Dear Olga,

One possible way is to consider both languages as two different registers rather than separate entities. You make your test at a specific n time. Then do the same test after. Instead of looking for the gap in results you may look for the increase in results.

Dear Olga,

For multilingual children it is important to consider that the languages are not separate. There will probably be words that are hybrid of the child's total linguistic repertoire. It is important to count these as well. So I would measure not only in each language but leave room for the possibility of hybrid forms. 

good point Aria!

Hello Olga,

This is a good question - you certainly cannot treat bilingual, or multilingual children, as two, or n-language, monolinguals. They have less input in each language than their monolinguals peers, so rate of vocabulary acquisition is likely to be slower in each language, but not when all languages of a multilingual child are taken together. And as Aria notes, children can be very creative in using their languages, such as using a term in one language when they don't know it in the more appropriate language for that context.

Note though, that 'vocabulary spurt' is a slippery thing, and many researchers have argued against this common textbook concept, e.g., Paul Bloom in 'How children learn the meaning of words' (precis here: http://groups.psych.northwestern.edu/waxman/wordextension_CommPBloom.pdf). If you are going to measure 'spurt' you really have to measure rate of vocabulary learning, not total vocabulary learned.

Dear Brahim, Aria and Elspeth! Thank you so much for your comments! You raise interesting points regarding my question. Indeed the differences in L1 and L2 input play an important role in determining VS. I also agree that looking at rate of vocabulary learning is a better  approach. In case you are interested, I found a good article where researchers are comparing vocabulary acquisition in bilingual (Spanish & English) kids. They also look at VS and strategies of language acquisition. Here is the link: http://onlinelibrary.wiley.com/doi/10.1111/j.1467-1770.1994.tb00633.x/abstract

Dear Elspeth, I am aware that VS is not an absolute term, and has been critiqued before. However I think it is a good idea to use it as one of many indicators of language acquisition. Although, clearly, not an absolute one, as not all kids experience it and exhibit more steady rate of vocabulary growth.

Thanks again!

Dear Elspeth,

The problem with considering the rate of acquisition is that it is not accurate enough. If we are dealing with motion we have to apply the principles of kinematics. Acceleration of acquisition, like any physical event is defined basically as the change of velocity over time. If what you mean by rate is VT then one should be able to find out the frequency and the wave length of this VS...Then with the proper equations we will be able to find the right answer to Olga's question.

Bilingualism -

• Learning a Second Language: Children in many developed countries learn a second language in school.
• Learning a second language, and learning correct pronunciation of the language, is easier for children than for adolescents or adults.

https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=64&ved=0ahUKEwjJtJejjOHKAhWBWY4KHQjHCWs4PBAWCCwwAw&url=http%3A%2F%2Ffacweb.bhc.edu%2Feastcampus%2Fleeb%2Fpsy262%2Fppts%2Fppt10.ppt&usg=AFQjCNGjMK4yLVhkaGIwUaL_RKdw6lTsCg&cad=rja

I see that some very good points have already been made, especially with respect to the issue that a child who learns multiple languages at a young age can (and frequently does) use them all simultaneously.

But I think there's a difference between learning the other languages purely in a class setting, as opposed to learning as you use them daily (best that this be in addition to class learning, in truth). If one learns in a class setting only, I don't think that alone will lead to fluency. Fluency, by my definition, is where you vocalize your thoughts in that language without having to translate in your head.

So, my siblings and I grew up with three languages. Then we took a fourth only in class (plus Latin as a fifth, but that doesn't count). To this day, it's always a mix and match among the first two or three. You vocalize in whatever language makes the point first in your head. Plus, not to diminish the fact that some languages can state certain concepts much more succinctly than others.

As to correct pronunciation, I think that is very much a function of how early in life one is exposed to the language. If you wait until say, the teen years, to learn the second language, forget it. You're unlikely to get the intonation completely right.

Movie actors can do a pretty amazing job sometimes, in imitating accents, but it seems to only work well if we're talking about accentuated variations of their native tongue. Not so much for other languages, as far as I've been able to determine. It takes a special skill to imitate other accents of a language well. I'm always impressed by those who can.