Statistically how many unconfirmed transactions get rejected? I need to analyse unconfirmed transactions and I'm interested in knowing what is the probability for a given unconfirmed transactions to be confirmed.

Depending on the model, I see two solutions:


checking for a relatively long period of time how many transactions had been submitted and checking how many had been confirmed.
searching pattern behavior in the structure of a transaction that will lead to a rejection.
For a transaction to be included in a block, the probability is related to the fee that transaction pays to the miner (higher fee = more likely) and varies over time based on demand of block space (more congested = higher fee needed). It's also unpredictable because miners can choose whatever transactions they want to, doesn't always have to be the x highest fees. 

Here's some stats on current fee vs confirmation time estimations: https://bitcoinfees.21.co