Three-site attachment experiment series: The pretest

To verify the suitability of the experiment items, a pretest was done. First, we threw out items where, according to our intuition, it was plausible that the first and the third NP would belong together. This can cause a bias due to the structure of parse trees of such sentences, so it had to be avoided.

The main pretest was an offline plausibility rating experiment: 23 people filled in a questionnaire on the web, giving the plausibility for each of the three attachments for each potential experiment item. The goal was to remove items that showed a clear bias towards one or two of the attachment possibilities.

Given the sentence De acteur in de film over de stuntman die populair was (the actor in the movie over the stuntman that was popular), the three cases to rank would be:

The ranking was done on a scale from one (bad, not plausible) to five (good, plausible) for each of the cases. The instructions asked to tell how good each of the "words" (we avoided the technical term NP) would fit the relative clause, and the subjects were encouraged to give the same score for more than one case if appropriate. The complete questionnaire can be found in the appendix and on the web. When gaps were found in the questionnaire, all three cases of the concerning sentences were removed from the data for that questionnaire.

After collecting enough results, a statistical measure was computed from the logged data of the questionnaires. For this, the data was first normalized: The mean for each subject was subtracted from all values, so that the new mean would be zero for each subject. Then, the values were scaled to give a standard deviation of one. In this way, individual differences in the general treating of the questionnaire were removed. If, for example, one subject considers ranking three as normal and one and five as bad and good, while another one thinks most cases are good for a ranking of four, and never worse than three, the further evaluation would have been biased without the normalization.

After the normalization, the mean and standard deviation were calculated for each case of each sentence, using the data of all questionnaires together. This data was used for additional checking of the results by hand: One sentence was removed by hand: Though all three attachment possibilities were similar in plausibility there, the overall plausibility of the cases (De puree/De schaal/De taart waar over gemorst is) was quite bad (average of -1.15).

The main check, however, was done automatically: For each sentence, an F-value telling how strongly the three cases differed was computed from the data. Then, the sentences were sorted using that value, giving a list with F-values from 0.02 to 6.17. All sentences with a value of more than 2.4 were removed, as well as the generally implausible sentence mentioned above. Our selection of p=0.1 was done to reject even items with a low possibility of bias towards one or two of the possible attachments. Finally we had 48 experiment items remaining for the use in the two production experiments:

*** end with ...experiments. and throw away the rest if you need! ***

One experiment task was to pronounce the sentences in a way making clear, for example, that it was the film that was popular. The cases were distributed over the subjects, so that each case of each sentence was spoken by the same number of subjects, and each subject did exactly one case of each sentence.

The other experiment task did not use the relative clauses, the subjects should only stress that, for example, the film was the most important part of the fragment the actor in the movie over the stuntman. As we shall see, this task of emphasizing an element in a list was a easier for the subjects and it was easier to perceive the intended case, compared to the experiment with the relative clauses.

Trying to perceive the intended attachment or stress and the search for a pattern were the last two "experiments" we did: For the perception experiment yet another questionnaire was used, using us as our own subjects (this was possible because we did not memorize the instruction lists for the subjects, but of course we did know what the experiment was about and that, for example, it was never the task to stress two of the NPs). It was indeed possible to correctly perceive the attachment by listening to the recorded speech of the subjects, but it was considerably harder for the second NP, especially in the experiment with the relative clauses (clearly lower mean and higher standard deviation for the condition "second NP intended, second NP perceived").