Response Rate Confusion

[Aaron Robinson]

Response rate is simply, (responses ÷ number of mail pieces sent out) right?

Soooo here's my issue with this... 

If I send 100 people a postcard (so that's 100 postcards), and 90 call back, that's a 90% response rate right?
But what happens when I mail the other 10 again next month? So now I've sent out 110... 

If I get 10 calls, is that a 100% response rate (10 calls divided by 10 mailings, or 100 calls divided by 100 people)?

or is that a 90% response rate (100 calls divided by 110 mailings)???

How do we calculate response rate? Per mailing or per person? By definition, response rate is based on how many people u invite vs. how many respond (not how many times u invite them). You can keep separate numbers to determine how many letters/touches it takes and how much your total campaign costs, but response rate seems like it should be per person (not per mailing). Thoughts???