ndenis - What does it mean to be "95%" sure of something?

If you are not sure, then you are unsure.

Can someone be "95%" alive?

It's statistical confidence over a range of samples, it's something you could apply to a population rather than an individual because 'confidence' depends on the natural variation there is in a range of samples.

I.e. part of my job is materials testing. I might be given 100 samples, 50 made using the current method, 50 made by some cheaper process or substitution and then be asked if there's a significant change in strength.

The answer isn't yes or no strictly. The answer is usually along the lines of 'yes/no with 65% confidence, if you want 90% confidence I'd have to test about another 200 samples'.

The level of confidence is statistical, depends on number of samples, variation between them, strength of the effect, etc. It's not like I take a straw poll around the Instron machine.