The simplest answer is “everything”.  Maybe the question should really be, “What Should Influence Decisions?”

I suppose the answers depend on who you are, but since this is my blog, I can only offer you my view.  Here is the list as I see it:

• Safety
• Performance
• Expected life
• Reliability
• Maintainability
• Ease of operation
• Cost

I would include legal requirements, such as government regulations, but they can affect any of the above.  And in that sense, they do affect decisions.  Weight is also a frequent requirement, so is power usage, range of operating conditions, simplicity, etc.  However, these types of requirements usually fall under one or more of the above categories.

What this should tell you is that writing the requirements for any particular design is by no means an easy task.  And getting them right is extremely important.

And what shouldn’t affect decisions, or at least should not significantly compromise a proper design?

• Politics
• Personal animosity
• Ignorance or stupidity on the part of the customer — that may take some polite convincing on your part.

We haven’t discussed schedule.  As they say, “time is money”.  I would caution you against swapping time for any number of real requirements, but don’t hang on to unnecessary requirements.  When you make a promise of time to a customer, put yourself in their shoes.  Quicker is better, as long as you meet all necessary requirements.

One last thing, aesthetics!

So, here is a little story from the past ––

We were in competition for particular device that was to be mounted to the Apollo Space Suit.  And as a part of that competition, all the potential suppliers were required to show prototypes of their designs to NASA.  We all complied, but there was one thing that our company did that a competitor did not do.  We paid all of our attention to performance and none to aesthetics — even in how we presented our design that day.  Our man on the scene said that he placed our fully compliant design on the table for NASA to see.  Ours was not bright and shiny, nor was it colored in anyway.  It simply worked as ordered.  Right after our man placed our device down, the competitor brought out a Tensor light and placed in on the table.  Then he put down a piece of black velvet.  And then he put down their design on the velvet — all shiny and color anodized.  And then he turned on the light.  The only thing their design lacked was compliance to the design requirements — not a small thing one would think.  And what went through the mind of our man?  “We are going to lose.” And so we did.

Not sure what to do with this imformation…