Here, is the number of measurements per observation, is the number of observations, and is the number of parameters being estimated. For a linear fit, is 2 (the slope and the intercept). For a quadratic fit, is 3. The 0.5 offset is added as a rule of thumb to produce an unbiased estimation of standard deviation for a normal distribution. can be used to indicate variability if the expected measurement noise covariance matrices, , are correct using the confidence interval, , which is