You can view an individual contact's stats by selecting its number from anywhere in your menu, then selecting Contact Stats.
The Stats tab holds all the quantitative information pertaining to a contact, including:
- First date of contact
- Last date of contact
- Number of messages sent
- Number of messages received
- Response rate
- Average response time
A response counts as any text a contact sends in response to a message you sent within a 24 hour period. Their reply only counts toward the last message you sent—so if you send multiple messages, and the contact only replies to the last one, the other messages aren't counted toward your response rate.
It's also worth noting that some of your messages will have naturally low response rates. For example, if you text your contacts, "Hey all, just wanted to let you know we'll be closed for Memorial Day," you're most likely not going to get a high response rate for that message.