def second_differences(signal):
	"""The mean of the absolute values of the second differences of the raw signal"""
	sec_diff = []
	for i in range(0,len(signal)-2):
		sec_diff.append(abs(signal[i+2]-signal[i]))

	fd_sum = sum(sec_diff)
	delta = float(fd_sum)/(len(signal)-2)
	return(delta)