Any research on a baseline traffic loss due to SERP crowding?

[NetvantageMarketing]

Hello all. I work with a number of ecommerce sites and in trying to gauge year over year performance I'm struggling to properly account for some of the organic shrinkage in SERPs. Many people have discussed the topic of SERPs with 6 & 7 results and the growing real estate being taken up by paid results. What I want to know is if anyone out there has done some large scale number crunching to find some averages on lowered CTR or traffic loss so I can better understand my numbers. Anyone have any input here?

[Debdulal]

_I can feel your pain here. Since we cannot count on the ranking only because of its unpredictable nature, you can compare and contrast things like the number of total visitors, total organic visitors, referral visitors etc. I think these metrics are good enough to figure out how things are faring. _