A software company emails a satisfaction survey to 1,000 customers drawn as a simple random sample from its complete customer registry.
400 customers reply. Of those 400, 70% report being satisfied.
Uneasy about the 600 who ignored the email, the company then phones them and eventually gets an answer from all 600. Among these 600 non-respondents, 40% are satisfied.
By how many percentage points does the reply-only figure of 70% overstate the true satisfaction rate across all 1,000 selected customers?
Select all that apply.