Friday, November 6, 2009
DPPI 2009
Beside some computer problems, one of the thing keeping me busy during that time was DPPI 2009. It turned out to be an enjoyable conference, with great food, and surprisingly well organized given the way these things are usually run in France. I have also met some nice people and had some interesting discussion but unfortunately did not find any presentation that would be directly relevant to my work.
One fascinating thing with design-oriented conferences is that organizers always try to do something a little bit « different », for example with the traditional conference bag or the badge given to each participant. DPPI added an interesting twist to the conference dinner: it was an assigned seated dinner and the seating plan was to be created automatically based on social contacts during the conference. Everyone had to get a ring with a RFID chip and the time spent together during the coffee breaks would be used to form groups. The result was displayed all the time on a big LCD screen in the hallway. A couple of conference attendees in fact used this screen for a clever hack, writing their name on tape and pasting it directly on a table.
At the end of the day, it did not work out very well, though. First, many people did not get a ring or even refuse to use the system, which was to be expected but is still a bit disappointing. Then, many technical constraints made the system a lot less cool than it might have been. In fact, the system would only notice a ring if it was placed in the middle of one of three tables in the hallway. This, combined with an apparently straightforward matching algorithm (the more you spent time with someone, the more likely you were to be seated together), made sure that no serendipitous discovery happened. It felt much more like telling the system whom you want to seat with rather than providing occasions to meet new people or reflect on social encounters during the conference.
But the last blow was dealt by the organizers themselves: instead of using numbers or little name signs to reflect the plan produced by the computer, they decided to print it on a poster, remove a seat from each table and let the tables unmarked. The idea was ostensibly to force people to mingle instead of sticking to the seating plan. The results was highly predictable: people sat together with acquaintances, ignoring the seating plan altogether. It was not all that important however and the diner turned out to be very enjoyable!
Sunday, September 20, 2009
Computing a confidence interval for ρ
Curiously, neither R nor SPSS seem to offer a simple way to compute a confidence interval for Pearson's correlation coefficient based on r and the sample size. R base package includes the cor.test function which does provide a confidence interval based on Fisher z transformation but it takes the full data set as input. Even then, the confidence interval depends only on the sample correlation and on the sample size so the extra information is not really needed, except to compute the sample correlation coefficient in the first place. The confidence interval can therefore just as well be computed from published correlation coefficients, without going back to the original data set.
The formula is relatively simple and can be found in any statistics textbook but tracking it down and computing it by hand every time can be somewhat cumbersome. Here is a short R function to do it easily.
r.cint <- function(r,n,level=.95) { z <- 0.5*log((1+r)/(1-r)) zse <- 1/sqrt(n-3) zmin <- z - zse * qnorm((1-level)/2,lower.tail=FALSE) zmax <- z + zse * qnorm((1-level)/2,lower.tail=FALSE) return(c((exp(2*zmin)-1)/(exp(2*zmin)+1),(exp(2*zmax)-1)/(exp(2*zmax)+1))) }
The result can also be used as an hypothesis test, by checking if the confidence interval includes 0 or any other constant. The conclusion is very similar but not identical to the tests reported by SPSS CORRELATIONS procedure or R cor.test, because these p values are based on another test statistic (and on the t distribution).
What's the point? As is plain to see from the formulas, the standard error of the z-transformed correlation depends only on the sample size (that's the point of the transformation), which means that you don't need any other information than the correlation coefficient and the sample size to perform a test.
Correlation are often reported without any discussion of sampling variability but with a very small sample size, the point estimate is going to be very imprecise and even an impressive r can hide a modest correlation. Similarly, a moderate observed correlation could reflect anything from a small correlation in the other direction to a strong correlation in the same direction. If nothing else, the confidence interval makes this imprecision visible and helps to interpret results based on experiments with a very small number of participants.
Bootstrapping techniques can also be used to construct a confidence interval for a correlation coefficient but they require access to the original data set and cannot be computed based only on typical research reports.
PS: This post was updated in 2013 to fix a layout problem and add some clarifications
Thursday, September 17, 2009
Salmon and voodoo
Anyway, I just discovered (via the neuroskeptic), that Craig Bennett (of Prefrontal.org) recently presented a poster illustrating how improper statistical analysis can lead to spurious detection of BOLD changes in a dead salmon. This really drives the point home cunningly.
Tuesday, September 15, 2009
Emotion slider
I presented a first article about it at the Design Research Society conference in Sheffield last year and I never uploaded it to my website, but I recently noticed that it is now available online. The paper is titled Designing a research tool and describes the development of the device from a design angle.
Since then, I have also written a more classical experimental psychology-type of paper, which I finally presented at ACII last week. Apparently it's not online yet but you can always contact me for more info. In the meantime, you can also download the slides from my presentation.
In a few words the conclusion of all this is that there is evidence of compatibility effects between movements on the slider and affective state (for example how good or bad you feel or how you evaluate a picture). Participants in the experiment were quicker to push to evaluate positive pictures than negative pictures and also quicker to pull to evaluate negative pictures than positive pictures. This difference in response time shows the one set of movements is more intuitive or easier than the other one.
However, unlike what some earlier reports suggested, this effect is very sensitive to the context (how the slider is positioned, what the instructions are, if there is some form of feedback, etc.) With "neutral" instructions (in my case I asked the participants to « push » and « pull » without any other precision) and a slider positioned between the screen and the user, the more natural mapping seems to be pushing for « positive » and pulling for « negative ».
Using the « wrong » movements also seems to have a small but noticeable effect on the number of errors people make but the evidence is not very strong.
Sunday, September 13, 2009
Back from ACII 2009
As far as I am concerned, the last session (« Guidelines for Affective Signal Processing: From Lab to Life ») was the most interesting but many other papers are well worth checking out. A few things that caught my attention are Rana el Kaliouby's emotion recognition system for children with autism spectrum disorders, Dimitry Tseterukou's affective haptics, Elisabeth Eichhorn's Recording Inner Life prototype, Jennifer Robison's paper on the consequences of affective feedback (she got a well-deserved best paper award).
A few of my colleagues from Delft also presented their work: Valentijn Visch had a paper on attribution of emotion by observers based on basic movement parameters and Miguel Bruns Alonso presented the last prototype that came out of his work on tangible interaction and stress reduction.
I don't know if the proceedings are online yet but in the mean time you can check out the conference website and contact the authors directly, most of them are really happy to send out copies of their article when asked.