[Grammar] the question AS TO why

[borocks]

I'm struggling with this sentence:

At the core, we investigate the question as to why the these industries have been characterized by poor labour productivity growth.
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Is the "as to" correct and appropriate for a formal academic text?

I'm uncertain because sometimes in academic papers you read

... we investigate the question of why ...
... we investigate the question why ...


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Any guidance is appreciated. Thanks!

 

[bhaisahab]

Yes, it's fine. It's OK with just "why" as well; I don't like it with "of", though.