We already know that #contentmarketing is an effective #outboundmarketing method.
However, trying to determine how effective that emailed article is in driving sales, you will probably compare it (that method or even the article itself) with the effectiveness of other marketing channels - but the results do not paint an accurate picture.
Your target audience might not be buying after they read the articles. But your brand awareness and inbound website traffic may have increased along with your search engine results rankings.
You might reinforce your brand position with the articles but sales may come in via other channels instead of the email’s direct call-to-action, resulting in a belief that the articles and your content marketing efforts are seemingly “ineffective”.
Don’t be fooled by oversimplified data analysis.