Average Failure Rate Of High-end Notebook Pcs?

Now that I'm involved in the IT purchasing decisions for a department of 700 employees, I have a question on what is the average (or acceptable) failure rate of business-grade notebook PCs.

Assume you purchased 100 Microsoft Surface Pro tablets (about $1300 each) at one time. Your goal is to keep each deployed for 4 years before replacing them. Excluding accidental damage caused by users (such as a cracked screen), what is considered an "acceptable" or "average" failure rate (the unit is unusable due to a bad board, dead Wi-Fi, etc.) at one year? Two years? Three years? Four years?

Having done the math, some batches of units our department has now seem to be averaging a failure rate of over 10% after two years and 15-20% within three years of deployment. Although under a 3-year warranty, Microsoft makes our (very qualified) techs jump through hoops when we have a warranty claim; it often feels like pulling teeth trying to get a unit with an obvious hardware failure repaired or replaced.

The first batches are scheduled to be retired and replaced by the end of this year; Lenovo Yogas and Dell XPS units are starting to look more attractive by the day.

Any thoughts on this from fellow IT Pros? Thanks in advance.

 

I am no IT Pro, either, but until they appeared to be stopping Toshiba production, I had great experiences with Toshiba. I have often had Dell XPS-No problems.
I have had Lenova for the last 4 years- I have the Lenova G70-70, core i7, and although the motherboard has always worked great- Ihave changed the top base unit twice, and base unit bottom part once, in 4 years--I would say this is rathe fragile for heavy office use, myself- but I like it.
Overall the most robust seemed to be Dell, but, as I said, I am not an expert.
Just my 4 pence worth,