$1.(25$ pts$)$ The data below are monthly of the head sets of Yonsei Electronics $\backslash$& Co$.$

$($a$)$ Determine the linear regression equation, MAD, and bias. What is forecast for September?

$($b$)$ Using the double exponential smoothing with $\backslash (\backslash$alpha$=0\mathrm{.}2\backslash )$ and $\backslash (\backslash$beta$=0\mathrm{.}2\backslash ),$ forecast monthly demand.

Assume that the actual demand in March is the same as the forecasted demand and the initial level and the initial trend for March both are $10.$ Determine the MAD and the bias. From these results, forecast the demand for October.

$($c$)$ For the analysis performed in $($a$)$ and $($b$),$ which technique is more desirable?