def first_differences(signal):
	"""The mean of the absolute values of the first differences of the raw signal"""
	# build the first differences list
	first_diff = []
	for i in range(0,len(signal)-1):
		first_diff.append(abs(signal[i+1]-signal[i]))

	fd_sum = sum(first_diff)
	delta = float(fd_sum)/(len(signal)-1)
	return(delta)