Vary the specified parameter(s) in order to estimate their uncertainties. Each specified parameter, in turn, is stepped and chi^2 is minimized. Stepping stops when the requested value of Delta chi^2 is obtained. "Uncertain" can take up to 3 numbers as arguments. If one number contains a decimal point, then that number is interpreted as the requested value of Delta chi^2 (which for the first time defaults to 2.7 and for later times defaults to the previous value). The remaining two numbers in the "Uncertain" command specify the lower and upper parameter numbers for which you want to estimate the error. If only one number is given (without a decimal point), then the error is generated only for a single parameter. If errors are currently turned off (or do not exist), then the routine works out a correction factor that converts the W-VAR to chi^2.