If your email marketing system transmits email in MIME format how do you know what format (HTML or Text) your recipients are reading? Most systems will calculate open rates however this is only regulated for HTML formats. In addition most systems will calculate clicks however they do not distinguish between a text click or HTML click.
If you're fortunate enough to have an email service provider like Gold Lasso that calculates unique opens and unique clicks that distinguishes between HTML and text formats you can easily calculate a rough distribution between the two.
Here are the formulas:
HTML format = Number of recipients who requested HTML formats + (Unique Opens - Number of recipients who requested HTML formats)
Text format = Number of recipients who requested text formats + (Unique Text Clicks - Number of recipients who requested text formats)
MIME Format = Number of transmissions - (HTML format + Text format)
The reason why you need to subtract the number of recipients who requested a specific format from the Unique stat is because you do not want to count them twice.
Hint: Over time if you see your text versions start to increase it is probably because your recipients are reading your email from a mobile device. Many of our B2B clients are observing the text stat increase.