Harry Cliff at Literary Hub:
The first thing to understand is that all measurements come with “uncertainties” or “errors,” two words often used interchangeably. The uncertainly on a measurement is an expression of the precision with which we think we have measured a particular quantity. Uncertainties come in two key types. There are statistical uncertainties and systematic uncertainties.
Let’s start with statistical uncertainty. To draw on the classic example, imagine I gave you a coin and asked you to determine if that coin is fair, or to put it more formally, if the probability of that coin coming up heads or tails is equal. To test this, you toss the coin twice and get one head and one tail. On the face of it, this might suggest that the coin is fair.
But you probably have the feeling that we can’t really be sure from only two tosses. Indeed, like all measurements, this one comes with an uncertainty, and with only two coin tosses this uncertainly is large. I’ll spare you the math and tell you that it’s around 26 percent.
More here.