    Calculates the reduced chi-squared, used to compare a model to observations. For example can be used to calculate how good a fit is by using fitted y values for y_model along with observed y values and error in those y values. Reduced chi-squared should be close to 1 for a good fit, lower than 1 suggests you are overestimating the measurement error (observation_error you entered is higher than the true error in the measurement). A value higher than 1 suggests either your model is a bad fit OR you are underestimating the error in the measurement (observation_error you entered is lower than the true error in the measurement). See https://en.wikipedia.org/wiki/Reduced_chi-squared_statistic for more detail.