Running experiment at two sites - match computers and refresh rates?

[David McFarlane]

Mary Kate,

Good question, I hope others weigh in.  Here are my thoughts.

Obviously, the more uniform the better.  So one might turn the 
question around and ask how much nonuniformity is too much.  And that 
will depend largely on the timing demands of the study, some studies 
have more stringent requirements than others.

Offhand, I would say that as long as each of your computer setups can 
robustly deliver millisecond-quality times, you should be OK.  You 
can and should test each one with the RefreshClockTest that you can 
download from PST, see Chapter 3 of the original User's Guide, or 
Chapter 4 of the revised version.  Bear in mind that RefreshClockTest 
just tests whether the system can keep up with the onboard 
millisecond clock, it does not test the accuracy of the clock per se, 
for that you would need to compare times with an external time 
standard.  In any case, you at least want all your machines to pass 
RefreshClockTest.

Matching refresh rates again depends on your timing requirements.  To 
take an example, If you have one display running at 60 Hz (refresh 
period = 16.7 ms) and another at 75 Hz (refresh period = 13.3 ms), 
and you ask for a Duration of 200 ms, then the one display will 
actually give you either 183 or 200 ms, while the other screen will 
give you either 199 ms or 213 ms.  You will have to decide whether 
that is acceptable.  Note that the production release (EP2.0.10.242) 
allows you to request a refresh rate 
(http://www.pstnet.com/support/kb.asp?TopicID=3465 ).  No guarantee 
that you will get what you ask, so still look at the value measured 
by E-Prime & reported in the .edat file.  (And go with the value 
measured by E-Prime, do not trust the refresh rate reported by Windows.)

I do not have any particular refresh rate to recommend, I do not run 
studies myself (I am a Systems Designer who helps others do their 
research), and I am embarrassed to say that I have never asked others 
what refresh rates they use, I will have to ask around.  Offhand it 
seems that faster is better as you get more exact times.  OTOH, I 
often tend toward lowest common denominators for greater 
compatibility, and for that reason might stick down at 60 Hz.  In 
truth, rightly or wrongly, I think we mostly just take whatever 
refresh rate we get and don't think about it.

You already plan to use the same size monitors at all sites, and I 
presume use the same display resolution.  I trust you will also seat 
subjects at the same distance so that visual stimuli subtend the same 
visual angle for all subjects.

Note that none of this affects the accuracy of response times, 
E-Prime has a very robust mechanism for gettting responses which is 
independent of the mechansims for presenting stimuli.

Now after all that, here is the short answer (Michiel, would like to 
chime in here?):  Chances are that whatever human behavior you 
measure has more variance than your measurement system.  That's not 
an excuse for getting sloppy when you can be exact, but maybe we need 
not fret too much about this; others have done statistical 
calculations to show that we can compensate for variance introduced 
by our measuring system merely by running a few more subjects (sorry, 
I don't have a citation handy).

So what do the rest of you think?  For that matter, what do others actually do?

-----
David McFarlane
E-Prime training 
online:  http://psychology.msu.edu/Workshops_Courses/eprime.aspx
Twitter:  @EPrimeMaster (https://twitter.com/EPrimeMaster)


At 11/15/2012 12:28 PM Thursday, MKT wrote:
>We will soon be running the same experiment on E-Prime 2.0 at two 
>sites (OSU and UPenn). We want to make sure we do not add any 
>additional noise in the data or run into problems.
>
>We plan to use the same size monitors at both sites and run on 
>Windows 7. Is it necessary to match computer type as well? How about 
>refresh rates? Is there a particular refresh rate you recommend?
>
>Thanks for your help!

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