In 2013, a study showed that 59% of UK residents would not use a company with a website that has poor grammar. It concluded to say that professionalism is of the utmost importance when communicating with a brand.
Now you might think with this statistic that big brands take extra care to have high quality content. You would be sorely mistaken.
In 2015, Acrolinx surveyed 150,000 pages from 340 companies across the world, all of which touted $250,000,000 or more in annual revenue. They assigned each company an "impact score," which was calculated by averaging the number of grammatical errors found in their content per every 1000 words. Fewer errors meant a higher score.
Interestingly, only 31% of companies received an impact score higher than 72. The telcom industry in particular tested very poorly.
These numbers should make us all take a step back and think about our content strategy, considering any language conventions we might be using or ignoring that might be harming our website's ability to convert. Not having proper grammar on your website costs you the trust of your users, and that is a statistic that cannot be measured with backlinks or keyword rankings. It is a lasting perception of your brand.