# Seasonal intervention effects

I have estimated an intervention model on the log of a seasonally differenced series of sales figures and have used a step function rather than a pulse function. My question is: how do I interpret the intervention effect ? If the coefficient is -0.05 am I correct in saying that the intervention resulted in a 5% decrease in sales? Is that interpretation correct even though the original series to which the intervention was applied is in seasonal differences (it's a weekly series)?

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Well, it's roughly a 5% (you may want to look at a confidence interval for that value to see how wide it is) dip in the average seasonal differences, $y_t - y_{t-s}$ (for seasonal period $s$). –  Glen_b Feb 17 '13 at 1:23