I actually had 10 minutes to do some research today and decided to get around to reading Kevin Edwards' presentation from the A4U Expo last year.
If you have not read it yet I recommend you do, with the snappy title The Truth Will Out: Disproving Common Misconceptions In Affiliate Marketing
2 big points are discussed:
Voucher code sites do not just grab the sale at the last minute with the last click
- Something I have seen myself quite clearly, but obviously there is a crossover at times.
The different models and behaviour of surfers make it hard to suggest 1 universal approach for commission attribution models based on first, middle and last click
- analysis performed by i-level among others shows the skiing concept I have to question how you include non-affiliate clicks when paying a CPA. I also think the growth of cash back sites makes it even harder to split commissions. Kevin has some good examples of how the models would work for different products.
Well worth a read.