in statistics, a confidence interval, abbreviated as ci, is a special interval for estimating a certain parameter, such as the population mean. with this method, a whole interval of acceptable values for the parameter is given instead of a single value—together with a likelihood that the real (unknown) value of the parameter will be in the interval. thus, if we're not sure of the exact number of vehicles that crossed a bridge, we can say 400 plus or minus 10 instead of just saying 400.