Ch4: Price Is Right error function query

[noelevans]

I'm working my way through the Price Is Right example in chapter 4. There is a function, error (below) incorporated in the pymc model. The tau value for this distribution is 1 / (3e3) ** 2 but I don't understand how the value 3000 has been chosen for the denominator. I see this is the SD of the holiday (as well as the mean for the snowblower). If it is the holiday's SD why is that value appropriate and not the SD of the snowblower?

@pm.potential
def error(true_price=true_price, price_estimate=price_estimate):
        return pm.normal_like(true_price, price_estimate, 1 / (3e3) ** 2)