Experimentation
Confidence interval
A range of plausible values for the true effect of a change, such as 'conversion up by between 0.4 and 2.1 per cent'.
A single number like '+1.2 per cent' hides how uncertain it is. The confidence interval shows the range the true effect probably falls in, given the data.
It is often more useful than the p-value. An interval that sits entirely above zero says the change helped; one that spans zero says you cannot tell yet; a narrow one says you measured precisely.