Suppose I run an experiment which tests which version of an email is most likely to cause people to click on a survey link inside the email.
There are two experimental manipulations: the email subject and the text within the email. For each experimental manipulation there are two levels (treatment and control). Both experimental manipulations are randomized independently of each other.
Normally (assuming no interaction effects), one would simply estimate treatment effects by running the following regression:
$ClickedLink_i = \beta_0 + \beta_1 EmailSubject_i + \beta_2 EmailText$
Where ClickedLink is a binary variable equal to 1 if user i clicked on the survey link, 0 otherwise. EmailSubject and EmailText are also binary variables, equal to 1 if the user received the Treatment email subject and email text respectively. (It may actually be more appropriate to use logit/probit models, and feel free to use those when answering, but this isn't the main point of the question)
I notice that email subject actually affects whether users actually see the email text. That is, if users don't click on the email after reading the subject line, the text of the email won't matter. How can I reformulate my regression to take this into account?