Q. Are you certain about that?

I have been asked in a deposition from time to time whether I was certain as to the truth of what I had said. 

My answer is a shortened version of this: 

No. I am not certain. I am not terribly uncertain either. I am openminded about this.  I describing empirical matters and expressing conclusions and opinions about them. 

Empirical propositions are always contingent and often slightly–or very slightly–wrong. Sometimes it is simply the function of the scientific and objective mind is to be ready to recognize errs and ready to revise descriptions, hypotheses, about them, conclusions from them, or opinions based upon them. 

I am usually right about what I actually assert since I built a little bit of rational and truthful vagueness in, or what I say is close enough to right that the error is of no consequence. 

Here is an example: usually, in measuring small distances an error of 1/100 of an inch is an actual error, but also actually, or almost completely negligible, except in unusual-to-rare, high precision situations. Still, the pervasiveness of negligible, indifferent errors is enough to justify the avoidance of certainty-of-mind and dogmatism both in what is said and in what is thought. Those who do not recognize this principle are not just dogmatic but epistemologically arrogant, and even dangerously prideful. 

At the same time, many errors might be described as clear errors. One can be pretty certain about them. Missing by 1/100 of an inch in ordinary circumstances is one thing; mismeasuring by 2 inches is quite another. 

In my experience, lawyers often ask a witness whether s/he is certain about what s/he just said.  At least sometimes it might be a good idea to help a witness  tin thinking about how to respond.