 .GT. 0)>;

This line of code tests whether the previous response for the item below it was Correct, Wrong, or a No Response (using a scaled down, 1-item-only version of the fancy math from before). It will only re-present the trial below if the previous response was a No Response. Otherwise, it will skip to the next 999 trial, check what the previous response for the item below that line was and so on. In effect, it is re-presenting No Response trials at the end of the block, and it will continue presenting No Response trials until the participant makes a (Correct or Wrong) response. Note the counters will change for each item.

Any item lines that are not presenting stimuli may be preceded the skip display ~ indicator so they only take a fraction of a millisecond to execute.

Here is the finished code:

<ep><fd 1><d 50><vm 800,600,600,8,60><dfs 36><dbc 210210210><cr><rcot><t 3000><id "keyboard"><s 4><g 2><cr><eop>
~100 <set c11=0> <set c12=0> <set c13=0> <set c14=0> <set c21=0> <set c22=0> <set c23=0> <set c24=0> <set c31=0> <set c32=0> <set c33=0> <set c34=0>;
11111 "Beginning of the block" <ms% 1200>;
~999 <branchif 999, (c11 + c21 - c31 .GT. 0)>;
+1 "Item 1" <ms% 1200 > * <incic 11> <inciw 21> <incinr 31>;

~999  <branchif 999, (c12 + c22 - c32 .GT. 0)>;
-2 "Item 2" <ms% 1200 > * <incic 12> <inciw 22> <incinr 32>;

~999  <branchif 999, (c13 + c23 - c33 .GT. 0)>;
-3 "Item 3" <ms% 1200 > * <incic 13> <inciw 23> <incinr 33>;

~999  <branchif 999, (c14 + c24 - c34 .GT. 0)>;
+4 "Item 4" <ms% 1200 > * <incic 14> <inciw 24> <incinr 34>;

~999 <branchif 11111, (c11 + c12 + c13 + c14 + c21 + c22 + c23 + c24 - c31 - c32 - c33 - c34 .LT. 4)>;

0 "THE END" l;

Blogged with Flock

08 October 2007

Unlocker: remove your USB flash drive when you want to

Have you ever tried to delete a file or folder but Windows didn't let you? It tells you that the file or folder is being used by another program.

Have you ever tried to eject a USB flash drive but Windows didn't let you? You usually receive an error like this:

Often, it is not clear which program is accessing your files. This can be time consuming and frustrating - often requiring you to perform a restart just to get your hands on your own USB flash drive! Well, I'm happy to say that those days are over :)

Unlocker is a free, tiny 191KB, program that allows you to "unlock" files, folders, or external drives that are being used by (often hidden) processes on your computer. Kiss those unnecessary restarts good bye.

You simply right click on the file or folder and select Unlocker from the menu. This will bring up the Unlocker program window which displays all of the programs that are accessing that file or folder. Click Unlock All and you're done.

Mac OSX users: I was unable to find an OSX equivalent to the excellent (and free and tiny) Unlocker :( However, I have found this article which describes how to deal with locked files in OSX.
Blogged with Flock

02 October 2007

Organise files on your hard disk

Keeping your files organised is very important... but hard. Some people argue that in our age of desktop search we don't need to worry about folder structures. I disagree.

Many times, when I ask people for a copy of a file, they cannot find it. They realise that they aren't organised. They swear that they will go through and organise their files asap. Of course, when I ask them for another file a few months later, they cannot find it. Keeping you files organised is hard.

So, here is my guide to file organisation.

Keep your desktop clean

The desktop should only contain a few shortcuts to often used programs and the recycled bin. No files should be permanently stored on your desktop. None. Think of the desktop as your workspace. You cannot work efficiently if it is cluttered. As your workspace, the desktop should temporarily house files that you are currently working on. When you are done with the files, they should be archived - moved off the desktop to another folder for permanent storage.

To My Documents or not to My Documents, that is the question
Some people do not like the My Documents folder, usually located here: C:\Documents and settings\YourUsername\My Documents\
There are a few problems with the default path. The spaces in the pathname can be problematic for scripts. It is a silly design choice by Microsoft. If you are going to use the default path, put double quotation marks around the pathname to ensure that your scripts will work. Another problem with the default My Documents location is that (by definition) it is a well known storage area and is often targeted by malicious programs, such as viruses and trojans.

You may change the location of the My Documents folder by right-clicking on it, selecting Properties, and changing the Target Folder Location. My advice is to create a folder called Docs (C:\docs\), and to use this as your My Documents folder.

One Folder to Rule them All
Placing all important documents and files in one folder:
1. makes backing up files easy - just sync one folder and you're done
2. makes files easy to find
3. makes archiving easy

I name my top level folder according to the year, eg., 2006, 2007 etc. All documents to do with work go into this folder (not audio or video files). My 2007 folder currently contains the subfolders DMDX, ERC, Experiment, Greece Trip, MMM, NCN, PDF library, Praat scripts, Reading group, Thesis, UWS Ethics. Of course, your subfolders will differ depending on your purpose. Every day before I go home, I sync the 2007 folder on my laptop with the 2007 folder on my USB flash drive. When I get home, I sync the 2007 folder on my USB flash drive with the 2007 folder on my home PC.

A Year folder allows you to archive files that are no longer being used - for e.g., when 2008 begins, create a new, empty 2008 folder and start organising yourself again. From this point on, sync the 2008 folder every day. There will be some files from 2007 that you will need to keep working on, such as thesis.doc. Simply, use file versioning and create a new file (e.g. thesis02.doc) in your new 2008\Thesis\ folder. Leave your 2007 files where they are.

Using a Year folder has the added bonus of archiving your important documents in an intuitive, chronological order. This folder structure allows you to see which experiments were conducted, pdf files read, documents written, MMMs given, presentations prepared, ethics amendments approved, and so on, during which year.

Blogged with Flock