Construct the control charts for the mean and the range, using the original $15$ samples.

On the control charts developed in part $($a$),$ plot the values from samples $16$ through $25$ and comment on whether the process is in control.

In part $($b$),$ if you concluded that the process was out of control, would you attribute it to a drift in the mean, an increase in the variability, or both? Explain your answer.