Final answer:
Response bias is a phenomenon that occurs when customers feel obligated to respond in a certain way to promotional materials, even if their true opinions or behaviors don't align. This can affect the accuracy of data collected in email marketing campaigns.
Step-by-step explanation:
The phenomenon that usually occurs when customers have subscribed to receive promotional materials is known as response bias. This bias refers to the tendency for individuals to respond in a certain way that may not accurately reflect their true opinions or behaviors. In the context of email campaigns, response bias can occur when customers feel obliged to respond positively, even if they are not genuinely interested or receptive to the promotional materials. For example, customers may feel pressured to subscribe or provide positive feedback to avoid disappointing the sender or to maintain a positive relationship. This bias can lead to inflated response rates and misleading data for businesses conducting email marketing campaigns.