The perennial concern about low quality questions and low quality answers is resurfacing again. It is my impression (not backed by data) that many VLQ answers are posted by rep 1 unregistered people. This seems to be true also of some low quality questions.

Is there data or could data be obtained to back up or refute this impression?

Could ELU run an experiment (for say, 3 months) requiring new users to register before posting?

This seems like an effort which might: (a) reduce the VLQ posts; (b) be unlikely to scare off people we want to attract; and (c) not require a lot of work to implement and analyze. 

**Addendum in response to comments so far**: Is it likely that ELU will be **permitted** by SE to require registration, either as a test or permanently? If not, we should move on.
['https://english.meta.stackexchange.com/questions/8191', 'https://english.meta.stackexchange.com', 'https://english.meta.stackexchange.com/users/121061/']
To answer your question of why it hasn't been done yet: It had not been requested before.

The number of downvotes here indicates that the community does not support this plan. Therefore, it is safe to conclude that it won't be implemented.
I can understand why you've proposed "taking mandatory English quiz". Yes, it is frustrating to see all those low-quality posts and find that we are not doing enough to prevent or slow down the influx of low-quality posts. My question is "Is there anything we can do?" 

One user once proposed all questions be *put on hold* automatically and users vote to open an on-topic question among them. This suggestion illustrates there have been so many off-topic and low-quality questions and answers posted and deleted on ELU. 

ELU is open to everybody in the world. Even a user who doesn't bother to register their user name can post a question and answer (except to closed and protected questions) and edit any post they want. 

We can't stop new users from asking, answering a question and editing a post. We need to 

1. Post a comment for clarification, additional information, request to show research.
2. Flag a question if you don't have 3,000 reputation points needed to vote to put a question on hold.
3. Downvote a post which is not useful and doesn't show research efforts.
4. Monitor suggested edits, especially those submitted by anonymous users, and review first posts and late answers carefully.

There are mechanisms we can use to deal with low-quality posts. I am one of the most aggressive users who are employing them to clean up the site. Even though asking new users to take mandatory English quiz might help reduce the number of low-quality posts, it is not an ideal solution considering the manpower and time it will take. 

Related questions: 

[What can we do to make this site more "intimidating"?](https://english.meta.stackexchange.com/questions/7429/what-can-we-do-to-make-this-site-more-intimidating)

[Does ELU Have Worse Questions Than Other Sites?](https://english.meta.stackexchange.com/questions/7535/does-elu-have-worse-questions-than-other-sites)

[Extraordinary spike in low-quality questions by 1 rep users](https://english.meta.stackexchange.com/questions/7576/extraordinary-spike-in-low-quality-questions-by-1-rep-users)