Very Likely

Amazon sent me another offer for diapers one size smaller than what I purchased last. This time, they were trying to sell me size 4 pull-ups, remembering that I bought size 5 pull-ups a while back. As a matter of fact, we just ran out, and realized too late. If they had sent me a reminder to buy size 5s a week ago, they might have sold me...

The likelihood of someone who buys size 5 pull-ups buying size 4 pull-ups is about as high as that same person shopping for

• a trip to the Heard and McDonald Islands,
  
• German chocolate, or
  
• guillotine replicas.
  

In other words, it's completely random, and actually rather unlikely since most families have children no more often than every 9 months, usually less often than that...

In my mind, this is elementary marketing. If you try to sell someone on related products, you need to take into account that some things are bought in sequence. Someone who bought an advanced engineering book won't be interested in "Elementary Principles of Engineering." Someone who bought size 4 diapers may be interested in size 4 and 5. You'd think Amazon would have this down.